Single crystal EPR spectroscopy, magnetic studies and catalytic activity of a self-assembled [2 × 2] CuII4 cluster obtained from a carbohydrazone based ligand

Article abstract of DOI:10.1016/j.poly.2014.11.038

A new tetranuclear [2 × 2] cluster of Cu(II) with a symmetric carbohydrazone based ligand, [Cu₄L₄](NO₃)₄·1.6(H₂O) (1), {where HL donates bis-[(E)-N'-(1-(pyridin-2-yl)ethylidene)]carbohydrazide} was synthesized and characterized by spectroscopic methods and X-ray analysis. The EPR spectra were performed on single crystals of complex 1 at various temperatures and allowed the identification and separation of two types of magnetic objects contributing to magnetism: single atoms of Cu(II) and a tetranuclear Cu₄ cluster. The main values of the g-factor and hyperfine structure were determined for single ions of Cu(II). The copper atoms in the tetramer are coupled antiferromagnetically with an isotropic antiferromagnetic exchange J = 215 K (149.4 cm^-1). A small anisotropic exchange of the order of 0.06 K (0.04 cm^-1) is responsible for the initial zero-field splitting of the energy levels in the tetramer spectrum. Magnetic measurements of complex 1 confirmed the existence of a strong antiferromagnetic exchange coupling between four Cu(II) ions. Complex 1 showed high potential for the catalytic and selective oxidation of cis-cyclooctene with aqueous H₂O₂.

Full text of DOI:10.1016/j.poly.2014.11.038

Polyhedron 88 (2015) 48–56
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Single crystal EPR spectroscopy, magnetic studies and catalytic activity
of a self-assembled [2 ꢀ 2] CuI4I cluster obtained from a carbohydrazone
based ligand
a
b,
b
c
Rahman Bikas a, , Hassan Hosseini-Monfared , Pavlo Aleshkevych , Ritta Szymczak , Milosz Siczek ,
⇑
⇑
Tadeusz Lis c
a Department of Chemistry, Faculty of Science, University of Zanjan, 45371-38791 Zanjan, Iran
b Institute of Physics, Polish Academy of Sciences (PAN), Al. Lotnikow 32/46, PL-02-668 Warsaw, Poland
c Faculty of Chemistry, University of Wroclaw, Joliot-Curie 14, 50-383 Wroclaw, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
A new tetranuclear [2 ꢀ 2] cluster of Cu(II) with a symmetric carbohydrazone based ligand, [Cu4L4]
(NO3)4ꢁ1.6(H2O) (1), {where HL donates bis-[(E)-N’-(1-(pyridin-2-yl)ethylidene)]carbohydrazide} was
synthesized and characterized by spectroscopic methods and X-ray analysis. The EPR spectra were
performed on single crystals of complex 1 at various temperatures and allowed the identification and
separation of two types of magnetic objects contributing to magnetism: single atoms of Cu(II) and a
tetranuclear Cu4 cluster. The main values of the g-factor and hyperfine structure were determined for
single ions of Cu(II). The copper atoms in the tetramer are coupled antiferromagnetically with an isotro-
pic antiferromagnetic exchange J = 215 K (149.4 cmꢂ1). A small anisotropic exchange of the order of
0.06 K (0.04 cmꢂ1) is responsible for the initial zero-field splitting of the energy levels in the tetramer
spectrum. Magnetic measurements of complex 1 confirmed the existence of a strong antiferromagnetic
exchange coupling between four Cu(II) ions. Complex 1 showed high potential for the catalytic and
selective oxidation of cis-cyclooctene with aqueous H2O2.
Received 27 September 2014
Accepted 28 November 2014
Available online 27 December 2014
Keywords:
Single crystal EPR
Magnetic studies
Catalytic epoxidation
[2ꢀ2] CuII cluster
4
Exchange coupling
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
as magnetism [9], biology [10] and medicine [11]. As catalysts,
for example, Cu(II) complexes show catalytic activities for the
The construction of multinuclear transition metal complexes
(MTMCs) remains a popular research area in the field of coordina-
tion and material chemistry [1]. This is mainly due to the peculiar
structure diversity of MTMCs and their potential application as
functional materials in fields such as catalysis [2], photochemistry
[3], ion exchange [4], sensor technology [5] and magnetic materials
[6]. In recent years polymetallic [2 ꢀ 2], [3 ꢀ 3] and [n ꢀ n] molec-
ular grid systems containing conformationally rigid tetranuclear
macrocyclic ring systems with approximately 90° bond angles
about each metallic corner have attracted the eyes of various
research groups [7]. Some of these clusters show strong magnetic
interactions and single molecule magnet (SMM) behavior [8]. In
this context, copper(II) complexes are of particular interest
because of the flexible coordination sphere of the Cu(II) ion and
the versatile applications of its complexes in various fields such
epoxidation of olefins [12], oxidation of alkanes [13], sulfoxidation
[14], cycloaddition [15] and polymerization reactions [16].
The controlled preparation of MTMCs with pre-established
structures, properties and functions is one of the challenges in syn-
thetic coordination chemistry. Among the various strategies for the
design of MTMCs, the self-assembly reaction between metal
sources and specifically designed precursors may lead to various
interesting polynuclear structures [17]. However, the self-assem-
bly of materials for this kind of complex does not occur by chance
and requires structural specificity together with particular multi-
dentate ligands [18]. The structure of the ligand plays the most
important role in the construction of MTMCs [19]. Some hydrazone
based ligands bearing multi-dentate donor atoms in specific
regions are good candidates for the design of grid-like metal ion
arrays, and [4 ꢀ 4] and [3 ꢀ 3] grid networks of Cu(II) have been
reported by a symmetric multidentate hydrazone ligand [20]. Car-
bohydrazide based ligands are a special type of symmetric ligand
in the hydrazone family which are ideal precursors for the design
of [2 ꢀ 2] clusters. Surprisingly, however, there are a few reports
⇑
Corresponding authors. Tel.: +98 241 5152576; fax: +98 241 2283203 (R. Bikas).
E-mail addresses: bikas@znu.ac.ir, bikas_r@yahoo.com (R. Bikas), pavloa@ifpan.
edu.pl (P. Aleshkevych).
http://dx.doi.org/10.1016/j.poly.2014.11.038
0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.

R. Bikas et al. / Polyhedron 88 (2015) 48–56
49
including Fe [21], Cd [22], Zn [23] and Co [24] examples concerning
structural characterization of this type of system. Recently, it has
been reported that lanthanide complexes of carbohydrazone based
ligands show SMM behavior [25]. To the best of our knowledge and
according to the reported structures in the Cambridge Structural
Database (CSD) [26], there are no reports regarding the structures
of [2 ꢀ 2] Cu complexes using symmetric hydrazone ligands. Fur-
thermore, understanding the magnetic interactions in novel
[2 ꢀ 2] clusters with symmetric ligands will be interesting. The
exchange clusters of magnetic ions in the MTMCs are the simplest
ferromagnets or antiferromagnets where exchange interactions
between a small numbers of magnetic ions has to be taken into
account. On the other hand, their magnetic properties depend
strongly on the symmetry and electronic nature of the magnetic
ions and the structures of the nearest neighboring ligands. These
factors strongly affect the EPR spectrum. The EPR technique is
known to be a useful tool in the study of exchange clusters, ensur-
ing its effectiveness because of the high sensitivity, selectivity and
large analytical background accumulated over many decades of
using EPR. In this article we report the synthesis, crystal structure,
EPR spectrum, magnetic behavior and catalytic activity of a tetra-
nuclear [2 ꢀ 2] cluster of Cu(II) with a symmetric carbohydrazone
based ligand. To achieve the full advantage of using EPR spectros-
copy, the presented EPR study was performed on single crystals.
60.73; H, 5.46; N, 28.42%. FT-IR (KBr, cmꢂ1): 3445 (m, br), 3355
(m), 3206 (m, N–H), 3061 (m), 2923 (m), 1700 (vs C@O), 1680
(vs), 1578 (m), 1518 (s), 1462 (s), 1433 (s), 1310 (w), 1261 (s),
1132 (s), 991 (w), 808 (m), 788 (s), 739 (m), 598 (w), 528 (w),
441 (w), 404 (w). 1H NMR (250.13 MHz, DMSO-d6, TMS) d (ppm):
10.87 (s, 2H, N–H), 8.61 (d, 2H, J = 7.25 Hz), 8.12 (d, 2H,
J = 7.5 Hz), 7.91 (m, 2H), 7.43 (t, 2H, J = 7.5 Hz), 2.45 (s, 6H). 1H
NMR (250.13 MHz, DMSO-d6 + D2O) d (ppm): 7.39–8.70 (10 H, aro-
matic), 2.59 (s, 6H). 13C NMR (62.90 MHz, DMSO-d6) d (ppm): 12.4,
120.8, 124.7, 136.4, 144.9, 149.1, 152.1, 157.3. UV–Vis (in CH3OH,
c = 5 ꢀ 10ꢂ5 mol dmꢂ3
, kmax [nm] with e
[Mꢂ1 cmꢂ1]): 214
(33000), 298 (36000), 380sh (5600).
2.3. Synthesis of the complex [Cu4L4](NO3)4ꢁ1.6(H2O) (1)
The appropriate amount of the ligand HL (0.296 g, 1 mmol) was
dissolved in methanol (30 mL) and Cu(NO3)2ꢁ3H2O (0.242 g,
1.00 mmol) was added. The solution was gently refluxed for 6 h.
After cooling, the resulting solid was filtered off, washed with
cooled absolute ethanol and dried at 100 °C. Purple crystals of 1
were prepared by slow solvent evaporation over one week. Yield
77% (0.33 g). Anal. Calc. for C60H63.2Cu4N28O17.6 (MW = 1712.34):
C, 42.09; H, 3.72; N, 22.90; Cu, 14.84. Found: C, 41.95; H, 3.75; N,
22.86; Cu, 14.78%. FT-IR (KBr, cmꢂ1): 3413 (m, br), 3058 (w),
1622 (m), 1601 (m), 1573 (s), 1548 (s), 1469 (m), 1384 (vs), 1353
(m), 1267 (m), 1155 (s), 1088 (m), 1011 (m), 789 (m), 775 (m),
699 (w), 570 (w), 403 (w). UV–Vis (in CH3OH, c = 2.5 ꢀ 10ꢂ5
2. Experimental
mol dmꢂ3, kmax [nm] with
e
[Mꢂ1 cmꢂ1]): 221 (100800), 369
2.1. Materials and instrumentations
(60600), 470 (32200), 695 (500).
All starting materials were purchased from Acros and used as
received. Solvents of the highest grade commercially available
(Merck) were used without further purification. IR spectra were
recorded as KBr disks with a Bruker FT-IR spectrophotometer
(model: TENSOR 27). UV–Vis spectra of solutions were recorded
on a thermo spectronic, Helios Alpha spectrophotometer. 1H and
13C NMR spectra of the ligand in DMSO-d6 solution were measured
on a Bruker 250 MHz spectrometer and chemical shifts are indi-
cated in ppm relative to tetramethylsilane. The atomic absorption
analysis was carried out using Varian Spectra AA-220 equipment.
The magnetic susceptibility and magnetization vs. magnetic field
were measured with a Quantum Design SQUID MPMS XL magne-
tometer in a magnetic field up to 5 T and in the temperature range
2–300 K. The magnetic susceptibility measurements were carried
out both in the zero-field-cooled (ZFC) and field-cooled (FC) modes.
No differences between the ZFC and FC results and no magnetic
phase transitions were observed. The EPR measurements were car-
ried out using a Bruker EMX spectrometer working at a fixed fre-
quency of 9.25 GHz (X-band) with an Oxford helium-flow
cryostat instrument operating in the temperature range from 3.8
to 300 K. A 100 kHz magnetic field modulation and phase sensitive
detection were used to record the derivative of the absorbed
microwave power. The samples were maintained in a rotatable
sample holder to record the angular variation of the EPR spectra.
2.4. X-ray crystallography
A summary of the crystal data and refinement details for com-
plex 1 are given in Table 1. Only special features of the analyses are
Table 1
Crystallographic data of 1.
Complex 1
Net formula
C60H60Cu4N24O4(NO3)4
1.6(H2O)
1712.34
Mr (g molꢂ1
Crystal size (mm)
Crystal shape
Color
)
0.25 ꢀ 0.20 ꢀ 0.15
block
purple
100(2)
T (K)
Radiation (Å)
Diffractometer
Crystal system
Space group
a, b (Å)
Mo Ka (0.71073)
Xcalibur PX with Onyx CCD
tetragonal
I41/a
13.929(3)
35.16(2)
6822(5)
4
1.667
1.32
analytical
33214
c (Å)
V (Å3)
Z
Dcalc (g cmꢂ3
)
l
(mmꢂ1
)
Absorption correction
Reflections measured
Rint
0.027
h, k, l
ꢂ19?19, ꢂ14?19, ꢂ50?50
2.2. Synthesis of bis-[(E)-N0-(1-(pyridin-2-yl)ethylidene)]
carbohydrazide (HL)
H
range
2.9–30.6
5242
Observed reflections
Hydrogen refinement
Reflections in refinement
Parameters/restraints
F(000)
mixed
4003
297/18
3504
0.028
0.081
1.03
0.001
0.39/ꢂ0.29
A
methanol (10 mL) solution of carbohydrazide (0.50 g,
5.55 mmol) was added dropwise to a methanol solution (10 mL)
of 2-acetylpyridine (1.34 g, 11.10 mmol) and the mixture was
refluxed for 6 h. The solution was evaporated on a steam bath to
5 mL and cooled to room temperature. The resulting white precip-
itate was separated and filtered off, washed with 5 mL of cooled
methanol and recrystallized. Yield: 95% (1.56 g). Anal. Calc. for
R(Fobs
)
Rw(F2)
S
Shift/errormax
Maximum/minimum electron density
(e Åꢂ3
)
C15H16N6O (MW = 296.33): C, 60.80; H, 5.44; N, 28.36. Found: C,

50
R. Bikas et al. / Polyhedron 88 (2015) 48–56
mentioned here. Single crystal data collection for 1 was performed
on an Xcalibur PX diffractometer with an Onyx CCD detector,
equipped with an Oxford Cryosystems open-flow nitrogen cryo-
band is seen in the IR spectrum of the complex, which suggests
enolization of the amide functionality of (L)ꢂ upon coordination
to the metal ions. This finding is also confirmed by the appearance
of a new band at 1622 cmꢂ1, which may be assigned to the
–C@N–N@C– moiety [30]. There is a weak band at 3058 cmꢂ1 in
the IR spectrum of the complex, which is due to the presence of
an N–H bond in 1 and indicates that one of the amidic hydrogen
atoms of (L)ꢂ remains intact during the complexation. The disap-
pearance of the carbonyl stretching vibration band and the
presence of the N–H band suggests the absence of delocalization
in the –N-C@O group in the coordinated (L)ꢂ ligand [31]. A broad
absorption band at 3413 cmꢂ1 (O–H stretching) in the infrared
spectrum of 1 is due to the presence of uncoordinated solvents.
The electronic spectra of complex 1 (green solution) and HL
(colorless solution) in a MeOH solution are shown in Fig. S1. The
hydrazone ligand displays strong bands at 214, 298 and 380 nm.
Based on their extinction coefficients, they are assigned to intrali-
stat, using
x scans and graphite-monochromated Mo Ka
(k = 0.71073 Å) radiation. The structure was solved by direct meth-
ods with SHELXS-97 [27], and refined with full-matrix least-squares
techniques on F2 with SHELXL-97 [28]. The C-bonded hydrogen
atoms were calculated in an idealized geometry riding on their
parent atoms. The O and N-bonded hydrogen atoms were located
from the difference map. The molecular structure plot was pre-
pared using Diamond [29].
2.5. General oxidation procedure
Liquid phase catalytic oxidation of cis-cyclooctene was carried
out under air (atmospheric pressure) in a 25 mL round bottom
flask equipped with a magnetic stirrer. In a typical experiment,
gand
p ?
p⁄ (214, 298 nm) and n ? p⁄ (380 nm) transitions. Shifts
H2O2 was added to a flask containing the catalyst (1 lmol) and
and changes of the bands in the UV–Vis spectrum of the complex
relative to the HL bands indicate the coordination of the ligand
to the Cu(II) ions. For the complex, the higher energy bands at
369 and 470 nm, with high extinction coefficient values, are due
to ligand to metal charge transfer (LMCT) transitions. The other
higher energy intense transition at 221 nm is assigned to the
cis-cyclooctene (1 mmol) in solvent (3 mL). The course of the reac-
tion was monitored using a gas chromatograph equipped with a
capillary column and a flame ionization detector. The oxidation
products were identified by comparing their retention times with
those of authentic samples or alternatively by 1H NMR and GC-
Mass analyses. Control reactions were carried out in the absence
of catalyst, under the same conditions as the catalytic runs. No
products were detected.
intraligand
p ?
p⁄ charge transfer transitions. The band for the
d-d transition appears as a broad weak peak near 700 nm.
3. Results and discussion
3.2. X-ray structure of [Cu4(L)4](NO3)4ꢁ1.6(H2O) (1)
3.1. Synthesis and spectroscopy
The structure of complex 1 and the coordination core about the
Cu(II) centers with selected bond lengths and angles are given in
Fig. 1. The core of the tetranuclear compound is composed of four
symmetry-related copper(II) ions, creating an almost square geom-
etry with a slight deformation towards a tetrahedral geometry.
Distortion from planarity can be noted in the value of the deviation
from the best plane created by the four copper atoms, which is per-
pendicular to the fourfold inversion axis (Fig. 1b). Two Cu atoms
are above the best plane by 0.2791(1) Å and the other two are
below the plane by -0.2791(1) Å.
The reaction of carbohydrazide with 2-acetylpyridine in meth-
anol gave the desired symmetric Schiff base ligand, HL, in excellent
yield and purity (Scheme 1). Keto-enol tautomerism of the aro-
ylhydrazone HL is illustrated in Scheme 1b. IR and NMR spectra
of the ligand confirmed the proposed structure and indicate that
the free ligand is stable in the keto form. In the 1H NMR spectrum
of the ligand the presence of a signal at d 10.87 ppm was assigned
to the common N-H group, which was consistent with the observa-
tion of a rapid loss of this signal when D2O was added to the solu-
tion. The aromatic hydrogen atoms appear at d 7.4–8.6 ppm and
the methyl hydrogens are located at d 2.45 ppm. The IR spectrum
of HL displays a characteristic band at 3206 cmꢂ1 which is due to
The tetranuclear Cu4L44+ [2 ꢀ 2] cation has four Cu(II) centers, to
which the ligands are coordinated in the mononegative form, (L)ꢂ,
by deprotonation of one N–H moiety. Four nitrate anions and H2O
molecules surround the cation. Complex 1 crystallized in the I41/a
space group and is isomorphous to a zinc complex with another
type of carbohydrazone based ligand [22]. The complex cation as
a whole has S4 symmetry. In the tetranuclear Cu4L4 [2 ꢀ 2] unit,
each Cu(II) atom is six-coordinated by two pyridyl nitrogen, two
azomethine nitrogen and two enolate oxygen atoms. The overall
geometry around the copper(II) ion is best described as a distorted
m
(N–H). In the IR spectrum of HL two strong bands appear at
1700 and 1680 cmꢂ1 due to
m
(C@O) stretching vibrations. Complex
1 was synthesized by the reaction of Cu(NO3)2ꢁ3H2O and HL in
equimolar ratios, with methanol as the solvent. A comparison of
the IR spectrum of complex 1 with that of the free ligand provides
evidence for the coordination mode of HL. Elimination of the C@O
R'
R'
H
N
H
N
H
N
H
N
N
N
HL in the keto form
R'
N
N
R'
N
O
N
O
- H+
- H+
Carbohydrazone
H
N
H
N
(a)
N
N
N
N
N
N
N
O
N
N
O
N
(L)— in the enol form
(b)
Scheme 1. (a) General formula of carbohydrazones and (b) the tautomeric forms of HL.

R. Bikas et al. / Polyhedron 88 (2015) 48–56
51
Jahn–Teller effect usually found in octahedral Cu(II) complexes
with a d9 electronic configuration [34].
Having H2O, NOꢂ3 and amidic nitrogen functionality, it is not
surprising that the crystal packing of 1 is dominated by strong
and directional hydrogen bonds. Water molecules and NOꢂ3 anions
form a 1D polymeric chain through O–Hꢁ ꢁ ꢁO hydrogen bonds. Also,
there are strong C–Hꢁ ꢁ ꢁ
p
and
p p interactions between the pyri-
ꢁ ꢁ ꢁ
dine rings [35] of the tetranuclear units. Some of these interactions
are shown in Fig. S2. Geometric parameters characterizing the
intermolecular interactions and hydrogen bonding connections
are collected in Table S1.
3.3. EPR spectrum
Examples of resonance spectra obtained for single crystals of
complex 1 recorded between 10 K and room temperature are
shown in Fig. 2. At low temperatures (T < 30 K) the spectra consist
of groups of four intense equidistant lines associated with a single
Cu(II) ion. The intensity of these lines decreases monotonically
upon increasing the temperature, until they completely disappear
at around 100 K. In the region of about 40–50 K new resonance
lines become visible. These lines are labelled as (1)–(4) in Fig. 2
and their intensities show non-monotonic behavior versus temper-
ature. It indicates that these lines belong to the exchange coupled
complex, while the four lines in the low temperature region are
due to isolated Cu(II) ions, with a clearly seen hyperfine structure.
The temperature dependencies of the integral intensities of all
the resonance lines are shown in Fig. 3. The open square symbol
shows the total intensity of the four lines of single Cu(II) ions,
which follows a Curie-like temperature dependence. On the other
hand, lines (1)–(4) change non-monotically with temperature. Line
(1) intensity at ꢃ115 K, lines (2) and (3) each reach a maximum at
ꢃ125 K, while line (3) reaches a maximum at ꢃ200 K. The solid cir-
cles represent the total intensity of all the resonance lines. The
presence of a maximum for the temperature dependence of inten-
sity indicates that the corresponding resonance transition occurs
between energy levels far above the non-magnetic ground state,
and thus is thermally activated. Based on the crystallographic data
and stoichiometry in the single crystals of complex 1, it is antici-
pated that the observed resonance lines (1)–(4) originate from
antiferromagnetically coupled
tetramers, with no
Fig. 1. (a) Molecular structure of complex 1 together with the numbering scheme;
(b) coordination environment around the metal cores which are placed alterna-
tively above and below the plane perpendicular to the fourfold inversion axis.
Selected bond lengths (Å) and angles (°): Cu–N3 = 1.9415(17); Cu–N31i =
2.0119(17); Cu–O1 = 2.0261(10); Cu–O1i = 2.2933(10); Cu–N2 = 2.0340(13); Cu–
N21i = 2.2535(13); O1–Cu–O1i = 91.03(5); N3–Cu–O1 = 78.98(5); N31i–Cu–O1 =
99.23(4); N3–Cu–N2 = 79.79(5); N31i–Cu–N2 = 102.37(5); O1–Cu–N2 = 157.77(5);
N31i–Cu–N21i = 75.61(5); O1–Cu–N21i = 91.76(4); N2–Cu–N21i = 98.63(5); N21i–
Cu–O1i = 150.09(4) where i = ꢂy + 5/4, x + 1/4, ꢂz + 1/4.
exchange between the different tetramers. According to the struc-
ture, the four Cu(II) atoms almost form a square with a Cuꢁ ꢁ ꢁCu dis-
tance equal to 4.070(1) Å. The local environment of each of the four
Cu(II) atoms is identical, as well as the nature of the bridging
chemical parts between nearest neighboring Cu(II) atoms. These
data allow the assumption that in the first approach, and to avoid
over parametrization, each of the nearest Cu–Cu pairs is
characterized by the same exchange coupling and the following
spin-Hamiltonian can be used to explain the EPR results:
octahedron with axial elongation due to the Jahn–Teller effect.
Each (L)ꢂ ligand binds to two Cu(II) cations, in which the oxygen
atom shares coordination to adjacent Cu(II) ions. These alternate
copper and oxygen atoms form a boat–boat conformation [32]
with an eight atom ring (Fig. 1b). Around each Cu(II) core, the
two enolate O atoms and two pyridyl N atoms are located cis with
respect to each other, whereas the azomethine N atoms are trans.
The adjacent Cuꢁ ꢁ ꢁCu distance is 4.070(1) Å and the Cu–O–Cu angle
is 140.76(5)°, values which are close to other reported tetranuclear
copper complexes with asymmetric hydrazone ligands [33]. The
Cuꢁ ꢁ ꢁCu distances across the diagonals are 5.701(1) Å and the
Cuꢁ ꢁ ꢁCuꢁ ꢁ ꢁCu angles are 88.922(5)°.
4
X
^
^
^
^
^
^
^
^
^
~ ~ ~ ~~ ~ ~~ ~ ~~ ~ ~~
^
H ¼
~
lBHgiSi þ S1JS2 þ S1JS4 þ S2JS3 þ S3JS4
ð1Þ
i¼1
~
where
l
B is the Bohr magneton, H – the external magnetic field vec-
^
~
~
tor, S – the spin operator, gi –the g-tensor of a particular Cu(II) atom
~
within the tetramer molecule, J – the second rank dyadic tensor
containing all the relevant exchange parameters involved in the
bilinear exchange. The schematic positions of spins 1–4 forming
the tetramer are shown in Fig. 4. For the EPR measurements and
analysis of the spectra, the Cartesian axis system (X,Y,Z) was defined
such that the axis Z is parallel to the fourfold inversion axis of the
tetramer complex. The principal values of the g-tensors for each
of the four Cu(II) atoms are assumed to be the same, but with
different orientations of the principal magnetic axes. The Cartesian
local site axis system for each of the four Cu(II) atoms is defined
The Cu–O and Cu–N distances are close to those found in other
Cu(II) complexes with N2O-donor hydrazone ligands [33]. The axial
Cu–O1i and Cu–N21i bond distances are 2.2933(10) and
2.2535(13) Å, respectively. These two bonds are considerably elon-
gated with respect to the other four, which is consistent with the

52
R. Bikas et al. / Polyhedron 88 (2015) 48–56
are parallel to the Z axis. The magnetic coupling between the copper
ions is mediated via the ligand bridges. It follows from the structure
that there are no ligands connecting atoms through diagonals,
therefore no cross-coupling is assumed. As any physical 2-rank ten-
~
~
~
~
~
sor, J could be split into 3 parts, J ¼ JE þ D þ A, where the isotropic
P
~
part of the exchange interaction J ¼ 1=3 TrJ ¼ 1=3ðJxx þ Jyy þ JzzÞ,
the asymmetrical part Aij ¼ 1=2ðJij ꢂ JjiÞ and the symmetrical part
Dij ¼ 1=2ðJij þ JjiÞ, (E is the unity matrix and the elements of the ten-
~
sor J are denoted as Jij, where the sub indexes i, j = x, y, z). The most
meaningful influence on the magnetic properties of the tetramer
~
comes from the diagonal elements of tensor J which are strictly
related to the isotropic part of exchange interaction J. In the case
of the tetramer, the isotropic exchange J splits 16-fold degenerated
levels into four multiplets (see diagram in Fig. 4) [36]. For the anti-
ferromagnetic interaction (J > 0), the ground state is a singlet. The
first excited multiplet is a triplet, which could be described by the
effective spin S0 = 1. The separation between the multiplets is pro-
portional to the isotropic exchange J. The slight deviation from a
square to the distorted Cu4 cluster does not change the multiplet
splitting qualitatively, but may lift the degeneration of multiplets
and could give rise to off-diagonal elements Jij, where i – j. The J
value can be found by analyzing the temperature dependence of
the integral intensity of the resonance lines. To identify correctly
the resonance transitions labelled as (1)–(4) on Fig. 2, the angular
variation of the resonance spectrum was measured at 120 K in the
XY plane (Fig. 5). As one can see, the positions of experimental lines
(2) and (4) resemble the fine structure that could be described by an
effective spin S0 = 1. Additionally, line (1), located at the ‘‘half-field’’
position (Hres = 1440 Oe), is a partially allowed transition within the
same triplet involving a change of the electron magnetic quantum
0
number by 2 (
D
MS = 2). The maximal intensity of lines (1), (2)
and (4) occurs at the same temperature, therefore these resonance
transitions belong to the same thermally activated spin triplet with
S0 = 1, which is the first excited multiplet of the tetramer. The line
(3) apparently originates from higher multiplets as it has maximal
intensity at nearly double the temperature (ꢃ200 K) as compared
with the first excited multiplet.
Fig. 2. Examples of the EPR spectra of single crystals of complex 1, recorded at
T = 10, 50, 100, 180, 300 K with H parallel (a) and perpendicular (b) to the Z axis. The
labels (1), (2), (3), (4) mark resonance transitions of the Cu4 tetramer. The group of 4
lines at 10 K shows the hyperfine structure of single 3d9 Cu(II) ions.
The presence of fine structure indicates that the degeneracy of
each of the multiplets is lifted because of zero-field splitting. Nei-
ther isotropic exchange nor a crystal field acting on Cu(II) can lift
the degeneracy within the multiplets and explain the presence of
the fine structure. We assume that the main contribution to the
zero-field splitting is caused by small off-diagonal elements of ten-
~
sor J (Jij, with i – j) related to anisotropic exchange [37]. The prop-
erly constructed spin Hamiltonian should be invariant upon the
cluster point group symmetry transformation. Due to the high S4
symmetry of the Cu4 cluster, we assume that the antisymmetric
part of the anisotropic exchange, related to Dzialoszynsky–Moria
interactions, is zero (Jij ꢂ Jji = 0) and in the first approach every
~
off-diagonal element of the elements of J can be reduced to the sin-
gle parameter Jij = Jji (i<>j) = D, accounting for the symmetrical part
of the anisotropic exchange. The exact numerical diagonalization
of the spin-Hamiltonian (1) was used to calculate the resonance
line positions by using a self-developed computer program. Using
the least-square procedure, the program fits simultaneously only
three spin-Hamiltonian parameters, i.e. gx, gz, D. This method
implicitly assumes the coincidence of the principal axes of the
g-tensor with the local site axis system for each Cu(II) atom. Of
course, this assumption is not strictly valid for monoclinic or
triclinic site symmetries. The calculated positions of eight reso-
nance transitions within the tetramer spectrum are shown by solid
lines in Fig. 5. The best agreement with the experimental positions
of the resonance lines was achieved for gx = 2.09 0.02,
gz = 2.28 0.01, D = 0.06 0.01 K (0.04 cmꢂ1). The calculated
resonance fields reproduce very well the experimental angular
Fig. 3. The temperature dependencies of the integral intensities of all resolved
resonance lines. The solid circles represent the total intensity of all the resonance
lines. The solid lines show numerically calculated line intensities vs.
T for
transitions within different multiplets of the tetramer spectrum for J = 215 K
(149 cmꢂ1). The inset shows the product T ꢀ Intensity vs. T for isolated Cu(II) (open
square) and the sum of the tetramer + Cu(II) intensities (open circle) and the best fit.
with respect to XYZ, as shown in Fig. 4a. The local zi axes were
chosen to be parallel to the most elongated Cu–O1i and Cu–N21i
bonds in the nearest environment of the Cu atom. The local yi axes

R. Bikas et al. / Polyhedron 88 (2015) 48–56
53
Fig. 4. (a) The schematic representation of the Cu4 tetramer. The dashed lines connect spins coupled by J. The X,Y,Z axes are the spin-Hamiltonian (1) Cartesian axis system.
The x,y,z axes are the local site axis system for each of the 4 Cu(II) atoms (y || Z). (b) The splitting of the 16-fold degenerated level of the tetramer under antiferromagnetic
isotropic exchange. The numbers in round bracket show the degeneracy of the multiplets without a magnetic field. The splitting between the multiplets is proportional to the
~
trace of the exchange coupling tensor J.
h
i
E
kT
E
q
kT
ꢀ
ꢁ
p
1
eꢂ
2
Ipq ¼ Hpq
eꢂ ꢂ eꢂ
ð2Þ
X
E
kT
i
i
where Hpq is the matrix element corresponding to the transition Ipq
.
The summation in the denominator runs over all populated levels in
the energy spectrum. Utilizing Eq. (2), the temperature dependenc-
es of the resonance transition intensities from different multiplets
of the tetramer as well as isolated Cu(II) ions were calculated and
are shown as solid lines in Fig. 3. For the single Cu(II) ion, the inten-
sity of one resonance transition, according to Eq. (2), is essentially
simplified to Curie-like, inversely proportional to temperature
dependence. The overall intensity was a weighted sum of the
resonance lines of the tetramer and the isolated Cu(II) ion: Isum
(1 ꢂ )Itetramer ICu(II). A good agreement between the experimen-
tal and calculated dependencies occurs for J = (215 15) K (149
12 cmꢂ1) and the fraction of isolated Cu(II) is
ꢄ (2.5 0.5)%. The
=
q
+ q
q
angular variation of the resonance fields of the isolated Cu(II) ion
at 30 K is shown in Fig. S3. As is seen, the dependence shows the
hyperfine structure due to the nuclear spin I = 3/2 of the 3d9 elec-
tron configuration of the Cu(II) ions. The experimental points could
be described by close to axial anisotropy of both g-factor and hyper-
fine splitting parameters: g\ = 2.09 0.04, g|| = 2.28 0.05,
A\ = 0.0019 0.0004 cmꢂ1, A|| = 0.0156 0.0010 cmꢂ1. This small
fraction of isolated Cu(II) ions are uncontrolled impurities associ-
ated with microcrystallites of copper nitrate precipitated on the
crystal faces.
Fig. 5. The symbols represent the angular variations of the experimental resonance
fields of lines measured at 120 K. The axis of rotation is the fourfold inversion axis of
the tetramer. The labels (1)–(4) correspond to resonance lines shown in Fig. 3. The
solid lines show the numerically calculated angular dependencies of the resonance
transitions of the tetramer best fitted to experimental points for: gx = 2.09 0.02,
gz = 2.28 0.01, D = 0.06 0.01 K. The red, green and blue lines correspond to
transitions from the first, second and third exited multiplet of the tetramer energy
spectrum, respectively. On the right side there are identifications for each of the
resonance transitions. (Color online.)
dependence. The experimental lines (2)–(4) are, in fact, the super-
position of resonance transitions from different exited multiplets
of the tetramer energy spectrum. Three resonance transitions
within the first exited multiplet of the tetramer spectrum (labelled
as S0 = 1 in Fig. 4b) are most intensive at T = 120 K and are shown
3.4. Magnetic studies
Magnetic measurements of complex 1 confirm the existence of
strong magnetic exchange coupling between four Cu(II) ions, also
observed by means of the EPR technique. The temperature depen-
dent magnetic susceptibility of powdered crystals of the complex
was examined on an MPMS-XL7 SQUID magnetometer over the
temperature range 2–300 K. A plot of the molar magnetic suscep-
tibility measured in magnetic field H = 6 kOe and corrected for dia-
magnetic contributions and TIP is displayed in Fig. 6 in the form of
by red lines in Fig. 5. The green line describes the nearly isotropic
0
allowed transitions (
D
MS = 1) within the second exited multiplet
(2ꢀS0 = 1, S = 0). There are also four transitions within the highest
exited multiplet (S0 = 2), shown by blue solid lines. These transi-
tions are still much less intensive at 120 K than transitions from
lower exited multiplets. The fine structure at H || Z is grouped into
a poorly resolved resonance line (see spectrum in Fig. 2a) centered
around Hres = 3200 Oe, which gives us the missing value of
gy = 2.094 0.004. Therefore, within the errors of calculation, each
of the Cu(II) atoms are characterized by an axial g-tensor with two
principal values gx = gy = g\ = 2.09, g|| = 2.28.
v
v
(T) and
vT(T). It is seen from this figure that at room temperature
T(T) is 1.7 cmꢂ3 K/mol, which is near (in the limit of experimental
error) to the theoretical value calculated for Cu in the Cu4 unit
(S = ½ and g = 2.15 from the EPR spectrum, which is considered
as an average value of the parallel and perpendicular components
of the anisotropic g tensor) without any interaction between ions.
As the temperature is lowered,
decrease, which is characteristic of antiferromagnetic coupling
The intensity of the resonance transition Ipq between two
energy levels Ep and Eq at a given temperature T was evaluated
by the following general formula [38]:
vT(T) keeps the continuous
between magnetic centers.

54
R. Bikas et al. / Polyhedron 88 (2015) 48–56
distances and bond angles in this tetranuclear unit are the same,
the exchange interactions can be considered to be equivalent,
and therefore J1 = J2 = J3 = J4 = J [39]. In the frame of this formalism
the magnetic susceptibility may be expressed as [39]:
2
vtetramer ¼ðNAðglBÞ =ðkBTÞÞðð2eꢂJ=T þ 4eꢂ2J=T þ 10ꢂ3J=T Þ=
ð1 þ 3eꢂJ=T þ 7eꢂ2J=T þ 5eꢂ3J=T ÞÞ;
vtotal ¼ ð1 ꢂ
q
Þ
vtetramer
þ
q
3=8g2T
ð3Þ
In this expression a Curie term is included in order to take into
account a % fraction of monomeric Cu ions, suggested by EPR
q
measurements. Using Eq. (3), the experimental data T (T) were fit-
v
ted in the temperature range 3–300 K with the best-fit parameters
J ꢄ 234 K (162 cmꢂ1), g = 1.91 and
q = 2.1%. The obtained parame-
ters J and g differ from those estimated from EPR data, because
Eq. (3) accounts for the isotropic exchange only, without aniso-
tropic exchange. Alternatively, the magnetic susceptibility was cal-
culated numerically using the exact diagonalization of the spin-
Fig. 6. Susceptibility
v and vT vs. temperature. The solid lines present two
theoretical calculations: using Eq. (3) and the spin-Hamiltonian (1).
Hamiltonian (1) with J = 215 K, D = 0.06 K,
g|| = 2.28. After averaging between the three principal directions
to approximate the powder behavior, the calculated T value is
q = 2.5%, g\ = 2.09,
v
shown in Fig. 6, labelled as ‘‘sH fit’’. Both fits are in good agreement
with the experimental data, but there is a more realistic value of
the g-factor when a small anisotropic exchange is taken into
account in the second case.
3.5. Catalytic activity
Since copper complexes of hydrazone ligands are good catalysts
for the epoxidation of olefins [40], the catalytic activity of the com-
plex was tested for the oxidation of cis-cyclooctene as a model sub-
strate. In this regard, we were particularly interested to use the
advantages of H2O2 as a green and cheap oxidant. The catalytic oxi-
dation of cis-cyclooctene was carried out with H2O2 to give cis-
cyclooctene oxide as the sole product. Control experiments
revealed that the presence of oxidant and catalyst are essential
for the oxidation.
To determine the optimal experimental conditions, the effects
of H2O2/cis-cyclooctene molar ratio, the reaction temperature and
influence of the solvent were studied. We have monitored the pro-
gress of cis-cyclooctene epoxidation for different molar ratios of
oxidant and substrate. Molar H2O2/cis-cyclooctene ratios of 1, 2,
3 and 4 were considered, while keeping a fixed amount of
Fig. 7. Effect of H2O2 concentration on the oxidation of cyclooctene by 1. Reaction
conditions: catalyst 1, 1
60 °C.
lmol; CH3CN, 3 mL; cyclooctene, 1 mmol; temperature,
In order to investigate quantitatively this coupling, an isotropic
Heisenberg-Dirac-van Vleck (HDVV) Hamiltonian formalism was
used (see eg., (1) in EPR analysis and also Ref. [39]). Since the bond
cis-cyclooctene (1.0 mmol) and catalyst (1
lmol) in 3 mL of
acetonitrile at 60 °C (Fig. 7). With a 1:1 ratio of H2O2:cyclooctene,
Fig. 8. Effect of solvent on the oxidation of cyclooctene by 1. Reaction conditions: catalyst 1, 1 lmol; solvent, 3 mL; cyclooctene, 1 mmol; H2O2, 3 mmol; temperature, 60 °C,
time; 4 h.

R. Bikas et al. / Polyhedron 88 (2015) 48–56
55
Acknowledgments
The authors are grateful to the University of Zanjan and Insti-
tute of Physics, Polish Academy of Sciences for financial support
of this study.
Appendix A. Supplementary data
CCDC 988074 contain the supplementary crystallographic data
for complex 1, respectively. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.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. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/10.
1016/j.poly.2014.11.038.
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Reaction conditions: Catalyst, complex 1,
1 mmol; H2O2, 3 mmol.
1 lmol; CH3CN, 3 mL; cyclooctene,
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