Trinuclear CuII Structural Isomers: Coordination, Magnetism, Electrochemistry and Catalytic Activity towards the Oxidation of Alkanes

Article abstract of DOI:10.1002/ejic.201500440

The reaction of the Schiff base (3,5-di-tert-butyl-2-hydroxybenzylidene)-2-hydroxybenzohydrazide (H₃L) with copper(II) nitrate, acetate or metaborate has led to the isomeric complexes [Cu₃(L)₂(MeOH)₄] (1), [Cu

Full text of DOI:10.1002/ejic.201500440

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DOI:10.1002/ejic.201500440
Trinuclear CuII Structural Isomers: Coordination,
Magnetism, Electrochemistry and Catalytic Activity
towards the Oxidation of Alkanes
Manas Sutradhar,*[a] Luísa M. D. R. S. Martins,[a,b]
M. Fátima C. Guedes da Silva,*[a] Kamran T. Mahmudov,[a,c]
Cai-Ming Liu,[d] and Armando J. L. Pombeiro*[a]
Keywords: N,O ligands / Copper / Magnetic properties / Electrochemistry / Oxidation
The reaction of the Schiff base (3,5-di-tert-butyl-2-hydroxy- plexes and that there exists very weak ferromagnetic inter-
benzylidene)-2-hydroxybenzohydrazide (H3L) with cop-
per(II) nitrate, acetate or metaborate has led to the isomeric
complexes [Cu3(L)2(MeOH)4] (1), [Cu3(L)2(MeOH)2]·2MeOH
(2) and [Cu3(L)2(MeOH)4] (3), respectively, in which the li-
gand L exhibits dianionic (HL2–, in 1) or trianionic (L3–, in
2 and 3) pentadentate 1κO,OЈ,N:2κNЈ,OЈЈ chelation modes.
Complexes 1–3 were characterized by elemental analysis, IR
spectroscopy, single-crystal X-ray crystallography, electro-
chemical methods and variable-temperature magnetic
susceptibility measurements, which indicated that the intra-
trimer antiferromagnetic coupling is strong in the three com-
molecular interactions in 1 but weak antiferromagnetic inter-
molecular interactions in both 2 and 3. Electrochemical ex-
periments showed that in complexes 1–3 the CuII ions can be
reduced, in distinct steps, to CuI and Cu0. All the complexes
act as efficient catalyst precursors under mild conditions for
the peroxidative oxidation of cyclohexane to cyclohexyl hy-
droperoxide, cyclohexanol and cyclohexanone, leading to
overall yields (based on the alkane) of up to 31% (TON =
1.55ϫ103) after 6 h in the presence of pyrazinecarboxylic
acid.
have important structural and magnetic properties in rela-
tion to their role in the active site of multi-copper ox-
idases,[6] whereas closed and linear triangle CuII ions have
been predominantly studied in order to investigate magnetic
interactions.[7] Although the magnetic interactions of CuII
complexes have been well explored, only a few Schiff-base
complexes containing the diazene bridge with a trinuclear
array of copper centres have been reported.[7c,7d,8]
The main aim of this work was thus to prepare other
types of trinuclear CuII complexes with a diazene bridge
and to examine the effect of different copper salts in reac-
tions with Schiff bases, which are known to stabilize metal
complexes in various oxidation states and nuclearities[9]
and, in particular, can play an important role in the synthe-
sis of multinuclear copper complexes.[8,10] Another aim was
to explore their possible catalytic applications in the field
of alkane functionalization (see below).[11–14]
Introduction
The chemistry of trinuclear copper(II) complexes has
been intensively studied owing to their potential biological
activities, for example, laccase,[1] methane monooxygen-
ase,[2] ascorbate oxidase,[3] as well as for their active role as
model systems to gain an understanding of the magnetic
interactions and magneto-structural correlations in molecu-
lar systems.[4] In general, trinuclear CuII complexes can be
classified into three types: 1) open triangular, 2) closed tri-
angular and 3) linear, depending upon the arrangement of
the metal centres.[5] Open triangle trinuclear CuII complexes
[a] Centro de Química Estrutural, Instituto Superior Técnico,
Universidade de Lisboa,
Av. Rovisco Pais, 1049-001 Lisboa, Portugal
E-mail: manaschem@yahoo.co.in
fatima.guedes@tecnico.ulisboa.pt
pombeiro@tecnico.ulisboa.pt
http://groups.ist.utl.pt/~cqe.daemon/members-contacts/
research-groups-2/coordination-chemistry-and-catalysis-ccc/
[b] Chemical Engineering Department, ISEL,
R. Conselheiro Emídio Navarro, 1959-007 Lisboa, Portugal
[c] Department of Chemistry, Baku State University,
Z. Xalilov Str. 23, Az 1148 Baku, Azerbaijan
[d] National Laboratory for Molecular Sciences, Center for
Molecular Science, Institute of Chemistry, Chinese Academy of
Sciences,
In addition to the biological and magnetic importance of
multinuclear copper complexes, their catalytic applications
have also been established.[10a,11g–1] In particular, the search
for mild, efficient and direct routes for the oxidative func-
tionalization of inert alkanes (relatively cheap and abun-
dant materials) to more valuable organic products is of high
industrial significance.[12] An example[11d–11i,12c–12f,12h–12j,13]
is the catalytic oxidation of cyclohexane to cyclohexanol
and cyclohexanone, which are important reagents for the
Beijing 100190, P. R. China
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201500440.
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production of adipic acid and caprolactam used for the copper(II) structural isomers, depending on the metal salt
manufacture of Nylon-6,6, for which the need for alterna- used (Scheme 1). The reaction of H3L in methanol at room
tives to the current industrial route, using more active and temperature with Cu(NO3)2·2.5H2O in the presence of tri-
selective catalytic systems under milder reaction conditions, ethylamine, Cu(OOCCH3)2·H2O or CuB2O4 (copper meta-
has been recognized.[11a,11d,12c,14]
borate) resulted in the formation of the green trinuclear
A promising approach consists of the design and devel- copper(II) complexes [Cu3(L)2(MeOH)4] (1), [Cu3(L)2-
opment of new bio-inspired catalysts.[2b,11e–11i,12h–12k,13h] (MeOH)2]·2MeOH (2) or [Cu3(L)2(MeOH)4] (3), respec-
Among various metals with a recognized biological func- tively. It is well known[18] that hydrazone-based ligands ex-
tion, copper is cheap and widespread in nature, being pres- hibit keto–enol tautomerism in solution (Scheme 1) and
ent in the active sites of many oxidation enzymes often as complex formation takes place by deprotonation of the enol
di-, tri- or polynuclear Cu centres,[15] such as particulate form. The trianionic (L3–) form of the ligand has two coor-
methane monooxygenase (pMMO), a multi-copper enzyme dination pockets, one bidentate and one tridentate, and
that catalyses the hydroxylation of alkanes.[16] Recently, sev- binds simultaneously to two metal ions, similarly to what
eral multi-copper(II) complexes have been found to exhibit has been found for N-substituted salicylhydrazide com-
a high catalytic activity in the oxidation of cycloalkanes by plexes[19] (Scheme 1).
hydrogen peroxide to the corresponding alkyl hydroperox-
In all the complexes, it is the trianionic form (L3–) of the
ligand that coordinates to the CuII centres. In the linear
ides, alcohols and ketones under mild conditions.[11e–11i,17]
Herein we report the synthesis of three structural isomers trinuclear complex 1, the copper cations exhibit square-
of new trinuclear CuII complexes with a Schiff base contain- pyramid and square planar geometries, in 2, the terminal
ing a diazene bridge by using three different copper salts and central CuII ions present square-planar and distorted
and explore their magnetic and electrochemical properties tetrahedral geometries, respectively, and in 3, the ligands
as well as their catalytic activity towards the oxidation of stabilize a linear trinuclear copper(II) complex in which the
alkanes.
metal ions are in square-planar and octahedral environ-
ments.
Results and Discussion
Crystal Structures
The Schiff base (3,5-di-tert-butyl-2-hydroxybenzylidene)-
2-hydroxybenzohydrazide (H3L) was obtained by the con-
densation reaction of 3,5-di-tert-butyl-2-hydroxybenzalde-
Overall Features
Crystals of compounds 1–3 suitable for X-ray diffraction
hyde with salicylhydrazide and was used to synthesize three were obtained upon slow evaporation of their methanolic
Scheme 1. Synthesis of complexes 1–3.
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Table 1. Crystal data and structure refinement details for complexes 1–3.
1
2
3
Empirical formula
Mr
Crystal system
Temperature [K]
C48H66Cu3N4O10
1049.66
triclinic
C48H74Cu3N4O14
1121.73
monoclinic
296(2)
C48H66Cu3N4O10
1049.66
monoclinic
150(2)
150(2)
¯
Space group
P1
C2/c
P21/c
a [Å]
b [Å]
c [Å]
α [°]
6.6851(9)
10.2750(11)
18.641(2)
83.106(7)
82.419(7)
79.292(9)
1241.1(3)
1
26.7119(10)
14.8620(5)
13.9824(4)
90
98.562(2)
90
5489.0(3)
4
1.357
19.7555(12)
11.3640(6)
11.1664(8)
90
100.038(2)
90
6991.5(7)
2
1.412
β [°]
γ [°]
V [Å3]
Z
Dcalcd. [gcm–3]
1.404
1.332
μ(Mo-Kα) [mm–1]
1.215
1.340
Reflections collected/unique/observed
Rint
7153/4131/2252
0.0873
0.1017, 0.2674
1.103
19578/5261/3877
0.0653
0.0533, 0.1374
1.027
20434/5186/3799
0.0633
0.0424, 0.0945
0.944
Final R1[a] , wR2[b] with I Ն 2σ(I)
Goodness-of-fit on F2
[a] R = Σ||Fo| – |Fc||/Σ|Fo|. [b] wR(F2) = [Σw(|Fo|2 – |Fc|2)2/Σw|Fo|4]½.
solutions at room temperature. The crystallographic data
and processing parameters are summarized in Table 1, and
selected dimensions and hydrogen-bonding geometries are
presented in Tables S1 and S2 in the Supporting Infor-
mation. Representative crystal structures are displayed in
Figures 1–3 and packing diagrams in Figures 4–6.
Figure 2. Structural representation of 2 with partial atomic label-
ling scheme. Hydrogen atoms have been omitted except for those
involved in hydrogen-bonding interactions (shown as dashed lines).
Symmetry codes to generate equivalent atoms: i) 1 – x, y, 1/2 – z;
ii) x, y, 1/2 + z; iii) 1 – x, –y, 1 – z; iv) x, –y, –1/2 + z.
Figure 1. Structural representation of 1 with partial atomic label-
ling scheme. Hydrogen atoms have been omitted except for those
involved in hydrogen-bonding interactions (dashed lines). Sym-
metry codes to generate equivalent atoms: i) 2 – x, 2 – y, 1 – z;
ii) 1 – x, 1 – y, 1 – z; iii) 1 – x, 2 – y,1 – z; iv) 1 + x, y, z; v) 1 + x,
1 + y, z.
Complexes 1–3 are trinuclear with H3L coordinating to one (in 2) or two (in 1 and 3) methanol molecules. One non-
the metal cations in the trianionic (L3–) form by means of coordinated methanol molecule could also be found in the
both phenolate O atoms, the enolate O atom and both the asymmetric unit of 2.
N atoms in a 1κO,OЈ,N:2κNЈ,OЈЈ-bridging chelate fashion.
In compounds 1 and 2, the central metal cations are in
In 1 and 3, the central metal cations are located at inversion N2O2 environments, assuming perfect square-planar and
centres and in 2 it lies on a two-fold axis. The asymmetric distorted tetrahedral geometries, respectively (τ4 = 0.00 and
units of the compounds contain half of the molecules, 0.83,[20a] respectively), whereas in 3, the central metal cation
namely one and a half copper cations, one L3– ligand and is in the middle of an almost perfect N2O4 octahedron
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moiety of a vicinal molecule, and as acceptors from a meth-
anol ligand of nearby molecules (Figures 1 and 4). The 1D
chain in 2 (Figure 5) results from non-coordinated meth-
anol molecules, which simultaneously act as donor to a
phenolate group of an adjacent trimer and as acceptor from
a coordinated methanol ligand of another close molecule.
Compound 3 also features a 2D network constructed from
the interaction of each trimer with neighbouring analogues
with each methanol ligand donating to phenolate groups
(Figure 6).
Figure 3. Structural representation of 3 with partial atomic label-
ling scheme. Hydrogen atoms have been omitted except for those
involved in hydrogen-bonding interactions (shown as dashed lines).
Symmetry codes to generate equivalent atoms: i) 2 – x, –y, 1 – z;
ii) x, 0.5 – y, 0.5 + z; iii) x, 0.5 – y, –0.5 + z; iv) x, –0.5 – y, 0.5 +
z; v) 2 – x, 0.5 + y, 0.5 – z; vi) 2 – x, –0.5 + y, 0.5 – z; vi) 2 – x,
–0.5 + y, 1.5 – z.
(quadratic elongation of 1.043 and angle variance of
19.04°2).[20b] The terminal metals are in N1O4 distorted
square-pyramidal (in 1, τ5 = 0.31)[20c] or slightly distorted
N1O3 square-planar environments (in 2 and 3, τ4 = 0.10).
The central copper cations are incorporated into two six-
membered CuOCCCN rings, whereas the terminal copper
cations feature in one five-membered CuOCNN ring and
one six-membered CuOCCCN ring. The minimum intra-
molecular Cu···Cu distance decreases in the order
4.662(1)Ͼ4.651(2)Ͼ4.643 Å (3), whereas the shortest
intermolecular distance between the metals decreases as
5.332(1)Ͼ5.210(3)Ͼ5.153 Å (2).
Figure 4. Fragment of the 2D network in compound 1, viewed
along the crystallographic a axis.
Each Schiff-base ligand is almost planar, the least-
squares planes of the aromatic rings making angles of 7.35
(1), 5.51 (2) and 7.50° (3), and the CNNC torsion angles
being 169.41 (1), 172.71 (2) and –168.16° (3). In compounds
1 and 3, the terminal metal cations are almost in the same
plane as the ligands, but the central copper atoms are tot-
ally out of plane, as indicated by the CCOCu and CCNCu
torsion angles of the six-membered metallacycles, which
range from 31.87 to 33.67°. In 2, however, these angles are
–17.81 and 11.13°, respectively, and, indeed, both metal cat-
ions lie in the plane of the Schiff-base ligand. These
variations in the coordination parameters of the metals also
affect other structural factors, as expressed by the Cu–
Figure 5. Fragment of the 1D chain in compound 2, viewed along
the crystallographic a axis. The non-coordinated methanol mo-
lecules are drawn by the space-filled model.
O
phenolate bond lengths (Table 1), which range from 1.878(8)
to 1.907(8) Å, the differences between these distances in
each compound being 0.034 (1), 0.012 (2) and 0.049 Å (3).
The Cu–Oenolate bond lengths average 1.938(4) Å and the
Cu–N lengths vary between 1.902(3) and 2.014(3) Å, the
values in each compound diverging by 0.094 (1), 0.031 (2)
and 0.116 Å (3).
Extensive hydrogen-bonding interactions were found in
all the structures (for compounds 2 and 3, see Table S2 in
the Supporting Information). In compound 1, a 2D net-
work results from contacts involving the methanol ligands;
Figure 6. Fragment of the 2D network in compound 3, viewed
they act as donors to a methanol ligand and a phenolate along the crystallographic a axis.
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Magnetism
J = θ = –0.37 K, ρ = 0.031, Nα =
–130.8 cm–1,
0.0044 cm3 mol–1 and R = 4.21ϫ10–6 for 3. Because the co-
ordination geometries of copper in the three complexes are
not always the same, it is hard to analyse the magneto-
structural correlations in detail. Nevertheless, the absolute
value of J increases in the order 1Ͻ2Ͻ3, ascribing approx-
imately to the order of decreasing Cu1···Cu2 distance: 1
(4.662 Å)Ͼ2 (4.651 Å)Ͼ3 (4.643 Å). The results confirm
that the shorter the Cu1···Cu2 distance, the stronger the
antiferromagnetic exchange.
Variable-temperature magnetic susceptibility determi-
nations were carried out on powdered samples of the three
complexes in a field of 1000 Oe in the temperature range 2–
300 K. As shown in Figure S1 in the Supporting Infor-
mation, the χMT value of 0.93 cm3 Kmol–1 for 1 at 300 K
is lower than the spin-only value expected for three isolated
copper(II) ions (1.125 cm3 Kmol–1, assuming g = 2.0).
Upon cooling, the χMT value clearly decreases from room
temperature down to about 100 K, which indicates strong
antiferromagnetic intramolecular interactions. Between 100
and 10 K, the χMT product plateaus at 0.53 cm3 Kmol–1
and then increases below 6 K, reaching a value of
0.63 cm3 Kmol–1 at 2 K. The magnetic data was analysed
by using a symmetric linear S = 1/2 trimer model. Equa-
Electrochemical Behaviour of 1–3
The redox properties of 1–3 were investigated by cyclic
voltammetry (CV) at a Pt disc electrode in a 0.2 m
[nBu4N][BF4]/MeCN solution at room temperature. All
compounds exhibit similar voltammograms: two single-
electron (per Cu ion, as measured by controlled potential
electrolysis) irreversible reduction waves (see Figure S3 in
the Supporting Information for complex 1), assigned to the
CuII ǞCuI (wave I) and CuI ǞCu0 (wave II) reductions, at
the reduction peak potentials given in Table 2 (IEpred ranges
ˆ ˆ
ˆ
tion (1) is derived from the Hamiltonian H = –2J(S1S2 +
ˆ ˆ
ˆ ˆ
S2S3)– 2JЈS1S3 [JЈ is assumed to be zero due to the large
distance between the external copper(II) ions].[8c] In Equa-
tion (1), N, k and β represent the usual constants. However,
this model could not reproduce the increase in χMT ob-
served below 6 K, so we included intermolecular interac-
tions (zjЈ) modelled with the molecular field approximation,
see Equation (2). With this improvement, the magnetic
properties could be well fitted over the whole temperature
range, with g = 2.01, J = –108.7 cm–1, zjЈ = 0.42 cm–1 and
R = 3.34ϫ10–5. The results indicate that the intratrimer
antiferromagnetic coupling is strong and that a very weak
ferromagnetic intermolecular interaction exists.
red
from –0.37 to –0.61 V vs. SCE, and IIEp between –1.21
and –1.69 V vs. SCE). A new irreversible anodic wave (wave
I
ox
III, Ep in the range –0.27 to –0.22 V vs. SCE) is also
observed upon scan reversal after the second cathodic pro-
cess (see Figure S4 for complex 2), which corresponds to
the oxidation of the cathodically generated Cu0 species. No
anodic waves were detected for any of the complexes in a
first anodic sweep without a previous reduction scan, which
indicates that neither a metal- nor a ligand-centred oxid-
ation is observed.
(1)
Table 2. Cyclic voltammetric data[a] for complexes 1–3.
Ep [V] (Ired
)
Ep [V] (IIred
)
Ep [V] (Iox [b]
)
(2)
1
2
3
–0.56
–0.37
–0.61
–1.53
–1.21
–1.69
–0.26
–0.27
–0.22
Similarly to 1, the room-temperature χMT values for 2
(0.87 cm3 Kmol–1) and 3 (0.96 cm3 Kmol–1) are also smaller
than those expected for three non-interacting copper(II)
ions (see Figure S2 in the Supporting Information). How-
ever, unlike 1, the decrease in the χMT product continues to
2 K, reaching 0.43 and 0.44 cm3 Kmol–1 for 2 and 3, respec-
tively. These results suggest intramolecular antiferromag-
netic exchange and an odd number of spins in the ground
state.[8c] The same Hamiltonian for 1 was used to describe
this symmetric linear S = 1/2 trimer magnetic coupling
model, however, it is necessary to consider the mole fraction
of the paramagnetic impurity (ρ), the Weiss-like tempera-
ture correction (θ) and temperature-independent paramag-
netism (Nα) for a better fitting, see Equation (3).[8c]
[a] Potential values in volts (Ϯ0.02 V) vs. SCE in a 0.2 m
[nBu4N][BF4]/MeCN solution at a Pt disc (d = 0.5 mm) working
electrode and determined by using the [Fe(κ5-C5H5)2]0/+ redox cou-
ple (E1/2 = 0.42 V vs. SCE)[21] as internal standard at a scan rate
ox
of 200 mVs–1. [b] Anodic wave generated upon scan reversal fol-
lowing the second reduction wave (IIred).
Similar behaviour has previously been reported[22] for tri-
nuclear triangular copper(II) compounds containing the
[Cu3(μ3-OH)(μ-pz)3]2+ (pz = pyrazolate) core stabilized by
different carboxylate ligands and additional solvent or Hpz
moieties.
The occurrence of a single-electron reduction per CuII
(or CuI) atom was confirmed by exhaustive controlled po-
tential electrolysis (CPE) at a potential slightly cathodic rel-
ative to that of the peak potential of wave I (or II), which
corresponds to a charge consumption of 3 Fmol–1. The
(3)
Analysis of the exchange by using Equation (3) gave g = CPE performed for the second reduction wave led to the
1.97, J = –126.4 cm–1, θ = –0.21 K, ρ = 0.013, Nα = deposition of metallic copper, which is clearly observed by
0.00016 cm3 mol–1 and R = 6.99ϫ10–6 for 2, and g = 2.02, its sharp oxidation (with desorption) wave at Epox in the
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range of –0.27 to –0.22 V versus SCE, as shown in Fig- up to 31% (relative to the alkane, Table 3, entry 3) after 6 h
ure S4 in the Supporting Information for compound 2. reaction time and overall turnover numbers (TON) of up
Because each of the cathodic waves involves the re- to 1.8ϫ103 (6ϫ102 per Cu atom, Table 3, entry 4). The
duction of three metal ions, without differentiation of dis- accelerating effect of pyrazinecarboxylic acid in metal-cata-
tinct waves at different potentials, no metal–metal electronic lysed oxidation reactions has been observed previous-
communication has been detected. Nevertheless, the occur- ly.[14f–14j] Complex 2 is also the easiest one to be reduced
rence of such an interaction, to a limited extent, cannot (Table 2), as shown by cyclic voltammetry (see above),
be ruled out in view of the considerable broadness of the which conceivably facilitates the CuII-catalysed oxidations
reduction waves, which may result from the overlap of other of H2O2 and CyOOH (see the mechanistic considerations
waves at similar, but distinct, reduction potentials below).
corresponding to sequential metal reductions, that is,
Table 3. Peroxidative oxidation of cyclohexane[a] with H2O2 (se-
CuIICuIICuII Ǟ CuICuIICuII Ǟ CuICuICuII ǞCuICuICuI
lected data) catalysed by 1–3.
for wave I, and similarly for further reductions to Cu0 at
Entry
Additive
Total yield [%][b] (Total TON[c])
wave II.[22] Moreover, cathodically induced Cu–ligand bond
cleavage can also occur.[23]
(amount [mol-%]
vs. substrate)
1
2
3
1
2
3
–
2.5 (26)
19 (190)
27 (270)
5.0 (50)
23 (230)
31 (310)
2.2 (22)
13 (130)
17 (170)
HNO3 (2.5)
Hpca (2.5)
Hpca (2.5)
TFA (2.5)
Ph2NH (100)
CBrCl3 (100)
Catalytic Activity
4[d]
5
12 (1.2ϫ103) 18 (1.8ϫ103) 10 (1.0ϫ103)
Complexes 1–3 exhibit good catalytic activity in the oxid-
ation of cyclohexane (CyH) with aqueous hydrogen perox-
ide (Table 3) under mild conditions to yield a mixture of
cyclohexyl hydroperoxide (CyOOH, main primary prod-
21 (210)
0.6 (6)
0.7 (7)
25 (250)
0.3 (3)
1.6 (16)
12 (120)
0.5 (5)
0.3 (3)
6
7
[a] Reaction conditions (unless stated otherwise): cyclohexane
uct), cyclohexanol and cyclohexanone (Scheme 2, a). The (5.0 mmol), 1–3 (5 μmol, 0.1 mol-% vs. substrate), H2O2
(10.0 mmol), room temp. [b] Molar yield (%) based on substrate,
alcohol and ketone are the final products after autodecom-
that is, moles of cyclohexanol + cyclohexanone per 100 mole of
position, the metal-assisted decomposition of the hydroper-
cyclohexane, determined by GC analysis (upon treatment with
oxide or, alternatively, following Shul’pin’s meth-
PPh3). [c] Turnover number (moles of products per mole of catalyst
od,[13a–13d,13g] upon reduction with PPh3 (Scheme 2, b).
precursor). [d] 1–3: 0.01 mol-% vs. cyclohexane.
Yields of up to 31% in the oxidation of an inert alkane
are considered high and are also much higher than that of
the industrial process (4% to reach a good selectivity, at ca.
150 °C)[11a,11d] in spite of the mild conditions of our system
(ambient temperature, atmospheric pressure, with an aque-
ous green oxidant and very low loads of catalyst). Similar
yields (28%) have been obtained with the trinuclear CuII
compound [Cu3(μ3-OH)(μ-pz)3(EtCOO)2(H2O)][17c] or the
mono-copper(II) complex [Cu(H2tea)(N3)] (H2tea = tri-
ethanolamine).[17f]
Moreover, a high selectivity towards the formation of cy-
clohexanol and cyclohexanone is exhibited by our systems,
because no traces of byproducts were detected by GC–MS
analysis of the final mixtures obtained under the optimized
conditions. These features are important for the establish-
ment of a greener catalytic process for cyclohexane oxid-
ation.
Scheme 2. Oxidation of cyclohexane to cyclohexyl hydroperoxide,
cyclohexanol and cyclohexanone with H2O2 catalysed by the Cu
complexes (a), and reduction of cyclohexyl hydroperoxide to cyclo-
hexanol in the presence of PPh3 (b).
The copper salts used for the synthesis of 1–3, Cu-
(NO3)2, Cu(OAc)2 or CuB2O4, exhibited poor activity
(maximum total yields of 4, 3 and 4%, respectively) under
the cyclohexane oxidation conditions of Table 3, which
points to the relevance of the polydentate N,O ligands, con-
The catalytic systems were based on the CuII complexes ceivably associated with their involvement in the proton-
1–3 with hydrogen peroxide (30% aqueous solution) as the transfer steps.[17,25d–25f]
oxidizing agent in acetonitrile in the presence of an acid co-
catalyst [nitric, trifluoroacetic, or pyrazinecarboxylic acid[13e,13f,17,22,24] on the peroxidative oxidation of alkanes
(Hpca)] under atmospheric pressure at room temperature.
catalysed by homogeneous or supported[24g–24j] metallic
The previously recognized promoting effect of an
Complex 2 provided the best activity, achieving overall species is also observed in these systems (Table 3). In fact,
yields, in the presence of pyrazinecarboxylic acid (Hpca), of the presence of an acid co-catalyst (either organic or min-
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IR spectra (4000–400 cm–1) were recorded with a Bruker Vertex 70
spectrometer in KBr pellets. 1H NMR spectra were recorded at
room temperature with a Bruker Avance II + 400.13 MHz (Ultra-
ShieldTM Magnet) spectrometer. The chemical shifts are reported
in ppm using tetramethylsilane as internal reference.
eral) has a strong promoting effect of the catalytic activity
(Table 3, entries 2–5) of 1–3. The role of the acid may be
associated with,[17] for example, the promotion of coordina-
tive unsaturation upon ligand protonation, its involvement
in proton-transfer steps, the acceleration of the oxidation
reaction, the facilitation of the formation of peroxo com-
plexes or the prevention of the decomposition of H2O2 to
water and oxygen (i.e., suppressing eventual catalase ac-
tivity in the acidic medium).
As observed for other copper and metal catalytic sys-
tems,[13e–13h,17,22,23] the introduction of a carbon or oxygen
radical trap (e.g., CBrCl3 or Ph2NH) into the reaction mix-
ture resulted in a marked decrease in the product yield. This
The magnetic susceptibility measurements were carried out on
polycrystalline samples with a Quantum Design MPMS-XL5
SQUID magnetometer in the temperature range of 2–300 K and at
an applied field of 2000 Oe. Diamagnetic corrections for all con-
stituent atoms were estimated from Pascal’s constants.[28] GC mea-
surements were carried out by using a FISONS Instruments GC
8000 series gas chromatograph with an FID detector and a capil-
lary column (DB-WAX, column length: 30 m; internal diameter:
0.32 mm) and the Jasco-Borwin v.1.50 software. The temperature
behaviour, along with the formation of cyclohexyl hydro- of injection was 240 °C. The initial temperature was maintained at
100 °C for 1 min, then raised at a rate of 10 °C/min to 180 °C and
held at this temperature for 1 min. Helium was used as the carrier
gas. GC–MS analyses were performed by using a Perkin–Elmer
Clarus 600 C instrument (He as the carrier gas) equipped with a
30 mϫ0.22 mmϫ25 μm BPX5 (SGE) capillary column. The elec-
trochemical experiments were performed with an EG&G PAR
273A potentiostat/galvanostat connected to a computer through a
GPIB interface. Cyclic voltammograms (CV) were obtained in
0.2 m solutions of [nBu4N][BF4] in MeCN at a platinum disc work-
ing electrode (d = 0.5 mm) at room temperature. Silver and plati-
num wires were employed as pseudo-reference electrode and
counter-electrode, respectively. Controlled potential electrolyses
(CPE) were carried out in electrolyte solutions with the above-men-
tioned composition in a three-electrode H-type cell. The compart-
ments were separated by a sintered glass frit and equipped with
platinum gauze working and counter electrodes. For both the CV
and CPE experiments, a Luggin capillary connected to a silver wire
pseudo-reference electrode was used to control the working elec-
trode potential. The CPE experiments were monitored regularly by
CV, thus assuring no significant potential drift occurred along the
electrolyses. The electrochemical experiments were performed un-
der N2 at room temperature. The potentials of the complexes were
measured by CV in the presence of ferrocene as the internal stan-
dard, and the redox potentials are normally quoted relative to the
peroxide (typical intermediate product in radical-type reac-
tions), supports a free-radical mechanism[25,26] for the oxid-
ation of cyclohexane in this study.
Conclusions
The different structural environments of the copper(II)
centres present in the starting materials for the synthesis of
the trinuclear complexes 1–3 result in different structural
isomers with distinct metal coordination environments.
Variable-temperature magnetic susceptibility measurements
showed that complexes 1–3 exhibit a strong antiferromag-
netic exchange interaction between the CuII ions. The order
of observed antiferromagnetic exchange interactions (J val-
ues) increases with decreasing distance between the copper
ions. The complexes exhibit irreversible CuII ǞCuI and
CuI ǞCu0 reduction waves in cyclic voltammetry measure-
ments, and therefore there is no evidence of any electronic
communication between the metals.
All of the complexes 1–3 behaved as catalyst precursors
for the efficient peroxidative oxidation of cyclohexane by
aqueous hydrogen peroxide in acetonitrile in the presence
of an acid co-catalyst at room temperature. The products
of the oxidation, cyclohexanol and cyclohexanone, were ob-
tained in good yields (overall yield of 31% for 2, maximum
TON 1.8ϫ103) via the formation of the corresponding cy-
cloalkyl hydroperoxide intermediate (CyOOH) following a
radical mechanism, as substantiated by radical-trap experi-
ments. Moreover, the use of an aqueous medium at room
temperature and of an environmentally friendly catalyst is
a significant step forward in the development of green cata-
lytic systems for cyclohexane oxidation.
SCE by using the [Fe(κ5-C6H5)2]0/+ redox couple (E1/2 = 0.42 V
ox
vs. SCE)[21] in a 0.2 m [nBu4N][BF4]/MeCN solution. Mass spectra
were recorded with a Varian 500-MS LC Ion Trap Mass Spectrom-
eter equipped with an electrospray (ESI) ion source. For electro-
spray ionization, the drying gas and flow rate were optimized ac-
cording to the particular sample with a nebulizer pressure of 35 psi.
Scanning was performed from m/z = 100–1200 in methanol solu-
tion. The compounds were observed in the positive mode (capillary
voltage = 80–105 V).
Typical Procedures for the Catalytic Oxidation of Cyclohexane and
Product Analysis: The peroxidative oxidation reactions were carried
out as follows: the catalyst precursor (0.5–10 μmol) and acid
(HNO3, Hpca or TFA; 125 μmol) were dissolved in MeCN
(3.00 mL) with vigorous stirring. Cyclohexane (0.54 mL,
5.00 mmol) and 30% H2O2 (1.02 mL, 10.00 mmol) were then added
and the reaction solution was stirred for 6 h at room temperature
and normal pressure. In the experiments with radical traps, CBrCl3
(5.00 mmol) or NHPh2 (5.00 mmol) was added to the reaction mix-
ture. The product analysis was carried out as follows: cyclohep-
tanone (internal standard, 90 μL), diethyl ether (to extract the sub-
strate and the organic products from the reaction mixture,
10.00 mL) and an excess of triphenylphosphine (to reduce the cy-
clohexyl hydroperoxide formed to the corresponding alcohol and
hydrogen peroxide to water, following a method developed by
Experimental Section
General Materials and Procedures: All the synthetic work was per-
formed in air. The reagents and solvents were obtained from com-
mercial sources and used as received, that is, without further purifi-
cation or drying. Three different metal sources, namely Cu-
(NO3)2·2.5H2O, Cu(COOCH3)2·H2O and CuB2O4, were used for
the synthesis of complexes 1–3. CuB2O4 was prepared according to
a literature method.[27] CHN elemental analyses were carried out
by the Microanalytical Service of the Instituto Superior Técnico.
Melting points were determined with a Leica Gallen III instrument.
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Shul’pin)[13a–13d,13g] were added. The mixture obtained was stirred
for 10 min and then a sample (1 μL) was taken from the organic
phase and analysed by GC using the internal standard method.
Blank tests indicated that no oxidation took place in the absence
of the Cu complex or the oxidant.
full sphere of data was obtained. Cell parameters were obtained by
using the SMART[30a] software and refined by using the SAINT[30b]
software on all the observed reflections. Absorption corrections
were applied by using SADABS.[30a] Structures were solved by di-
rect methods by using SHELXS-97[30c] and refined with SHELXL-
97.[30c] Calculations were performed by using WinGX (Version
1.80.03).[30d] The hydrogen atoms attached to carbon atoms were
inserted at geometrically calculated positions and included in the
refinement by using the riding-model approximation; Uiso(H) was
defined as 1.2Ueq of the parent carbon atoms for phenyl residues
and 1.5Ueq of the parent carbon atoms for the methyl groups. The
hydroxy H-atoms of methanol molecules were located from the fi-
nal difference Fourier map, and the isotropic thermal parameters
were set at 1.5 times the average thermal parameters of the O-
atoms. Disordered molecules are present in the structure of 2. Be-
cause no obvious major site occupations were found for those
molecules, it was not possible to model them. PLATON/
SQUEEZE[30e] was used to correct the data and a potential volume
of 788.5 Å3 was found with 131 electrons per unit cell worth of
scattering. These were removed from the model and included in the
empirical formula as one water molecule per asymmetric unit.
Least-squares refinements with anisotropic thermal motion param-
eters for all the non-hydrogen atoms and isotropic parameters for
the remaining atoms were employed. The crystallographic data are
summarized in Table 1 and selected bond lengths and angles are
presented in Table S1 in the Supporting Information.
Synthesis of the Pro-ligand H3L: The Schiff base pro-ligand (3,5-
di-tert-butyl-2-hydroxybenzylidene)-2-hydroxybenzohydrazide
(H3L) (Scheme 1) was prepared by condensation of the correspond-
ing salicylhydrazide with 3,5-di-tert-butyl-2-hydroxybenzaldehyde
according to a previously reported procedure.[29] Yield 86%. IR
(KBr): ν = 3328 [ν(OH)], 2957 [ν(NH)], 1658 [ν(C=O)], 1590
˜
[ν(C=N)] cm–1. 1H NMR ([D6]DMSO): δ = 12.20 (s, 1 H, OH),
12.08 (s, 1 H, OH), 11.65 (s, 1 H, NH), 8.62 (s, 1 H, –CH=N), 7.89–
6.98 (m, 6 H, C6H4), 1.41 (s, 9 H, CH3), 1.28 (s, 9 H, CH3) ppm.
C22H28N2O3 (368.47): calcd. C 71.71, H 7.66, N 7.60; found C
71.67, H 7.62, N 7.57.
Synthesis of the Trinuclear CuII Complexes
[Cu3(L)2(MeOH)4] (1): An excess of Cu(NO3)2·2.5H2O (0.93 g,
4.00 mmol) was added to a methanolic solution (30 mL) of H3L
(0.368 g, 1.00 mmol) and the reaction mixture was stirred for
15 min at 50 °C. Triethylamine (0.202 g, 2 mmol) was then added
to the reaction mixture and stirring was continued for a further
10 min. The resultant dark-green solution was filtered and the fil-
trate was left to stand in air. After 2 d, green single crystals suitable
for X-ray diffraction analysis were isolated, washed three times
with cold methanol and dried in open air, yield 0.264 g (72%, with
CCDC-1042432 (for 1), -1042433 (for 2), and -1042434 (for 3 )
contain the supplementary crystallographic data for this paper.
This data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
respect to H L). IR (KBr): ν = 3441 [ν(OH)], 1609 [ν(C=N)], 1251
˜
3
[ν(C–O) enolic], 1167 [ν(N–N)] cm–1. MS (ESI, +): m/z (%) = 470
(100) [NaCu(L)(H2O)]+. C48H66Cu3N4O10 (1): C 54.92, H 6.34, N
5.34; found C 54.87, H 6.30, N 5.31.
[Cu3(L)2(MeOH)2]·2MeOH (2): An excess of Cu(OOCCH3)2·H2O
(0.80 g, 4.00 mmol) was added to a methanolic solution (30 mL) of
H3L (0.368 g, 1.00 mmol) and the reaction mixture was stirred for
30 min at 50 °C. The resultant dark-green solution was filtered and
the filtrate was left to stand in air. After 3 d, green single crystals
suitable for X-ray diffraction analysis were isolated, washed three
times with cold methanol and dried in open air, yield 0.28 g (76%,
Acknowledgments
The authors are grateful to the Fundação para a Ciência e a Tecno-
logia, Portugal (FCT) (project number PEst-OE/QUI/UI0100/
2013) for financial support. M. S. acknowledges FCT for a post-
doctoral fellowship (grant number SFRH/BPD/86067/2012). The
authors are also thankful to the Portuguese NMR Network (IST-
UL Centre) for access to the NMR facility.
with respect to H L). IR (KBr): ν = 3436 [ν(OH)]. 1609 [ν(C=N)],
˜
3
1252 [ν(C–O) enolic], 1170 [ν(N–N)] cm–1. MS (ESI, +): m/z (%) =
461 (100) [Cu(L)(MeOH)]+. C48H66Cu3N4O10 (2): C 54.92, H 6.34,
N 5.34; found C 54.84, H 6.28, N 5.29.
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1.00 mmol) and the reaction mixture was heated at reflux for 1 h.
The resultant dark-green solution was filtered and the filtrate was
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with graphite-monochromated Mo-Kα (λ = 0.71073 Å) radiation.
Data were collected by using omega scans of 0.5° per frame and a
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Received: April 22, 2015
Published Online: July 17, 2015
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