A [2 × 2] Cu4 molecular grid and a Mn5 cluster derived from a 1-(2-pyridyl)pyrazole based polytopic ligand - Synthesis, structure, magnetic properties and catalytic activity in the allylic oxidation of cyclohexene

Article abstract of DOI:10.1016/j.poly.2013.05.056

The homoleptic [2 × 2] tetranuclear compound [Cu₄(PyPzCAP) ₄](NO₃)₄·8H₂O (1) and the pentanuclear Mn(II) cluster [Mn₅(PyPzCAP)₆](ClO ₄)₄·4C₆H₆ (2) are self-assembled from the ditopic pro-ligand HPyPz-CAP [(5-methyl-1-(pyridin-2-yl) -N′-[1-(pyridin-2-yl)ethylidene]-1H-pyrazole-3-carbohydrazide)]. Complex 1 is almost a square grid, and all the Cu(II) centers show a distorted octahedral environment with an N₄O₂ chromophore. Complex 2 has a pentanuclear core with a trigonal bipyramidal arrangement of the Mn(II) atoms. The axial Mn centers bear an N₃O₃ chromophore and the equatorial centers have an N₄O₂ distorted octahedral arrangement. Complex 1 shows a ferromagnetic exchange interaction (J= +4.62 cm ¹ and g= 2.05), whereas 2 shows weak an antiferromagnetic interaction (J= -2.73 cm ¹ and g = 2.02). Both complexes 1 and 2 catalyze the allylic oxidation of cyclohexene to cyclohex-2-ene-1-one with good selectivity and an excellent overall turnover number.

Full text of DOI:10.1016/j.poly.2013.05.056

Polyhedron 62 (2013) 51–60
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
A [2 ꢀ 2] Cu molecular grid and a Mn cluster derived from
4
5
a 1-(2-pyridyl)pyrazole based polytopic ligand – Synthesis, structure,
magnetic properties and catalytic activity in the allylic oxidation
of cyclohexene
a
a
b
a
c
c
Atanu Jana , Saugata Konar , Kinsuk Das , Sangita Ray , James A. Golen , Arnold L. Rheingold ,
d,
⇑
e
e
e,
⇑
Mohamed Salah El Fallah , Sanghamitra Mukherjee , Samik Gupta , Armando J.L. Pombeiro ,
Susanta Kumar Kar
Department of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009, India
Department of Chemistry, Haldia Government College, Debhog, Purba, Midnapur 721657, India
Department of Chemistry and Biochemistry, University of California, San Diego, CA 92093, USA
a,
⇑
a
b
c
d
e
Departament de Química Inorgànica, Facultat de Química, Universitat de Barcelona, Martí i Franquès 1-11, 08028 Barcelona, Spain
Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Universidade Técnica de Lisboa, Av. Rovisco Pais, 1049-001 Lisbon, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
The homoleptic [2 ꢀ 2] tetranuclear compound [Cu (PyPzCAP) ](NO ) ꢁ8H O (1) and the pentanuclear
4
4
3 4
2
Received 6 March 2013
Accepted 25 May 2013
Available online 17 June 2013
Mn(II) cluster [Mn
5
(PyPzCAP)
6
](ClO
4 4
) ꢁ4C
6 6
H (2) are self-assembled from the ditopic pro-ligand HPyPz-
0
CAP [(5-methyl-1-(pyridin-2-yl)-N -[1-(pyridin-2-yl)ethylidene]-1H-pyrazole-3-carbohydrazide)]. Com-
plex 1 is almost a square grid, and all the Cu(II) centers show a distorted octahedral environment with
an N
the Mn(II) atoms. The axial Mn centers bear an N
distorted octahedral arrangement. Complex 1 shows a ferromagnetic exchange interaction
4
O
2
chromophore. Complex 2 has a pentanuclear core with a trigonal bipyramidal arrangement of
Keywords:
3 3
O
chromophore and the equatorial centers have an
Tetranuclear Cu(II)
Pentanuclear Mn(II)
Ferromagnetic
Antiferromagnetic
Turnover number
4 2
N O
ꢂ1
ꢂ1
(
J = +4.62 cm and g = 2.05), whereas 2 shows weak an antiferromagnetic interaction (J = ꢂ2.73 cm
and g = 2.02). Both complexes 1 and 2 catalyze the allylic oxidation of cyclohexene to cyclohex-2-ene-
-one with good selectivity and an excellent overall turnover number.
Ó 2013 Elsevier Ltd. All rights reserved.
1
1
. Introduction
exchange interactions can occur between the paramagnetic metal
ions through the direct bridging (e.g. -O, -N–N) functionalities
l
l
A variety of aesthetical architectures, like clusters [1], grids
2–5], helicates [6], cages [7], ladders [8] and racks [9], have been
of the ligands. Moreover, depending on the packing, such assem-
blages have the unique potential to exhibit either long-range order
or single molecule magnetic behavior. Transition metal ions tem-
[
synthesized through self-assembly processes. Researchers are
becoming more enthusiastic towards the synthesis of these struc-
tures because of their enormous range of applications in different
challenging fields due to their electronic, redox, magnetic, photo-
physical and catalytic properties [10–13]. The self-assembly of
transition metals with multifunctional organic ligands represents
a successful domain for the single step synthesis of new clusters
of well-defined and predesigned architectures. A major challenging
goal in this area is the design and synthesis of new polytopic ligands
which can be used for the synthesis of molecular metal grids and
clusters. Hence, with the proper choice of paramagnetic metal ions
which interact with these polytopic ligands, the magnetic coupling
can be fine-tuned. In these self-assembled grids and clusters, spin
plated cyclizations have produced many tetranuclear [2 ꢀ 2] Mn
4
,
Co , Ni , Cu and Zn grid complexes [14–16], in which the bridging
4
4
4
4
arrays among the metal ions involve oxygen donor groups. Most of
the so far reported alkoxo-bridged polynuclear grids [Mn(II), Co(II)
and Ni(II)] show anti-ferromagnetic coupling, while the ferromag-
netic interaction prevails only in Cu(II) grids. Manganese is suitable
for self-assembling high-nuclearity clusters primarily based on li-
gands with oxygen donors due to its oxophilicity. The easy avail-
5
4
3
ability of at least three oxidation states, +II (d ),+III (d ),+IV (d ),
with the highest number of unpaired electrons possible in the 3d
orbital of the coordinated metal center, makes the study of self-
assembled Mn clusters interesting. The study of these Mn clusters
has helped us gain knowledge about the structure and function of
manganese-containing biological systems [17], as well as supplying
⇑
Corresponding authors. Tel.: +91 9831077929; fax: +91 03323508386 (S.K. Kar).
E-mail address: skkar_cu@yahoo.co.in (S.K. Kar).
0
277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.05.056

5
2
A. Jana et al. / Polyhedron 62 (2013) 51–60
a wealth of data for species that are able to function as ‘‘single mol-
ecule magnets’’ (SMMs).
Caution! Although we have not encountered any problems, it
should be kept in mind that only small amounts of the perchlorate
compound should be prepared and it should be handled with care.
Immense applications of
a,b-unsaturated ketones are found in
different fields, for example as building blocks and products of
pharmaceutical interest [18–19], chemo sensors for metal ions
[
20–21], intermediates of many natural products and bioactive
2
.2. Physical measurements
compounds [22–28]. Conventionally, they are prepared by the al-
dol condensation of aldehydes and ketones [29–30] or elimination
reactions of precursors such as
Elemental analyses (carbon, hydrogen and nitrogen) of the
a-halide ketones [31–32]. Some
,b-unsaturated ketones prepared through a C–C bond forming
proligand PyPzCAP and the metal complexes 1 and 2 were carried
out with a Perkin Elmer CHNS/O analyzer 2400. The electronic
spectra of 1 in purified CH OH solution were recorded on a Hitachi
3
model U-3501 spectrophotometer. IR spectra (KBr pellet, 4000–
a
reaction between primary alcohols and ketones in the presence
of gold nanoparticles [33] have been reported by Kim et al. Yadav
et al. have reported a one-pot process for the synthesis of
unsaturated ketones with a cation exchange resin-promoted cou-
a,b-
ꢂ1
4
00 cm ) were recorded on a Perkin Elmer model 883 infrared
spectrophotometer. The mass spectrum of the ligand was per-
formed at the Indian Institute for Chemical Biology, Kolkata with
a JEOL JMS-AX 500 mass spectrometer. Room temperature mag-
netic susceptibility was measured with a PAR 155 vibrating sample
magnetometer. Magnetic susceptibility measurements were car-
ried out on polycrystalline samples, at the Servei de Magnetoquí-
mica of the Universitat de Barcelona, with a Quantum Design
SQUID MPMS-XL susceptometer apparatus working in the range
pling reaction of aldehydes with alkynes [34]. The technique for
the preparation of cyclohex-2-ene-1-one, an
a,b-unsaturated ke-
tone, by the transition metal catalyzed selective oxidation of cyclo-
hexene has being studied for a long time [35–43]. Copper(II) and
Mn(II) complexes have featured as catalysts in a few of these meth-
ods. However, the use of multinuclear complexes is still little ex-
plored and deserves further attention.
In this work, we report the use of a new non-linear unsymmet-
2
–300 K under a magnetic field of approximately 500 G (2–30 K)
rical polytopic ligand, PyPzCAP (Scheme 1) to prepare a [2 ꢀ 2] Cu
molecular grid and a Mn cluster. The Schiff base ligand precursor
HPyPzCAP has been prepared by the condensation of 5-methyl 1-
2-pyridyl) 3-pyrazole carbohydrazide and 2-acetyl pyridine. The
magnetic studies reveal that the Cu grid is ferromagnetically cou-
pled and the Mn cluster is anti-ferromagnetically coupled. Apart
4
and 1000 G (35–300 K). Diamagnetic corrections were estimated
from Pascal Tables.
5
(
4
5
2.3. Synthesis of the ligand
from the magnetic properties, we have explored the catalytic prop-
erties of these multinuclear complexes towards the allylic oxida-
tion of cyclohexene. This is our first attempt in studying the
5
-Methyl-1-(2-pyridyl) pyrazole-3-carbohydrazide was pre-
pared following an established method. The ligand HPyPzCAP
was synthesized by refluxing an ethanolic solution (30 mL) of 5-
4 5
catalytic properties of a Cu grid and a Mn cluster. It has been
found that these two complexes catalyze the allylic oxidation of
cyclohexene to cyclohex-2-ene-1-one with good selectivity and a
remarkable turnover number (TON).
methyl-1-(2-pyridyl)
pyrazole-3-carbohydrazide
(2.17 g,
1
0 mmol) with 1-(pyridin-2-yl) ethanone (1.21 g, 10 mmol), also
taken in ethanol (10 mL). Refluxing was continued for ca. 30 min
at water bath temperature. After slow solvent evaporation of the
reaction mixture, a solid microcrystalline compound separated. It
was filtered off, washed several times with cold ethanol and dried
2
. Experimental
.1. Materials
-Acetyl pyridine was purchased from Aldrich. Other commer-
2
in a vacuum over fused CaCl
2
.
HPyPzCAP Yield: 2.37 g (70%); m.p. 232 °C. Anal. Calc. for
2
C
16
H
16
N
6
O: C, 62.34; H, 5.19; N, 27.27. Found: C, 62.29; H, 5.13;
ꢂ1
cially available chemicals and solvents were used and purified by
standard procedures.
N, 27.20%. IR/cm
:
mNH 3360;
m
CO 1650(s);
m
CN 1555(s); mN–
Npz 1062(s);
m
py 1015(s).
O
CH3
CH3
NH2
O
HO
N
N
N
NH
NH
R
R
N
O
CH
3
N
1:1 EtOH
+
H C
3
N
N
N
R
R
H3C
H3C
;
R
N
R
N
R
HPyPzCAP
Cu(NO ) . 6H O
4 2 2
Mn(ClO ) . 6H O
3
2
2
[
Cu (PyPzCAP) ](NO ) . 8H O
5 6 4 4 6 6
[Mn (PyPzCAP) ](ClO ) . 4(C H )
4
4
3 4
2
(2)
(
1)
Scheme 1. Preparation of complexes 1 and 2.

A. Jana et al. / Polyhedron 62 (2013) 51–60
53
2
2
.4. Synthesis of the complexes
Table 1
Experimental data for crystallographic analysis of 1 and 2.
.4.1. Preparation of the complex [Cu
4
(PyPzCAP)
4
](NO
3
)
4
ꢁ8H
2
O (1)
Compound
1
2
The ligand HPyPzCAP (0.122 g, 1 mmol) was added to a hot
solution of Cu(NO O (0.148 g, 1 mmol) in CH OH (30 mL).
Empirical formula
Formula weight
T (K)
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
C
126
H
112
C
l4Mn
N
5 36
O
22
C
68 4 28 24
H76Cu N O
3
)
2
ꢁ6H
2
3
2899.02
100
1.54178
monoclinic
C2/c
1923.73
100
0.71073
monoclinic
P2(1)/c
The resulting suspension was stirred with heating (ꢃ60 °C) until
complete dissolution of the ligand occurred. The resulting deep
green solution was filtered to remove any undissolved ligand and
left at room temperature. Dark green crystals suitable for X-ray dif-
fraction were isolated from the filtrate after standing for several
29.0264(10)
18.9144(10)
22.8805(9)
90.00
95.520(3)
90.00
12503.5(9)
4
1.457
13.7957(12)
40.881(3)
14.2670(12)
90.00
93.610(2)
90.00
8030.4(12)
4
1.472
b (Å)
c (Å)
days. Yield: 40%. Anal. Calc. for C68
H76Cu
4
N
28
O
24: C, 42.46; H,
ꢂ1
3
m
.98; N, 20.39. Found: C, 42.47; H, 3.95; N, 20.41%. IR/cm
NH/H O 3379; 3179; CO 1670(s); 15429(s); CN 1542(s);
Npz 1053(s); py 1027(s). UV–Vis (kmax/nm): 675.
:
a
(°)
b (°)
(°)
2
m
m
m
N-
m
c
3
V (Å )
z
D
2
.4.2. Preparation of the complex [Mn
The ligand HPyPzCAP (0.122 g, 1 mmol) was dissolved in meth-
anol. Mn(ClO O (0.361 g, 1 mmol) dissolved in methanol was
5
(PyPzCAP)
6
](ClO
4
)
4
ꢁ4(C
6 6
H ) (2)
ꢂ3
calc (Mg m
)
Absorption coefficient
5.511
1.140
4
)
2
ꢁ6H
2
ꢂ1
(
mm
)
added to this solution. Then it was stirred for 5 min. The pH of the
solution was kept at 7–8 by adding a few drops of a methanolic
solution of NEt . The solution was stirred for additional half an
3
hour and its color was reddish. The resulting solution was filtered
and kept for slow evaporation at room temperature. A red crystal-
line compound was isolated from the filtrate after standing for sev-
eral days. Crystals suitable for X-ray diffraction were grown by
slow diffusion of benzene into an acetonitrile solution of the com-
F (000)
h Range (°) for data
collection
5956
2.50–24.50
3952
1.51–27.90
Index ranges
ꢂ33 6 h 6 34
ꢂ22 6 k 6 21
ꢂ26 6 l 6 26
ꢂ17 6 h 6 17
ꢂ51 6 k 6 53
ꢂ18 6 l 6 18
1.056
Goodness-of-fit (GOF) on F2 1.035
Independent reflections
[R ]
int
Absorption correction
Refinement method
10465(0.044)
18085[0.075]
multi-scan
multi-scan
plex. Yield: 60%. Anal. Calc. for C114
4
3
100
H C
l4Mn
5
N
36
O22: C, 54.97; H,
full-matrix least
squares on F2
10465/40/770
46720
full-matrix least
squares on F2
18085/36/1054
120313
ꢂ1
.05; N, 10.12. Found: C, 54.92; H, 4.09; N, 10.13%. IR/cm
365; CO 1660(s); CN 1540(s); N-Npz 1080(s);
RT 3.42 B.M.
: mNH
Data/restraints/parameters
Reflections collected
Final R indices [I > 2
m
m
m
mpy 1030(s).
UV–Vis (kmax/nm): 670.
m
r
(I)]
R = 0.0690,
R = 0.0729,
wR
2
= 0.2016
wR
ꢂ0.90, 3.31
2
= 0.1848
2.5. Single crystal X-ray crystallography
Largest difference peak and ꢂ1.64, 1.61
ꢂ3
hole (e Å
)
Selected crystal data of 1 and 2 are given in Table 1. Data of
complexes 1 and 2 were collected on a Bruker APEX II CCD sys-
tem with Cu radiation and corrected for absorption with SADABS
.
The structures were solved by direct methods and all non-hydro-
gen atoms of the cation were refined as anisotropic by Fourier full
matrix least squares on F . SHELXS-97 and SHELXL-97 [44] programs
cyclohexen-1-one using aqueous tert-butyl hydroperoxide (TBHP)
under mild conditions (Scheme 3). Cyclohexene has been studied
as a model substrate in view of the importance of the products
2
were used for structure solution and refinement respectively. In
(i.e.,
additive reactions) [18–19]. The reaction mixtures were prepared
as follows: to 0.5–20 mol (preferably 10 mol) of complex 1or 2,
contained in a reaction flask, were added 3 mL of MeCN, 5.0 mmol
cyclohexene and 1–10.0 mmol TBHP (70% aqueous solution), in
this order. The reaction mixture was stirred for 10 h at the de-
sired temperature (ca. 30–60 °C) under an air (atmospheric pres-
a,b-unsaturated allylic oxidation products used for further
1, three of the four nitrate ions were disordered and were treated
using the two part model (56.1/43.9 ratio). Atoms of two of the
nitrate anions were refined isotropically and EADP constraints
were used for each pair of atoms of the anions. In addition, unre-
fined electron density was treated by using Platon SQUEEZE. This
program found a void of 893 cubic angstroms with 334 electrons
and was treated as 32 water molecules, which were added to the
chemical formula to account for the molecular mass, density and
F000 value. In 2, the perchlorate anions were disordered and one
was refined as anisotropic, the other as isotropic and both anions
had their (29.5/70.5 and 51.2/49.8 ratio) distances and angles
fixed using DFIX and DANG commands, and their amplitudes
were matched using EADP commands. A disordered benzene mol-
ecule (59.9/40.1) was located and refined isotropically as a fixed
hexagonal ring and with a common EADP command. The Platon
program SQUEEZE was then used to adjust for unrefined solvent.
The program found a void of 1120 cubic angstroms and 320 elec-
trons – associated with eight benzene molecules, which were
added to the unit card to adjust the molecular mass, density
and F000 value.
l
l
sure), then 50 lL of cyclopentanone (as an internal standard) and
9.5 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. The GC analyses showed the presence
of only traces (less than 3%) of oxidation products. The reaction
under pressurized dioxygen was carried out in a 13.0 mL stainless
steel autoclave, equipped with a Teflon-coated magnetic stirring
bar. The autoclave was closed and flushed with dioxygen three
times to remove the air, and finally pressurized with 5 atm of
dioxygen. The reaction mixture was stirred for 10 h at 50 °C using
a magnetic stirrer and an oil bath, whereupon it was cooled in an
ice bath, degassed, opened and transferred to a flask. Diethy ether
(9.5 mL) and 50 lL of cyclopentanone (GC internal standard) were
added. The obtained mixture was vigorously stirred for 10 min,
2
2
.6. Catalytic method
and the organic layer was analyzed by gas chromatography
(
internal standard method). Blank experiments were performed
.6.1. Oxidation of cyclohexene
Compounds 1 and 2 were investigated as catalyst precursors
and confirmed that no cyclohexene oxidation products (or only
traces, below 0.5%) were obtained in the absence of the metal
catalyst.
for the oxidation of cyclohexene to 2-cyclohexen-1-ol and 2-

5
4
A. Jana et al. / Polyhedron 62 (2013) 51–60
Fig. 1. Structural representation and atom numbering scheme of the cationic complex 1.
3
. Results and discussion
N5–Cu2–N21, N9–Cu4–N23 and N11–Cu3–N15. The tetranuclear
core structure is shown in Fig. 2.
3.1. Description of the structure of [Cu
4 4 3 4 2
(PyPzCAP) ](NO ) ꢁ8H O (1)
5 6 4 6 6
3.2. Description of the structure of [Mn (PyPzCAP) ](ClO )4.4(C H ) (2)
The structure of the cation in 1 is illustrated in Fig. 1 and impor-
tant bond distances and angles are listed in Table 2. The lattice is
The structure of the pentanuclear cation cluster of 2 is illus-
monoclinic with P21/C symmetry and the unit cell consists of four
trated in Fig. 3, and important bond distances and angles are
listed in Table 2. The pentanuclear Mn(II) core is best described
as a trigonal bipyramid. The lattice is monoclinic in nature with
molecules. The complex 1 is a homoleptic Cu
4
(II) [2 ꢀ 2] almost
square grid. The ligand PyPzCAP has two coordination pockets. It
can in principle, adopt two different geometries, (a) and (b)
5
C2/c symmetry. The Mn cluster consists of six ligands, four per-
(
Scheme 2) [2], but only conformation (a) is present in 1. The over-
chlorate anions and four benzene molecules. The pentanuclear
core structure is shown in Fig. 4. The Mn2, Mn3 and Mn3_a cen-
ters are situated in the equatorial plane of the trigonal bypyr-
amid, while the Mn1 and Mn1_a centers occupy the axial
positions. All the angles in the Mn2–Mn3–Mn3_a equatorial tri-
angle fall within the range 59.23–60.38°. The coordination sphere
all structural arrangement shows a tetranuclear cation, four nitrate
anions and eight water molecules (the nitrates and water mole-
cules are uncoordinated). The grid consists of four six coordinate
copper centers arranged in a square with alkoxide bridges connect-
ing the Cu(II) ions. Four ligands are arranged in parallel pairs in an
opposite arrangement above and below the mean Cu
head-to-tail arrangement of the ligand strands lead to the forma-
tion of only one type of chromophore, N , around each Cu(II)
4
plane. The
of Mn1 and Mn1_a consists of an N
gen atoms from pyrazole and three alkoxide oxygens), whereas
for Mn2, Mn3 and Mn3_a it consists of an N chromophore
3 3
O chromophore (three nitro-
4
O
2
4 2
O
ion, which consists of four nitrogen atoms (two nitrogens from
pyridyl–pyrazole, one imine nitrogen, one pyridine nitrogen and
two alkoxide ions). The coordination sphere around each Cu(II) is
distorted octahedral. The Cu–O and Cu–N bond lengths and N–
Cu–N, O–Cu–N and O–Cu–O bond angles (Table 2) are very similar
to those previously reported for octahedral Cu2 complexes with
(two imine nitrogens, two pyrazole nitrogens and two alkoxide
oxygens). It is noteworthy that all the pyridine nitrogens from
the pyridyl–pyrazole part remain uncoordinated. The distance be-
tween the Mn1 and Mn1_a atoms is 5.311 Å and the distances be-
tween the three equatorial atoms fall in the range 5.010–5.06 Å.
The distance of the two axial Mn atoms from equatorial plane
is 2.656 Å. All the Mn atoms are in a distorted octahedral environ-
ment. The metal–oxygen bond distances fall within the range
2.199–2.167 Å. The Mn–O–Mn bridge angles are in the range
128.22–128.74°.
+
amide and pyridine containing ligands [3]. The Cu
4
square is
unsymmetrical with four unequal axial contacts from copper to
the oxygen bridges. The Cu–Cu distances fall in the range 4.115–
4
.412 Å. All the Cu–O bond distances are different, consistent with
a distorted octahedral geometry. The Cu–O–Cu angles fall in the
range 137.96(14)–144.31(14)°. Cu2 and Cu3 are below, while Cu1
and Cu4 are above the mean plane defined by the four copper
atoms. The deviation of the Cu ions from their mean plane range
between 0.414 (for Cu4) and 0.426 Å (for Cu1). The Jahn–Teller
axes of Cu1, Cu2, Cu3 and Cu4 are defined as N3–Cu1–N17,
3.3. Magnetic properties
3.3.1. Complex 1
The variable temperature magnetic susceptibility data for the
compound were recorded between 300 and 2 K. A plot of vM
T

A. Jana et al. / Polyhedron 62 (2013) 51–60
55
Table 2
Selected bond distances (Å) and angles (°) for complexes 1 and 2.
Complex
Bonds
Distances (Å)
Bond angles
Values(o)
1
Mn1–O1
Mn1–O2
Mn1–O3
Mn1–N3
Mn1–N9
Mn1–N14
Mn2–O2
Mn2–N11
Mn2–N12
Mn2–O2_a
Mn2–N11_a
Mn2–N12_a
Mn3–O1
2.205(3)
2.167(3)
2.193(3)
2.215(4)
2.248(4)
2.247(4)
2.180(3)
2.179(4)
2.295(4)
2.180(3)
2.179(4)
2.295(4)
2.197(3)
2.201(4)
2.310(4)
2.317(4)
2.192(4)
2.199(3)
O1–Mn1–O2
O1–Mn1–O3
O1–Mn1–N3
O1–Mn1–N9
O1–Mn1–N14
O2–Mn1–O3
O2–Mn1–N3
O2–Mn1–N9
O2–Mn1–N14
O3–Mn1–N3
90.81(11)
89.71(11)
74.93(12)
88.55(12)
164.69(13)
91.63(11)
165.70(12)
75.43(12)
88.92(13)
89.38(12)
166.91(12)
96.25(13)
100.96(11)
141.13(14)
151.68(15)
141.13(14)
128.24(13)
128.74(14)
O3–Mn1–N9
O2–Mn2–N12
O2–Mn2–O2_a
O2–Mn2–N12_a
N11–Mn2–N11_a
O2_a–Mn2–N12
Mn1–O1–Mn3
Mn1–O3–Mn3_a
Mn3–N5
Mn3–N6
Mn3–N17
Mn3–N18
Mn3–O3_a
2
Cu1–O1
Cu1–O3
Cu1–N1
Cu1–N3
Cu1–N17
Cu1–N18
Cu2–O1
Cu2–O4
Cu2–N5
Cu2–N6
Cu2–N19
Cu2–N21
Cu3–O2
Cu3–N11
Cu3–N12
Cu3–N13
Cu3–N15
Cu4–O2
Cu4–O4
Cu4–N7
Cu4–N9
Cu4–N23
Cu4–N24
2.435(3)
1.986(3)
2.355(3)
1.957(3)
1.916(3)
2.028(3)
1.971(3)
2.639(3)
1.940(4)
2.011(3)
2.384(3)
1.974(3)
1.980(3)
1.926(3)
2.018(3)
2.380(3)
1.959(3)
2.434(3)
1.992(3)
2.350(4)
1.954(4)
1.927(4)
2.039(3)
O1–Cu1–O3
O1–Cu1–N1
O1–Cu1–N3
O1–Cu1–N17
O1–Cu1–N18
O3–Cu1–N1
O3–Cu1–N3
O3–Cu1–N17
O3–Cu1–N18
N1–Cu1–N3
N1–Cu1–N17
N1–Cu1–N18
N3–Cu1–N17
N3–Cu1–N18
N17–Cu1–N18
O1–Cu2–O4
O1–Cu2–N5
O3–Cu3–N11
O3–Cu3–N12
O2–Cu4–O4
O2–Cu4–N7
O4–Cu4–N24
N7–Cu4–N9
92.47(10)
142.91(10)
71.12(11)
111.64(11)
88.81(11)
90.70(11)
Fig. 2. Structural representation and atom numbering scheme of the tetranuclear
core in 1.
96.92(12)
80.09(12)
159.27(13)
71.81(12)
105.32(12)
100.69(11)
175.92(12)
103.05(13)
80.21(13)
89.32(10)
78.97(12)
109.75(11)
93.28(10)
90.73(10)
143.78(11)
159.09(13)
72.48(13)
OH
O
1
or 2 as catalyst
+
CH CN, aq TBHP
3
Scheme 3. Oxidation of cyclohexene into 2-cyclohexene-1-ol and 2-cyclohexen-1-
one.
versus T in Fig. 5 shows ferromagnetic behavior. The
1
v
M
T value is
.597 cm mol K at 300 K, being close to that expected for four
uncoupled S = 1/2 spins (with g = 2.1). T increases slightly when
the temperature is lowered and reaches a maximum value of
3
ꢂ1
v
M
3
ꢂ1
2
.950 cm mol K at 2 K. This maximum is consistent with that
CH3
expected for ferromagnetically coupled copper(II) ions. The struc-
ture of 1 consists of copper ions linked by the ligand PyPzCAP, giv-
ing a tetranuclear compound. Taking into account the compound
topology (a slight distortion from a square array, see crystallo-
graphic data), three coupling parameters J , J and J (Fig. 6) can
1 2 3
be considered to interpret the magnetic interactions in the com-
plex using the Hamiltonian:
CH3
N
N
N
N
N
M
O
M
N
H ¼ ꢂJ ðS
1
ꢁS
2
þS
3
ꢁS
4
ÞꢂJ ðS
1
ꢁS
4
þS
2
ꢁS
3
ÞꢂJ ðS
1
ꢁS
3
þS
2
ꢁS
4
Þ
ð1Þ
1
2
3
(
a)
Based on the structural data of the [Cu
sidered that J = J = J and J
4
–O
4
] core, we have con-
1
2
3
= 0 (no cross-coupling connection). For
this square disposition, the E value can be obtained by using the
Kambe method [45] from the Hamiltonian:
n
CH
3
O
H ¼ ꢂJ ðS
1
ꢁ S
2
þ S
2
ꢁ S
3
þ S
3
ꢁ S
4
þ S
1
ꢁ S
4
Þ
ð2Þ
1
M
N
Analysis of the experimental susceptibility data has been performed
by the use of the expression:
N
N
N
N
M
2
vmT ¼ 0:125g fðJ; TÞ
where
ð3Þ
N
CH3
(
b)
fðJ; TÞ ¼ 6
expðJ=KTÞ þ 12 expð2J=KTÞ þ 30 expð3J=KTÞ
Scheme 2. Two different conformations of the ditopic ligand PyPzCAP: (a)
represents the common coordination mode; (b) represents a very uncommon
coordination mode. Only conformation (a) is present in 1.
1 þ 3 expðJ=KTÞ þ 7 expð2J=KTÞ þ 5 expð3J=KTÞ

5
6
A. Jana et al. / Polyhedron 62 (2013) 51–60
Fig. 3. Structural representation and atom numbering scheme of the cationic complex 2.
Fig. 4. Structural representation and atom numbering scheme of the pentanuclear core in 2.
ꢂ1
The best fit parameters found are: J = +4.62 cm and g = 2.05
compound 1 should be considered as normal, taking into account
the structural and magnetic data reported recently in our works
[4,5].
P ꢀ
ꢂ5
2
with an error R = 4.7 ꢀ 10 , where R ¼
ð
vMTÞ ꢂ ðvMTÞ
ꢄ
obs
calc
P
2
obs
=
ðv
TÞ . The field dependence of the magnetization (0–5 T)
M
measured at 2 K, shown in Fig. 7 (inset) in the form of M/N
mb
(
per Cu
4
entity) verus H, suggests that the magnetization tends
3.3.2. Complex 2
to 4 Nb (experimental value = 4.16 M/Nb). This feature agrees with
the global weak ferromagnetic coupling within the four copper(II)
The variable temperature magnetic susceptibility data for com-
plex 2 were recorded between 300 and 2 K. The plot of vMT versus
T of Fig. 7 shows typical antiferromagnetic behavior, decreasing
atoms (fit of the experimental data: T = 2, S = 2 and g = 2.08). The
ꢂ1
3
ꢂ1
value of the superexchange parameter J = +4.62 cm
for
from 20.378 cm K mol at 300 K, which is slightly lower than

A. Jana et al. / Polyhedron 62 (2013) 51–60
57
that expected for five uncoupled S = 5/2 spin centers (21.9 with
3
ꢂ1
g = 2.0), to 4.538 cm K mol at 2 K, which corresponds to one
uncoupled S = 5/2 spin center (4.38 with g = 2.0) due to the odd
number of spin centers in the complex. The magnetization mea-
surements at 2 K up to an external field of 5 T in form M/Nb versus
H show the saturation of the complex 2 tending to 4.96 Nb (elec-
trons), corresponding to a one uncoupled S = 5/2 system (Fig. 7,
inset).
As shown in the crystallographic part, compound 2 is made up
of six ligands bonded in parallel pairs around the Mn
5 6
(l-O) core,
with the ligands opposed (anti) in each pair, similar to the Mn
5
compounds reported by Thompson et al. [16] Thus, six of one kind
of coupling parameter J must be considered between two neighbor
manganese ions to interpret the magnetic properties of 2. We have
not considered the slight differences observed between the angles
and bond lengths of two neighbor manganese ions. On the basis of
this assumption, the experimental magnetic data have been fitted
using the isotropic Heisenberg Hamiltonian:
Fig. 5. Plots of the
corresponds to the best fit.
M
v T vs. T and M/Nb vs. H at T = 2 K (inset) for 1. Solid line
H ¼ ꢂJðS
1
S
2
þ S
1
S
3
þ S
1
S
5
þ S
2
S
4
þ S
3
S
4
þ S
4
S
5
Þ
ð4Þ
ꢂ1
The best fit parameters obtained values are J = ꢂ2.73 cm and
h
P
ꢂ6
g = 2.02 with an error R = 2.6 ꢀ 10
(R ¼
ð
v
TÞ
ꢂ
M
exp
P
2
2
ð
vMTÞ ꢄ = ½ðvMTÞ ꢄ ). The series of calculations were made
calc
exp
using the computer program CLUMAG that uses the irreducible tensor
operator formalism (ITO) [46].
3.4. Catalytic activity
Compounds 1 and 2 were investigated as catalytic precursors
for the oxidation of cyclohexene into 2-cyclohexene-1-ol and 2-
cyclohexen-1-one (Scheme 3) under mild conditions with aque-
ous tert-butyl hydroperoxide (TBHP). Cyclohexene has been stud-
ied as the model substrate in view of the importance of the
products (i.e., the allylic oxidation products,
tionalized substrates, are used for further additive reactions) [18–
8].
a,b-unsaturated fuc-
Fig. 6. Structural representation of the [Cu
constants in 1.
4 4
O ] core and exchange coupling
2
Fig. 7.
M
v T vs. T and M/Nb vs. H (inset) for complex 2.

5
8
A. Jana et al. / Polyhedron 62 (2013) 51–60
Table 3
Peroxidative oxidation of cyclohexene to 2-cyclohexene-1-one and 2-cyclohexene-2-ol catalyzed by complexes 1 and 2.a
(
a) Variation of oxidant
Entry
Catalyst
Oxidant
Oxidant/substrate
Selectivity of productb (%)
Total conv. c (%)
Ketone/alcohold
TONe
Ketone
Alcohol
01
02
03
04
1
2
1
2
TBHP
TBHP
2.0
2.0
2.0
2.0
86
70
–
12
26
–
67
66
–
7.1
2.6
–
1675
1650
–
H
H
2
O
O
2
2
2
–
–
–
–
–
(
b) Variation of temperature
Entry
Catalyst
Temp (°C)
Oxidant/substrate
Selectivity of productb (%)
Total conv. c (%)
Ketone/alcohold
TONe
Ketone
Alcohol
05
06
07
08
09
10
11
12
1
1
1
1
2
2
2
2
30
40
50
60
30
40
50
60
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
80
82
86
67
75
74
75
70
17
17
12
30
22
24
20
26
19
53
67
71
12
48
66
68
4.7
4.8
7.1
2.2
3.4
3.0
3.7
2.6
475
1325
1675
1775
300
1200
1650
1700
(
c) Variation of catalyst amount
Entry
Catalyst
n(Catalyst)
l
mol
Oxidant/substrate
Selectivity of productb (%)
Total conv.c (%)
Ketone/alcohold
TONe
Ketone
Alcohol
13
14
15
16
17
18
19
20
21
1
1
1
1
1
1
1
1
1
0.0
0.5
1.0
2.5
5.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
–
–
03
08
29
48
56
61
67
66
62
–
–
86
82
79
81
86
86
88
87
11
13
12
10
11
12
11
09
7.8
6.3
6.5
8.1
7.8
7.1
8.0
9.6
4000
7250
4800
2800
2033
1675
1116
775
7.5
10.0
15.0
20.0
(
d) Variation of oxidant amount
Entry
Catalyst
n(TBHP) mmol
Oxidant/substrate
Selectivity of productb (%)
Total conv. c (%)
Ketone/alcohold
TONe
Ketone
Alcohol
22
23
24
25
26
27
28
29
30
1
1
1
1
1
2
2
2
2
1.0
2.5
5.0
7.5
10.0
2.0
5.0
10.0
12.0
0.1
0.5
1.0
1.5
2.0
0.4
1.0
2.0
2.4
80
80
80
81
86
68
70
70
68
14
15
15
15
12
25
23
24
20
55
56
56
57
67
33
52
66
59
5.7
5.3
5.3
5.4
7.1
2.7
3.0
2.9
3.0
1375
1400
1400
1425
1675
825
1300
1650
1475
(
e) Variation of reaction conditions
Entry
Catalyst
Condition
Oxidant/substrate
Selectivity of productb (%)
Total conv. c (%)
Ketone/alcohold
TONe
Ketone
Alcohol
31
32
33
34
35
36
37
38
1
1
1
1
1
2
1
1
N
N
O
2
2.0
0.1
0.0
0.1
0.1
2.0
2.0
2.0
80
80
–
83
89
68
72
67
15
17
–
12
10
24
12
11
18
05
–
56
72
64
13
22
5.3
4.7
–
6.9
8.9
2.8
6.0
6.0
450
125
–
1400
1800
1600
325
550
2
f
2
Air
f
O
N
2
2
Ph
CBrCl
2
NH
3
a
Effects of oxidant (a), temperature (b), amount of catalyst (c), amount of oxidant (d) and other conditions (e). Reaction conditions (unless stated otherwise): catalyst
precursor (0.01 mmol), MeCN (3 mL), cyclohexene (5 mmol), oxidant (10 mmol), temperature (50 °C), reaction time 8 h, under air.
b
(
(
Moles of product/mole of reacted cyclohexene) ꢀ 100.
c
d
e
f
Initial substrate–final substrate/initial substrate) ꢀ 100, amounts in moles.
Ketone (2-cyclohexene-1-one)/Alcohol (2-cyclohexene-1-ol) molar ratio.
Overall TON values (moles of products/mole of metal complex).
Pressurised O at 5 atm.
2

A. Jana et al. / Polyhedron 62 (2013) 51–60
59
The reactions were carried out in acetonitrile medium under air
or pressurized dioxygen and the effects of various factors, such as
relative amounts of catalyst, oxidant and temperature on the cata-
lytic activity, have been studied. The obtained results are presented
in Table 3. No oxidation products (or only traces) are obtained in
the absence of any component of the catalytic system, namely
the metal catalyst or TBHP.
Both the pre-catalysts 1 and 2 are inactive with hydrogen per-
oxide (Table 3a, entries 3, 4) but they are quite active with tert-bu-
tyl hydroperoxide. The ketone (2-cyclohexen-1-one) is always the
main product and the ketone/alcohol molar ratio can reach values
up to ca. 10 (Table 3c, entry 21). Overall conversions of cyclohex-
ene up to 67% can be achieved at 50 °C (e.g. for TBHP/catalyst 1
and TBHP/substrate molar ratios of 1000 and 2.0, respectively,
Table 3a entry 1). Increasing the temperature to 60 °C leads to a
higher conversion (71%, Table 3b, entry 8), but with a decrease in
selectivity (the ketone/alcohol molar ratio lowers to 2.2). Similar
behaviors are observed for both pre-catalysts 1 and 2.
In presence of CBrCl
3
, 3-bromo cyclohexene is formed, confirm-
ing the allylic C-centered radical formation. The stability of this
radical is enhanced by the presence of the conjugated double bond,
which preferably helps in forming the allylic oxidation products
(trace amounts of 2-cyclohexene-1,4-dione and 2-cyclohexene-
1,4-diol are also identified by GC–MS) without affecting the ole-
finic bond. These observations justify the absence of epoxide in
the product mixture, the reaction proceeding selectively toward
the allylic oxidation products.
Recently, Lashanizadegan et. al. [56] reported that homogenous
(and the heterogeneous analogues) azo based Schiff-base com-
plexes of copper and manganese show high conversions of cyclo-
hexene with good selectivity but with low TON values (105–149
range). A similar allylic oxidation of cyclohexene was carried out
with a Schiff-base copper complex as a soluble or immobilized cat-
alyst [37], where the product selectivity was quite low, along with
a low TON of 47 or 146, respectively.
The above high conversions (67–71%) also correspond to good
turnover numbers (TON, moles of products/mole of catalyst pre-
4
. Conclusion
3
cursor) of 1.7 ꢀ 10 (Table 3a, entries 1, 2; Table 3b, entries 7, 8).
3
Furthermore, higher TON values (up to 7.3 ꢀ 10 , Table 3c, entry
The two new polynuclear complexes reported herein, a tetranu-
1
5) can be obtained for lower catalyst loadings, but with a decrease
clear Cu
4
[2 ꢀ 2] grid (1) and a pentanuclear Mn(II) cluster (2), are
of the conversion (29% for 1 lmol 1 versus 67% for 10 lmol 1,
Table 3c, entry 15 versus 19, respectively). Hence, in order to oper-
ate with good conversions, the latter amount of catalyst precursor
produced by a well known self-assembly technique using a new
polytopic ligand, PyPzCAP. They provide important new additions
to an already rich family of such compounds reported by our group
and others. The ligand shows different binding modes to the metal
ions. Thus, Cu(II) directs four ligand moieties to assemble and form
a square grid while Mn(II) directs six ligand moieties to assemble a
pentanuclear metal cluster with a trigonal bipyramidal core. The
square copper grid (1) exhibits intramolecular ferromagnetic spin
exchange which is associated with the orthogonal alkoxide bridg-
ing arrangement and the close proximity of the Cu(II) centers.
The pentanuclear Mn(II) cluster, on the other hand, shows antifer-
romagnetic behavior.
(
10 lmol, i.e. a catalyst /substrate molar ratio of 1:500) was se-
lected for the typical reactions (Table 3, footnote a).
The effect of the amount of oxidant was also investigated
(Table 3d) and commonly an increase tends to lead to a higher con-
version, without a substantial change in selectivity. This is fol-
lowed clearly with the precatalyst 2 (range of 0.4–2.0 for the
oxidant/substrate molar ratio, Table 3d, entries 27–29), but a fur-
ther increase in the oxidant amount resulted in a lowering of the
conversion (entry 30), which may be due to the enhancement of
the water content in the reaction mixture [47–48]. However, with
the pre-catalyst 1 the increase in conversion is only observed for an
oxidant/substrate molar ratio above 1.5 (Table 3e, entries 22–26).
When the reaction was carried out with the absence of TBHP, the
conversion was negligible (Table 3e, entry 33), but for the TBHP/
cyclohexene molar ratio of only 0.1, the conversion was already
The catalytic properties of these complexes were investigated to
open up a new applicability for this class of self-assembled clus-
ters. The complexes were found to be rather active catalyst precur-
sors for the selective allylic oxidation of cyclohexene, leading to a
3
high overall TON value up to 7.3 ꢀ 10 and a yield of up to 71%,
which disclose a higher activity than those reported earlier for
other copper(II) catalysts. It was shown that the allylic oxidation
proceeds via a C- and O-centered radical mechanism, where both
TBHP and dioxygen are shown to play relevant roles.
5
6% (Table 3e, entry 34). The reaction was also carried out in a
dioxygen atmosphere (5 atmospheric pressure), both in the pres-
ence (Table 3e, entry 35) and absence (Table 3e, entry 33) of TBHP,
and the catalytic activity was found only when TBHP was present.
Although the detailed mechanism of the catalytic activity has
not been studied, we observed that in the presence of a radical
trap, a remarkable fall of the catalytic activity occurred for complex
Acknowledgements
1
(Table 3e, entries 37, 38). This concerns either an O-centered rad-
ical trap (Ph NH) [49] or a C-centered radical scavenger (CBrCl
50], implying that the reaction proceeds mainly via radical paths
SKKar [Project No. 01(2401)/10/EMRII] and Mr. Atanu Jana
2
3
)
[
CSIR File No: 09/028(0719)/2008-EMR-I, dated 19/01/2009]
thanks CSIR, New Delhi, India for financial support. Samik Gupta
SFRH/BPD/69415/2010) and Sanghamitra Mukherjee (SFRH/BPD/
3764/2008) thank FCT–Portugal for post doctoral grants. The
studies were partially supported by the project PEst-OE/QUI/
UI0100/2011 (FCT).
[
involving both C- and O-centered radicals, as with other multi-cop-
per systems [51–53]. In case of complex 1, the reaction did not pro-
ceed well in a dinitrogen atmosphere (Table 3e, entries 31, 32),
thus suggesting the involvement of dissolved dioxygen. Hence,
H-abstraction from cyclohexene conceivably by the tert-butyl per-
oxo radical TbOOꢀ or the hydroxyl radical HOꢀ (formed by metal-as-
sisted decomposition of TBHP) leads to the cyclohexenyl radical
which, as suggested in other cases [54], can react with dissolved
(
4
Appendix A. Supplementary data
O
2
to give the 3-cyclohexenyl peroxy radical, or with a metal-
hydroperoxo species to give tert-butyl peroxy-2-cyclohexene
found as a trace by GC–MS). The conversions of the 3-cyclohexe-
nyl peroxy radical and tert-butyl peroxy-2-cyclohexene into the
products can also be metal-assisted, conceivably involving dismu-
tation of the 3-cyclohexenenyl peroxy radical to both 2-cyclohex-
ene-1-ol and 2-cyclohexene-1-one [55].
CCDC 863686 and 863687 contains the supplementary crystal-
lographic data for complexes 1 and 2. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
ing.html, or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336 033; or
e-mail: deposit@ccdc.cam.ac.uk.
(

6
0
A. Jana et al. / Polyhedron 62 (2013) 51–60
[
25] T. Kajiwara, T. Terabayashi, M. Yamashita, K. Nozak, Angew. Chem., Int. Ed. 47
2008) 6606.
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