Supramolecular dimer formation through hydrogen bond extensions of carboxylate ligands - Path for magnetic exchange

Article abstract of DOI:10.1016/j.poly.2009.06.056

A new complex of unusual composition [Cu(3-O₂Nbz)₂(nia)(H₂O)₂] (1) (nia = nicotinamide, 3-O₂Nbz = 3-nitrobenzoate) has been prepared and studied together with two other complexes of composition [Cu(4-O₂Nbz)₂(nia)₂(H₂O)₂] (2) and [Cu(4-O₂Nbz)₂(nia)₂]{bullet operator}(4-O₂NbzH)₂ (3) (4-O₂Nbz = 4-nitrobenzoate). The composition of all complexes has been determined by elemental analysis, the complexes have been studied by electronic, infrared and EPR spectroscopy, as well as by magnetization measurements over the temperature range 1.8-300 K, and their structures have been solved. The structure of complex (1) consists of molecules, where Cu(II) atom is monodentately coordinated by the pair of 3-nitrobenzoato anions in trans-positions together with water and nicotinamide molecules, forming nearly tetragonal basal plane, and by another water molecule in axial position of tetragonal-pyramidal coordination polyhedron. The neighboring molecule coordination polyhedron basal planes are coplanar and allow formation of supramolecular dimers with strong H-bonds between hydrogen atoms from equatorially coordinated water molecules and uncoordinated carboxylate oxygen atoms thus giving the nearest Cu?Cu distance of 4.886(2) ?. Magnetization measurements showed that complex (1) exhibits maximum of magnetic susceptibility at 6.5 K and a fit to Bleaney-Bowers equation gave singlet-triplet energy gap 2J = -6.25 cm^-1, and zJ′ = -0.03 cm^-1. This might be an experimental proof that the carboxylate bridges extended with hydrogen bonds are the pathway of the spin-spin interactions. The temperature dependence of changes in EPR spectra of (1) and the spectrum at 4.2 K have confirmed its hydrogen bonded dimeric structure. The calculated Cu?Cu distance 4.8 ? is in very good agreement with the value obtained from crystal structure. The complexes (2) and (3) at 300 K exhibit magnetic moment μ_eff = 1.98 B.M. and μ_eff = 1.84 B.M., respectively. These values practically do not change with lowering the temperature up to 5 K and only small drops to μ_eff = 1.87 B.M. (for (2)) and μ_eff = 1.79 B.M. (for (3)) at 1.8 K have been observed. The EPR spectra of complex (2) at room temperature as well as at 77 K are of axial type with g_⊥ = 2.062 and g_|| = 2.285 and exhibit resolved parallel hyperfine splitting with A_|| = 160 Gauss. The EPR spectra of complex (3) at room temperature as well as at 77 K are of axial type with g_⊥ = 2.065 and g_|| = 2.235 and exhibit unresolved parallel hyperfine splitting. EPR spectra of (2) and (3) are consistent with the X-ray structure.

Full text of DOI:10.1016/j.poly.2009.06.056

Polyhedron 29 (2010) 154–163
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Supramolecular dimer formation through hydrogen bond extensions
of carboxylate ligands – Path for magnetic exchange
a
b
a
Zuzana Vasková a, , Jan Moncol , Maria Korabik , Dušan Valigura ,
*
Jozef Švorec a, Tadeusz Lis b, Marian Valko c, Milan Melník a
a Department of Inorganic Chemistry, Slovak Technical University, Radlinského 9, 812 37 Bratislava, Slovakia
b Faculty of Chemistry, University of Wrocław, 50-383 Wrocław, Poland
c Department of Physical Chemistry, Slovak Technical University, Radlinského 9, 812 37 Bratislava, Slovakia
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 27 June 2009
A new complex of unusual composition [Cu(3-O2Nbz)2(nia)(H2O)2] (1) (nia = nicotinamide, 3-O2Nbz = 3-
nitrobenzoate) has been prepared and studied together with two other complexes of composition [Cu(4-
O2Nbz)2(nia)2(H2O)2] (2) and [Cu(4-O2Nbz)2(nia)2]?(4-O2NbzH)2 (3) (4-O2Nbz = 4-nitrobenzoate). The
composition of all complexes has been determined by elemental analysis, the complexes have been stud-
ied by electronic, infrared and EPR spectroscopy, as well as by magnetization measurements over the
temperature range 1.8–300 K, and their structures have been solved. The structure of complex (1) con-
sists of molecules, where Cu(II) atom is monodentately coordinated by the pair of 3-nitrobenzoato anions
in trans-positions together with water and nicotinamide molecules, forming nearly tetragonal basal
plane, and by another water molecule in axial position of tetragonal-pyramidal coordination polyhedron.
The neighboring molecule coordination polyhedron basal planes are coplanar and allow formation of
supramolecular dimers with strong H-bonds between hydrogen atoms from equatorially coordinated
water molecules and uncoordinated carboxylate oxygen atoms thus giving the nearest Cuꢀ ꢀ ꢀCu distance
of 4.886(2) Å. Magnetization measurements showed that complex (1) exhibits maximum of magnetic
Keywords:
Copper complexes
Crystal structure
Hydrogen bonds
Supramolecular structure
Magnetic properties
susceptibility at 6.5 K and
a fit to Bleaney-Bowers equation gave singlet–triplet energy gap
2J = ꢁ6.25 cmꢁ1, and zJ0 = ꢁ0.03 cmꢁ1. This might be an experimental proof that the carboxylate bridges
extended with hydrogen bonds are the pathway of the spin–spin interactions. The temperature depen-
dence of changes in EPR spectra of (1) and the spectrum at 4.2 K have confirmed its hydrogen bonded
dimeric structure. The calculated Cuꢀ ꢀ ꢀCu distance 4.8 Å is in very good agreement with the value
obtained from crystal structure. The complexes (2) and (3) at 300 K exhibit magnetic moment
l
eff = 1.98 B.M. and
l
eff = 1.84 B.M., respectively. These values practically do not change with lowering
eff = 1.87 B.M. (for (2)) and eff = 1.79 B.M. (for (3))
the temperature up to 5 K and only small drops to
l
l
at 1.8 K have been observed. The EPR spectra of complex (2) at room temperature as well as at 77 K
are of axial type with g\ = 2.062 and g|| = 2.285 and exhibit resolved parallel hyperfine splitting with
A|| = 160 Gauss. The EPR spectra of complex (3) at room temperature as well as at 77 K are of axial type
with g\ = 2.065 and g|| = 2.235 and exhibit unresolved parallel hyperfine splitting. EPR spectra of (2) and
(3) are consistent with the X-ray structure.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
have a great potential of applications in different fields [3] such as
heterogeneous catalysis, molecular electronics, and magnetic or
Hydrogen bonds are used as interesting tools for crystal engi-
neering of organic substances while the coordination bonds are
considered as the main tool for coordination chemists [1] to build
up building blocks that possess suitable geometry for their applica-
tion in supramolecular structures. Hydrogen bonds, from very
strong to very weak, play an important role in supramolecular
structure formation of metal complexes [2] and consequently they
optical materials. Unusual dimeric complex [Cu(5-MeOsal)2(
nia)(H2O)]2 (where 5-MeOsal = 5-methoxysalicylate anion,
l-
nia = nicotinamide) published recently [4] demonstrated interest-
ing magnetic properties with maximum susceptibility at around
6 K. Antiferromagnetism, characterized by singlet–triplet energy
gap (2J = ꢁ6.83 cmꢁ1), has been attributed for [Cu(5-MeOsal)2(
l-
nia)(H2O)]2 to interdimer Cuꢀ ꢀ ꢀCu interactions through carboxylate
bridges extended via O–Hꢀ ꢀ ꢀO-hydrogen bonds. With the aim to
obtain a proof for a suggested pathway of a mentioned antiferro-
magnetic interaction we have analyzed structures and investigated
* Corresponding author. Tel.: +421 2 59325611; fax: +421 2 52493198.
E-mail address: zuzana.vaskova@stuba.sk (Z. Vasková).
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.06.056

Z. Vasková et al. / Polyhedron 29 (2010) 154–163
155
magnetic properties of several other carboxylatocopper(II) com-
plexes that exhibit unusual composition. In this paper we present
preparation, structure, spectral and magnetic properties of [Cu(3-
O2Nbz)2(nia)(H2O)2] (1), together with complexes [Cu(4-
O2Nbz)2(nia)2(H2O)2] (2) and [Cu(4-O2Nbz)2(nia)2]ꢀ2(4-O2NbzH)
(3) (where O2Nbz = nitrobenzoate anionic ligands).
V = 30 mL. After several minutes, 3-nitrobenzoic acid (336 mg,
2 mmol) was added to the dark blue solution. The reaction mixture
was stirred for a few days at ambient temperature. The light blue
microcrystalline product which precipitated was filtered off
(350 mg, 63%), and mother liquid was left to crystallize at ambient
temperature. The dark blue crystals suitable for X-ray structure
determination were separated after several days and dried at
ambient temperature. Anal. Calc. for [Cu(3-O2Nbz)2(nia)(H2O)2]
(1): Cu, 11.47; C, 43.37; H, 3.28; N, 10.11. Found: Cu, 11.82; C,
42.86; H, 3.18; N, 9.70%.
2. Experimental
2.1. Materials and methods
The complex (2) was prepared by the same procedure as (1)
replacing 3-nitrobenzoic acid by 4-nitrobenzoic acid. Nicotinamide
(244 mg, 2.0 mmol) was added under stirring to aqueous solution
of copper(II) acetate (200 mg, 1 mmol) in V = 30 mL. After several
minutes, 4-nitrobenzoic acid (336 mg, 2 mmol) was added to the
dark blue solution. The reaction mixture was stirred for a few days
at ambient temperature. The light blue microcrystalline product
which precipitated was filtered off (450 mg, 66%), and mother li-
quid was left to crystallize at ambient temperature. The dark blue
crystals suitable for X-ray structure determination were separated
after several days and dried at ambient temperature. Anal. Calc. for
[Cu(4-O2Nbz)2(nia)2(H2O)2] (2): Cu, 9.40; C, 46.19; H, 3.58; N,
12.43. Found: Cu, 9.35; C, 46.94; H, 3.62; N, 12.36%.
The complex (3) was prepared in similar manner as complex (2)
by changing Cu(II):nia molar ratio in water or in acetonitrile solu-
tions. Nicotinamide (122 mg, 1.0 mmol) was added under stirring
to aqueous solution of copper(II) acetate (200 mg, 1 mmol) in
V = 30 mL. After several minutes, 4-nitrobenzoic acid (336 mg,
2 mmol) was added to the resulting dark blue solution. The reac-
tion mixture was stirred for a few days at ambient temperature.
The light purple–blue microcrystalline product which precipitated
was filtered off (150 mg, 56%), and mother liquid was left to crys-
tallize at ambient temperature. The dark blue crystals suitable for
X-ray structure determination were separated after several days
and dried at ambient temperature. Anal. Calc. for [Cu(4-
O2Nbz)2(nia)2]ꢀ(4-O2NbzH)2 (3): Cu, 6.52; C, 49.31; H, 3.10; N,
11.50. Found: Cu, 6.75; C, 48.97; H, 2.92; N, 11.57%.
All used chemicals were of analytical grade (Aldrich, or Sigma)
and were used without further purification. Carbon, hydrogen
and nitrogen analyses were carried out on a CHNSO FlashEATM
1112 Automatic Elemental Analyzer. The copper content was
determined by electrolysis of water solution obtained by the sam-
ple mineralization with a mixture of sulfuric acid and potassium
peroxodisulfate.
The electronic spectra (190–1100 nm) of the complexes were
measured in nujol suspension with a SPECORD 200 (Carl Zeiss Jena)
spectrophotometer at room temperature. The infrared spectra
(4000–100 cmꢁ1) were recorded on a MAGNA 750 IR (Nicolet) spec-
trophotometer at room temperature. EPR spectra of the powdered
samples were recorded with a spectrometer Brucker ESP 300 oper-
ating at X-band equipped with an ER 035 Bruker NMR gaussmeter
and HP 5350B Helwett Packard microwave frequency counter.
Magnetisation measurements in the temperature range of 1.8–
300 K were carried out on powdered samples of complexes, at the
magnetic field 0.5 T, using a Quantum Design SQUID Magnetome-
ter (type MPMS-XL5). Corrections for diamagnetism of the consti-
tuting atoms were calculated using the Pascal constants [5], the
value of 60 ꢂ 10ꢁ6 cm3 molꢁ1 was used as the temperature-inde-
pendent paramagnetism of copper (II) ion. The best exchange
parameters were obtained by fitting with a good agreement factor
R defined:
2
n
X
ð
vei xpT ꢁ vci alcTÞ
R ¼
2
ð
vei xpTÞ
i¼1
2.3. X-ray crystallography
2.2. Syntheses of [Cu(3-O2Nbz)2(nia)(H2O)2] (1), [Cu(4-O2Nbz)2-
(nia)2(H2O)2] (2) and [Cu(4-O2Nbz)2(nia)2]ꢀ(4-O2NbzH)2 (3)
Data collection and cell refinement were carried out using a
axis diffractometers Kuma KM-4 CCD or Oxford Diffraction Gemini
R CCD with graphite monochromated Mo K radiation, using CRY-
SALIS software package [6]. The diffraction intensities were cor-
j-
a
Nicotinamide (244 mg, 2.0 mmol) was added under stirring to
aqueous solution of copper(II) acetate (200 mg, 1 mmol) in
Table 1
Crystallographic data for compounds 1, 2 and 3.
1
2
3
Code
Chemical formula
Mr
wal10/ZV62o
C20H18CuN4O11
553.93
wal11/ZV6
C26H24CuN6O12
676.06
exp275/ZV5o
C26H20CuN6O10ꢀ2(C7H5NO4)
974.27
Cell setting, space group
triclinic, P1
100(2)
triclinic, P1
100(2)
triclinic, P1
293(2)
Temperature (K)
a (Å)
b (Å)
c (Å)
6.461(2)
13.257(3)
13.854(3)
76.44(3)
77.17(3)
78.90(3)
1112.3(5)
2, 1.654
7.703(2)
9.866(3)
9.871(3)
77.15 (3)
73.59(3)
88.34(3)
701.1(4)
1, 1.601
7.475(2)
8.995(2)
15.718(3)
74.90(3)
89.61(3)
76.25(3)
989.4(4)
1, 1.635
a
(°)
b (°)
c
(°)
V (Å3)
Z, Dx (Mg mꢁ3
)
Radiation type
Mo K
1.05
a
Mo K
0.86
a
Mo K
0.65
a
l
(mmꢁ1
)
Crystal size (mm)
Number of measured, independent and observed reflections
0.40 ꢂ 0.35 ꢂ 0.25
21482, 8933, 7600
0.019, 35.0
0.030, 0.081, 1.07
0.57, ꢁ0.41
0.40 ꢂ 0.40 ꢂ 0.12
11486, 3910, 3540
0.019, 30.0
0.028, 0.076, 1.06
0.45, ꢁ0.24
0.67 ꢂ 0.17 ꢂ 0.11
30638, 4717, 3926
0.020, 28.0
0.029, 0.072, 1.00
0.23, ꢁ0.34
Rint, hmax (°)
R[F2 > 2
D
r
(F2)], wR(F2), S
qmax, D )
qmin (e Åꢁ3

156
Z. Vasková et al. / Polyhedron 29 (2010) 154–163
Table 2
rected for Lorentz and polarization factors. The structures were
solved by direct methods using SIR-97 [7] or SHELXS-97 [8] and re-
fined by the full-matrix least-squares procedure with SHELXL-97
[8]. The analytical absorption correction was made by using CRY-
SALIS-RED [6]. Geometrical analyses were performed with SHELXL-97.
The structures were drawn with ORTEP-3 for Windows [9] and XP
in SHELXTL [8]. Crystal data and conditions of data collection and
refinement are reported in Table 1.
0
Selected bond lengths (ÅA) and angles (°) for (1).
Cu1–O12
Cu1–O1W
Cu1–O13
O13–Cu1–N1
O1W–Cu1–O13
O12–Cu1–N1
O1W–Cu1–N1
O12–Cu1–O2W
1.951(1)
1.972(1)
1.973(1)
89.39(4)
88.99(4)
92.60(4)
175.79(3)
87.71(5)
Cu1–N1
Cu1–O2W
2.016(2)
2.246(1)
88.95(4)
177.72(3)
89.38(4)
93.23(5)
94.59(4)
O12–Cu1–O1W
O12–Cu1–O13
O1W–Cu1–O2W
O13–Cu1–O2W
N1–Cu1–O2W
3. Results and discussion
3.1. [Cu(3-O2Nbz)2(nia)(H2O)2] complex
There are some other copper(II) carboxylato-complexes where
hydrogen bonding synthons and formation of supramolecular di-
meric units were observed [12]. Those are two supramolecular
polymorphs of coordination polymers of copper(II) 5-nitroisophta-
The principal structural feature and atom numbering scheme of
complex (1) are depicted in Fig. 1 and selected bond distances and
angles are listed in Table 2. The independent part of structure (1)
consists of [Cu(3-O2Nbz)2(nia)(H2O)2] molecules with elongated
tetragonal-pyramidal coordination polyhedron around Cu atom.
The coordination sphere of copper(II) atom in trans-basal plane
is formed by two 3-nitrobenzoate anions bonded via O12 and O13
atoms (Cu–O12 distance is of 1.951(1) Å and Cu–O13 is of
1.973(1) Å), one nia molecule bonded via N1 atom (Cu–N1 distance
is of 2.016(1) Å) and one water molecule bonded via O1W atom
(Cu–O1W distance is of 1.972(1) Å). Another water molecule is
bonded in an apical position of square-pyramidal polyhedron via
O2W donor atom with slightly longer bond distance Cu–O2W of
2.246(2) Å.
late [12] and heterometallic tetranuclear complex [Cu2L1
-
2
(py)2(DMF)2(H2O)2], where H2L1 = 1,10-ferrocenedicarboxylic acid
[13]. Additional stacking interactions between anion aro-
pꢀ ꢀ ꢀp
matic rings stabilize the supramolecular structure. The other
hydrogen bonds (Fig. 2 and Table 3) are formed by the axial water
molecules of supramolecular dimers to form infinite 1D linear
chain which is parallel to
a
axis (O2Wꢀ ꢀ ꢀO23ii distance is
2.714(2) Å). In addition, there is a network of weak hydrogen
bonds that stabilizes 1D chain structure (with N4ꢀ ꢀ ꢀO2Wv and
N4ꢀ ꢀ ꢀO12v distances of 3.014(2) and 2.986(2) Å, respectively). The
chains of supramolecular dimers are interconnected (Fig. 2) via
amidic group head-to-head R22(8) hydrogen bonds to complete
the 2D layer which is parallel with ab-plane formation (N4ꢀ ꢀ ꢀO1iv
distance is 2.906(2) Å). The strongest interlayer hydrogen bond
contacts are formed by axial water molecule hydrogen atoms and
the O43 nitro group oxygen atoms (O2Wꢀ ꢀ ꢀO43iii distance
2.848(2) Å).
The tetragonality parameter value [10] is about 0.03 indicating
nearly ideal square-pyramidal coordination sphere. The basal
planes of the neighboring complex molecules are coplanar and
the distance between two basal planes is 3.457(3) Å. Moreover,
the water molecules in basal position O1W form strong hydrogen
bonds (Fig. 2 and Table 3) with pairs of carboxylate oxygen atoms
O22 and O23 (O1Wꢀ ꢀ ꢀO22i and O1Wꢀ ꢀ ꢀO23i distances are 2.692(2)
and 2.724(2) Å, respectively) which causes that the Cuꢀ ꢀ ꢀCui dis-
tance of neighboring copper atoms is 4.886(2) Å.
The similarity of supramolecular structure formation between
(1) and the [Cu(5-MeOsal)2(l-nia)(H2O)]2 complex [4] has its con-
sequence in magnetic properties of the complex under study. Mag-
netization measurements showed that sample of complex (1)
exhibits similar maximum of magnetic susceptibility at 6.5 K
(Fig. 3).
These pairs of supramolecular synthons [11] R22(10) and
R22(12) form supramolecular dimers from complex molecules (1)
which are very similar to those found in complex [Cu(5-MeO-
The carboxylate bridges extended with O–Hꢀ ꢀ ꢀO hydrogen bond
sal)2(l-nia)(H2O)]2 [4] (Scheme 1).
(Fig. 2) are the pathway of the unpaired spin antiferromagnetic
Fig. 1. Perspective view of [Cu(3-O2Nbz)2(nia)(H2O)2] (1), with the atom numbering scheme. Thermal ellipsoids are drawn at the 30% probability level.

Z. Vasková et al. / Polyhedron 29 (2010) 154–163
157
0.05
0.04
0.03
0.02
0.5
0.4
0.3
0.2
0.1
0.0
χ
χ
0.01
0.00
0
50
100 150 200 250 300
T(K)
Fig. 3. Temperature dependence of vm (d) and
vmT (s) of (1). The solid line
represents the calculated curve, according to Bleaney-Bowers equation involving
the molecular field corrections, with the parameters: g = 2.24, 2J = ꢁ6.26 cmꢁ1
,
zJ0 = ꢁ0.03 cmꢁ1, R = 1.4 ꢂ 10ꢁ4; Tmax = 6.5 K.
interactions, giving magnetic phase transitions at the susceptibility
maximum temperature. It is known from experiment and DFT cal-
culations [14] that CuII compounds where copper ions are con-
nected by H-bonding through coplanar equatorial ligands have
negative J values characteristic of antiferromagnetic coupling. In
contrast when an equatorial ligand is hydrogen bonded to an axial
ligand – compound has been found practically uncoupled (J = +0.3
and ꢁ0.02 cmꢁ1).
Logically the magnetic data of the complex (1) were fit into a
susceptibility equation derived from spin Hamiltonian H = ꢁ2JS1S2,
with Bleaney-Bowers equation [15] to characterize interaction (2J)
within the supramolecular dimer:
ꢂ
ꢀ
ꢁꢃꢁ1
Nb2g2
3kT
1
1 þ exp
3
ꢁ2J
vm
¼
kT
Taking into account above mentioned supramolecular structure
and other but weaker and for magnetic interactions less relevant
H-bonds [14], a molecular field model [16] was added to describe
interdimer interactions:
Fig. 2. ‘‘Supramolecular dimers” formation (above) and their linking into 1D chain
parallel to a axes (below) in crystal structure of (1).
vm
1 ꢁ
vcmorr
¼
2zJ’vm
Nb2g2
Table 3
Selected hydrogen bonds parameters of (1) (Å, °).
where vm is the molar susceptibility given by Bleaney-Bowers
equation, zJ0 characterizes interdimer interactions, and z is a num-
ber of the nearest neighbors. Other symbols have their usual mean-
ing: N is the Avogadro number, g is the spectroscopic splitting
factor, b is the Bohr magneton, and k is the Boltzmann constant.
The best fit parameters were obtained with a good agreement factor
R = 1.40 ꢂ 10ꢁ4 (Fig. 3). Both obtained best fit parameters, the sin-
D–Hꢀ ꢀ ꢀA
D–H
Hꢀ ꢀ ꢀA
Dꢀ ꢀ ꢀA
D–Hꢀ ꢀ ꢀA
O1W–H2Wꢀ ꢀ ꢀO22i
O1W–H1Wꢀ ꢀ ꢀO23i
O2W–H3Wꢀ ꢀ ꢀO23ii
O2W–H4Wꢀ ꢀ ꢀO43iii
N4–H4Aꢀ ꢀ ꢀO1iv
0.84
0.84
0.84
0.84
0.88
0.88
0.88
1.86
1.90
1.90
2.02
2.03
2.33
2.49
2.692(2)
2.724(2)
2.714(2)
2.848(2)
2.906(2)
3.014(2)
2.986(2)
171
166
163
167
174
135
116
N4–H4Bꢀ ꢀ ꢀO2Wv
N4–H4Bꢀ ꢀ ꢀO12v
glet–triplet energy gap 2J = ꢁ6.25 cmꢁ1, as well as zJ0 = ꢁ0.03 cmꢁ1
,
are similar to the values obtained for the [Cu(5-MeOsal)2(l-nia)-
Symmetry codes: (i) ꢁx, ꢁy + 1, ꢁz + 1; (ii) x + 1, y, z; (iii) ꢁx + 1, ꢁy + 1, ꢁz; (iv)
ꢁxꢁ1, ꢁy, ꢁz + 1; (v) xꢁ1, y, z.
(H2O)]2 complex (2J = ꢁ6.83 cmꢁ1 and zJ0 = ꢁ0.28 cmꢁ1 [4]).
Obtained small value zJ0 = ꢁ0.03 cmꢁ1 for complex (1) confirms that
interdimer magnetic interactions are very weak. It should be
stressed that the main difference between complex (1) and the
[Cu(5-MeOsal)2(l-nia)(H2O)]2 complex is that the supramolecular
O
O
O
O
O
H
structure of (1) is built up from monomeric complex species. Thus
H
H
H
O
Cu
O
the difference in the zJ0 values could be explained by existence of li-
Cu
O
O
Cu
O
N
O
H
O
N
gand bridge in the case of [Cu(5-MeOsal)2(l-nia)(H2O)]2 complex,
H
H
whereas in the complex (1) there are only additional weaker hydro-
gen bonds and van der Waals interactions between nearest neigh-
boring supramolecular dimers as building blocks.
The polycrystalline EPR spectra of (1) measured at different
temperatures are in agreement with the dimeric character of (1)
that was derived from magnetic susceptibility measurements. Both
spectra (at room and at liquid nitrogen temperature – see Fig. 4)
O
H
R22(8)
R22(10)
R22(12)
Scheme 1. Supramolecular dimers synthons (R22(10) and R22(12)), and amidic
head-to-head R22(8) synthon.

158
Z. Vasková et al. / Polyhedron 29 (2010) 154–163
The quantitative method of relative intensities of half-field EPR
transitions [19] was used for evaluation of spin–spin distance. Dis-
tance between Cu(II) ions can be obtained using Eaton‘s dipolar
expression:
areað
D
D
Ms ¼ 2Þ
A
IðrelativeÞ ¼
¼
areað
Ms ¼ 1Þ r6
where r is an interspin distance and A is a constant dependent on
the g values of the two molecules and the microwave frequency
of the experiment. Constant A can be computed from formula:
ꢀ
ꢁ
2
9:100
t
A ¼ ð19:5 þ 10:9
D
gÞ
where
D
g = |gmaxꢁgmin| and
t
stands for spectrometer frequency
[20]. In our case A equals 20.14 Å6, the relative intensity is
1.65 ꢂ 10ꢁ3, and calculated Cuꢀ ꢀ ꢀCu distance r = 4.8 Å, which is in
excellent agreement with the value obtained from crystal structure
4.886(2) Å. Similar results were obtained from EPR spectra for some
carboxylato-copper(II) complexes containing chelating nitrogen do-
nor ligands [21] (bipyridine, phenanthroline or bis(2-pyri-
dyl)amine). Using those chelating ligands might be the reason
why, even at 4.2 K, the observed EPR spectra revealed overlapped
signals of both monomeric and dimeric species.
Fig. 4. EPR spectrum of (1) complex at 77 K (inset is half-field transition with
increased gain).
exhibit broad features, characteristic of a triplet spin state with
non-negligible zero-field splitting due to the coupling of the two
S = ½ spins within the dinuclear unit.
Lowering the temperature leaves the pattern of EPR spectra
practically unchanged. In addition to that, another confirmation
of dimeric nature of EPR spectra comes from the observation of for-
In the IR spectrum of the complex (1), the bands corresponding
to
m m
as(COOꢁ) and s(COOꢁ) are present at 1610 cmꢁ1 and
1392 cmꢁ1, respectively and the
D
m
value (D
m
=
mas m )
s = 218 cmꢁ1
ꢁ
bidden half-field (DMS = 2) transitions at about 1500 Gauss which
arises as a result of anisotropic dipole- dipole interaction. The
intensities of both EPR signals of (1) at room and at liquid nitrogen
temperature are very similar, and also markedly lower when ob-
served at 4.2 K due to the depopulation of triplet state (antiferro-
magnetic interaction). At 4.2 K temperature, the triplet state
signal is still observed together with presence of half-field transi-
tion (Fig. 5). Triplet EPR spectra can be analyzed within spin Ham-
iltonian formalism in the form:
corresponds to unidentate mode of coordination [22]. The band
positions at 3525, 3357, 3280 and 3158 cmꢁ1 were attributed to
O–H and/or N–H vibrations of water or nicotinamide. The hydro-
gen bond vibration are positioned between 2530 and 2910 cmꢁ1
.
The vibration of the C@N within pyridine ring is shifted from
1596 to 1606 cmꢁ1, thus confirming its coordination to Cu(II).
The electronic spectrum of (1) shows a broad asymmetrical band
at about 670 nm, typical of tetragonal-pyramidal copper(II), and
is consistent with an essentially dx2
orbital ground state.
ꢁy2
1
3
2
^
2
^
2
^
^
^
^
^
H ¼ gjjbHzSz þ g?bðHxSx þ HySyÞ þ D½Sz ꢁ SðS þ 1Þꢃ þ EðSx ꢁ SyÞ
Finally, it should be stressed that complex (1) exhibits unusual
composition within the group of carboxylatocopper(II) complexes
with nicotinamide and it was obtained in all our experiments done
with different stoichiometric ratios of copper(II) salt to nicotin-
amide. We believe that its specific structure – presence of supra-
molecular dimers – is the reason, why we were not able to
prepare complexes [Cu(3-O2Nbz)2(nia)2(H2O)2] or [Cu(3-O2Nbz)2
(nia)2].
where the symbols have their usual meaning [17]. The spectra were
simulated using SimFonia program developed by Bruker [18]. Be-
cause SimFonia does not permit calculation of the position and
the intensity of half-field transition, only allowed EPR transitions
are simulated. The simulation of the most resolved EPR spectrum
of (1) measured at 4.2 K gave the best results for spectroscopic split-
ting factors g\ = 2.070 and g|| = 2.220, zero-field splitting parameter
D = 0.0282 cmꢁ1 and the rhombic parameter E = 0.0028 cmꢁ1
(Fig. 5).
3.2. [Cu(4-O2Nbz)2(nia)2(H2O)2] and [Cu(4-O2Nbz)2(nia)2]ꢀ
(4-O2NbzH)2 complexes
The structure of (2) consists of centrosymmetric molecules
(Fig. 6) with elongated tetragonal-bipyramidal coordination poly-
hedron around Cu atom (Table 1S).
The coordination sphere of copper(II) atom in trans-basal plane
is formed by two 4-nitrobenzoate anions bonded via O2 atoms
(Cu1–O2 distance is of 2.005(2) Å), by two nicotinamide molecules
bonded via N1 atoms (Cu1–N1 distance is of 2.006(2) Å) and by
two water molecules (Cu1–O1W distance is of 2.409(2) Å) in apical
positions. The water molecules are slightly shifted from precise
apical positions due to intramolecular and intermolecular hydro-
gen bonds (O1W–H1Wꢀ ꢀ ꢀO3 with O1Wꢀ ꢀ ꢀO3 distance of
2.724(2) Å and O1W–H2Wꢀ ꢀ ꢀO1vi with O1Wꢀ ꢀ ꢀO1vi distance of
2.899(2) Å, respectively), and the O2–Cu1–O1W and N1—Cu1—
O1W angles are 95.48(4) and 93.49(5)°, respectively. The mole-
cules of (2) are linked to adjacent molecules by hydrogen bonds
(Table 4) of nicotinamide carboxamide groups (R22(8) head-to-
head synthons
–
Scheme 1) with N3ꢀ ꢀ ꢀO1vii distance of
Fig. 5. EPR spectrum of (1) complex at 4.2 K (inset is half-field transition with
increased gain).
2.868(2) Å in 1D chains (Fig. 7). The chains are additional

Z. Vasková et al. / Polyhedron 29 (2010) 154–163
159
Fig. 6. Perspective view of [Cu(4-O2Nbz)2(nia)2(H2O)2] (2), with the atom numbering scheme. Thermal ellipsoids are drawn at the 30% probability level.
should be noted that the last three complexes of this sequence
(2-nitrobenzoate [26], 3-hydroxybenzoate [27] and (2)) form sim-
ilar system of hydrogen bonds as is shown in Fig. 7.
Table 4
Selected hydrogen bonds parameters of (2) (Å, °).
D–Hꢀ ꢀ ꢀA
D–H
Hꢀ ꢀ ꢀA
Dꢀ ꢀ ꢀA
D–Hꢀ ꢀ ꢀA
The
leff = 1.98 B.M.) at room temperature and practically does not
v
mT value of the complex (2) is equal to 0.488 cm3 K molꢁ1
O1W–H1Wꢀ ꢀ ꢀO3
O1W–H2Wꢀ ꢀ ꢀO1vi
N3–H3Aꢀ ꢀ ꢀO1vii
N3–H3Bꢀ ꢀ ꢀO3iii
0.84
0.84
0.88
0.88
1.90
2.10
2.02
2.16
2.724(2)
2.899(2)
2.868(2)
2.886(2)
167
158
162
140
(
change with lowering of the temperature up to 5 K. Only
small change is observed at the lowest temperatures to
0.435 cm3 K molꢁ1
(leff = 1.87 B.M.) at 1.8 K (Fig. 3S). The inverse
Symmetry codes: (iii) ꢁx + 1, ꢁy + 1, ꢁz; (vi) x + 1, y, z; (vii) ꢁx, ꢁy + 1, ꢁz.
magnetic susceptibility vs temperature of complex (2) is in good
agreement with the Curie-Weiss law,
v
= C/(Tꢁh) [16] and gives
C = 0.487 cm3 K molꢁ1, and h = ꢁ1 K. In an order to obtain value
of the magnetic exchange parameters, the present magnetic data
of complex (2) have been fitted using magnetic centers (S = 1/2)
into the molecular field model of magnetic interactions [16].
intermolecular hydrogen bonds (O1Wꢀ ꢀ ꢀO1vi and N3ꢀ ꢀ ꢀO3iii dis-
tances are of 2.724(2) and 2.886(2) Å, respectively) into 2D puck-
ered sheets parallel to ac-plane (Fig. 7).
vm
1 ꢁ
vcmorr
¼
The structure of (2) can be compared to other complexes of gen-
eral formula [Cu(RCOO)2(nia)2(H2O)2] [23–27]. The average Cu–Oax
distance (Cu–Oax where Oax stands for water oxygen) in complex
(2) is found as the shortest within the group of those complexes
with known structure (2.626(3) Å for RCOO = formate [23],
2.575(2) Å for RCOO = 4-hydroxybenzoate [24], 2.373(3) or
2.894(4) Å for RCOO = salicylate [25], 2.466(2) Å for RCOO = 2-
nitrobenzoate [26], 2.569(2) Å for RCOO = 3-hydroxybenzoate
[27]). On the other hand the average Cu–Oeq distance (Cu–Oeq
where Oeq is the oxygen of carboxylate group) in complex (2) is
greater than these distances for majority of other known com-
plexes (1.981(2) Å for RCOO = formate [23], 1.956(2) Å for
RCOO = 4-hydroxybenzoate [24], 1.966(2) or 1.976(2) Å for
RCOO = salicylate [25], 1.971(1) Å for RCOO = 3-hydroxybenzoate
[27], 2.005(1) Å for RCOO = 2-nitrobenzoate [26]). Moreover the
Cu–Neq (where Neq is the nitrogen of nicotinamide pyridine ring)
distance is greater than those found for the other complexes. This
means that the complex (2) shows the smallest axial distortion of
2zJ’vm
Nb2g2
Nb2g2
3kT
vm
¼
SðS þ 1Þ
where zJ0 is the molecular exchange parameter, and z is the number
of the nearest neighbors. Spectroscopic splitting factor g was ob-
tained in the fitting process. Satisfactory least-squares fit to the
experimental results (solid lines in Fig. 3S) was obtained for
zJ0 = ꢁ0.34 cmꢁ1, and g = 2.26, with agreement factor R = 2.3 ꢂ
10ꢁ4. Small value of zJ0 parameter is in good agreement with the
monomeric structure of complex (2) and with the shortest Cuꢀ ꢀ ꢀCu
distance of 7.703(3) Å (Fig. 7). However, presence of antiferromag-
netic interaction could be taken as a confirmation that
p p stack-
ꢀ ꢀ ꢀ
ing interactions offer a route for the transmission of magnetic
effects (Fig. 4S). Similar properties, albeit with slightly smaller
zJ0 values, were observed for complexes [Cu(2-NO2bz)2(nia)2-
(H2O)2] (zJ0 = ꢁ0.06 cmꢁ1
)
[26], [Cu(2-NO2Bz)2(denia)2(H2O)2]
and [Cu(2-NO2bz)2(3-pyme)2(H2O)2]
octahedron from this group. The value of
s parameter [28] (s = RS/
(zJ0 = +0.05 cmꢁ1
)
[29]
RL) indicating the degree of tetragonally elongated octahedron in-
creases in the order 0.758 (salicylate) [25] < 0.762 (formate)
[23] < 0.770 (4-hydroxybenzoate) [24] < 0.775 (3-hydroxybenzo-
ate) [27] < 0.813 (2-nitrobenzoate) [26] < 0.818 for complex (2). It
(zJ0 = ꢁ0.08 cmꢁ1) [30].
The EPR spectrum of complex (2) is of an axial type with hyper-
fine splitting in parallel area that was detected both at room and

160
Z. Vasková et al. / Polyhedron 29 (2010) 154–163
Fig. 7. Hydrogen bonds through amide-amide R22(8) supramolecular synthons and by R42(8) synthons forming 2D sheets of molecules (2) parallel to ac-plane (4-O2Nbz
groups are omitted for clarity).
even at liquid nitrogen temperature. The spectrum was interpreted
using spin Hamiltonian in the form:
tance is of 2.480(1) Å) that are shifted due to asymmetric chelate
bonding mode from precise apical positions giving the O2–Cu1–
O1 angle of 57.95(4)°. The Cu1–N1 bond is nearly perpendicular
to O1–Cu1–O2 plane.
^
^
^
^
^ ^
H ¼ g bHzSz þ g bðHxSx þ HySyÞ þ A SzIz
jj
jj
?
where the symbols have again their usual meanings which can be
found in literature [20]. The simulation of spectrum at 77 K gave
the best fit with the spectroscopic splitting factors g\ = 2.062 and
Another unique feature of the structure of (3) is the presence of
the 4-nitrobenzoic acid molecules bonded to the core of the com-
plex by strong hydrogen bonds (Table 5) to form amide-carboxylic
R22(8) supramolecular synthons (Fig. 9). The amidic H2N atom of
nicotinamide is bonded to O6 of carboxylate oxygen (N2–
H2Nꢀ ꢀ ꢀO6 with N2ꢀ ꢀ ꢀO6 distance of 3.030(2) Å) and vice-versa the
carboxylic H7O hydrogen atom forms hydrogen bond with carbox-
amide O3 oxygen atom (O7–H7Oꢀ ꢀ ꢀO3 with O7ꢀ ꢀ ꢀO3 distance of
2.568(2) Å). The coordinated nicotinamide molecules of neighbor-
ing complex molecules are coplanar (plane-to-plane distance is of
g|| = 2.285, and
a well resolved parallel hyperfine splitting
A|| = 160 Gauss resulting from the interaction of the unpaired elec-
tron of copper center with the spin of copper nucleus I = 3/2
(Fig. 5S). Hyperfine splitting of powdered samples of CuII complexes
is observed when copper centers are magnetically isolated in the
crystal lattice [20,31]. The value of gav = 2.136 obtained from the
above splitting factors values is somewhat lower than the g param-
eter obtained from magnetic measurements. It could be attributed
to the fact that g values obtained from magnetic measurements
are only refined isotropically [32]. The value of parameter G = 4.59
(G = (g||ꢁ2)/(g\ꢁ2)) [33] is slightly above the value of G = 4 and sug-
gests that the local tetragonal axes are probably only slightly misa-
ligned [33]. The electronic and IR spectra of the complex (2) are
fully consistent with structural data (Table 2S).
In conclusion of this part it should be pointed out that the inter-
esting properties of complex (1) led us to a great effort in finding
the experimental conditions for preparation of similar product
which would contain the 4-nitrobenzoate anion. In spite of the
large number of different experiments (with different stoichiome-
try of reactants, using different solvents, etc) only the complex (3)
has been obtained (for details see Section 2).
3.619 Å) and the
p p stacking interactions can occur due to their
ꢀ ꢀ ꢀ
overlapped positions (Fig. 9). The hydrogen bonded 4-nitrobenzoic
acid molecules are nearly coplanar to nicotinamide ones thus
forming another
and 2D net is formed (Fig. 9).
p p stacking contacts in perpendicular direction
ꢀ ꢀ ꢀ
The core of the structure of 3 could be compared with the struc-
tures of other [Cu(RCOO)2(nia)2] complexes: aspirinate [34], two
salicylate polymorphs (orthorhombic Pbca form) [35] and (mono-
clinic P21/n form) [36], clofibrate [37], 2-chloro-6-fluorobenzoate
[38], 2-bromobenzoate [41], benzoate [39], valerate [40], vanilliate
[42]. The Cu–Oeq, Cu–Neq and Cu–Oax distances (where eq stands
for donor atom in equatorial position and ax stands for semicoor-
dinated oxygen in quasi apical position) are found for complex
(3) within the group of those complexes with known structure
The main features of the structure of complex (3) are depicted
in Fig. 8. The structure consists of centrosymmetric molecules with
highly distorted tetragonal-bipyramidal coordination polyhedron
around Cu atom (Table 1S).
The basal plane of the coordination sphere of copper(II) atom is
formed by two 4-nitrobenzoate anions bonded via O1 atoms (Cu1–
O1 distance is of 1.990(1) Å), two nia molecules bonded via N1
atoms (Cu1–N1 distance is of 2.012(1) Å). The apical positions
are occupied by two carboxylate oxygen atoms O2 (Cu1–O2 dis-
[31–42]. According to the value of s parameter[28] (s = RS/RL) –
indicating the degree of tetragonally elongated octahedron – which
increases in the order 0.630 (2-chloro-6-fluorobenzoate)
[38] < 0.673 (2-bromobenzoate) [41] < 0.714 (salicylate P21/n
form) [36] < 0.753 (salicylate Pbca form) [35] < 0.763 (clofibrate)
[37] 0.768 (vanilliate) [42] < 0.794 (valerate) [40] < 0.799 (aspiri-
nate) [34] < 0.807 for complex (3) < 0.948 (benzoate) [39], the
complex (3) belongs to the subgroup of low tetragonal elongation.
It should be noted that very similar intermolecular hydrogen bonds

Z. Vasková et al. / Polyhedron 29 (2010) 154–163
161
Fig. 8. Perspective view of [Cu(4-O2Nbz)2(nia)2]?2(4-O2NbzH) (3), with the atom numbering scheme. Thermal ellipsoids are drawn at the 30% probability level.
– Table 5 – which are responsible for formation of vertical chains
– See Fig. 9) were found only in complexes with clofibrate [37],
vanilliate [42] or valerate [40] anions.
Table 5
Selected hydrogen bonds parameters of (3) (Å, °).
D–Hꢀ ꢀ ꢀA
D–H
Hꢀ ꢀ ꢀA
2.26
2.22
1.75
Dꢀ ꢀ ꢀA
3.060(2)
3.030(2)
2.568(2)
D–Hꢀ ꢀ ꢀA
156
157
171
The magnetic properties of complex (3) are very similar to com-
N2–H2Nꢀ ꢀ ꢀO6
N2–H1Nꢀ ꢀ ꢀO2viii
O7–H7Oꢀ ꢀ ꢀO3
0.86
0.86
0.82
plex (2). At room temperature, the
0.422 cm3 K molꢁ1
eff = 1.84 B.M.). It does not change with
decreasing the temperature to 5 K, and at 1.8 K is observed mT va-
lue of 0.399 cm3 K molꢁ1
eff = 1.79 B.M.). Similarly, temperature
vmT value is equal to
(l
v
Symmetry codes: (viii): ꢁx + 1, ꢁy, ꢁz + 1.
(l
dependence of the inverse susceptibility obeys the Curie-Weiss
law [16] and gives C = 0.422 cm3 K molꢁ1, and h = ꢁ0.03 K. The
data (3) have been fit similarly as for (2) using magnetic centers
(formed in (3) between H1N hydrogen atom of nicotinamide and
O2viii carboxylate oxygen atom of neighboring complex molecule
Fig. 9. Hydrogen bonds through amide-carboxylic R22(8) supramolecular synthons and
direction) and between pyridine rings of nicotinamide molecules (vertical direction) in crystal structure of (3) (4-O2Nbz groups of the anions are omitted for clarity).
pꢀ ꢀ ꢀp stacking interactions between aromatic rings of 4-nitrobenzoic acids (horizontal

162
Z. Vasková et al. / Polyhedron 29 (2010) 154–163
Table 6
Selected geometric data of (2) and (3) in relation to other complexes.
Complex
zJ0
Cuꢀ ꢀ ꢀCu
P/Ca
Reference
(2)
(3)
ꢁ0.34
ꢁ0.16
ꢁ0.08
ꢁ0.06
+0.05
9.871
7.475
7.605
7.872
9.403
3.319/3.923
This work
This work
[27]
[25]
[26]
3.619/3.935b 3.357/3.831c
3.516/3.745 3.552/4.903d
3.674/4.539
[Cu(2-NO2bz)2(3-pyme)2(H2O)2]f
[Cu(2-NO2bz)2(nia)2(H2O)2]
[Cu(2-NO2bz)2(denia)2(H2O)2]g
N/Ae
a
Plane-to-Plane/Centroid-to-Centroid distances.
For nicotinamide molecules.
For 4-O2NbzH molecules.
b
c
d
There are two non-equivalent
p
ꢀ ꢀ ꢀ
ꢀ ꢀ ꢀ
p
stacking interactions.
e
f
Not applicable – there is no
p
p
stacking interaction in this structure.
3-pyme is 3-pyridylmethanol.
g
Denia is N,N-diethylnicotinamide.
(S = 1/2) in the molecular field model of magnetic interactions [16].
cerning the magnetic interaction pathway. Similar experiments
using the 4-nitrobenzoic acid instead of 3-nitrobenzoic acid gave
a classical molecular complex, so-called hydrate type, [Cu(4-
O2Nbz)2(nia)2(H2O)2] (2) or a complex of composition [Cu(4-
O2Nbz)2(nia)2]ꢀ(4-O2NbzH)2 (3).
Satisfactory least-squares fit to the experimental resultꢁs1(solid
lines in the Fig. 6S) was obtained for zJ0 = ꢁ0.16 cm
, and
g = 2.13 for R = 4.5 ꢂ 10ꢁ5
.
The EPR spectrum (Fig. 7S) of complex (3) is of an axial type
with unresolved hyperfine splitting in parallel area which was de-
tected both at room and even at liquid nitrogen temperature. The
simulation of spectrum at 77 K by SimFonia-WinEPR [18] program
gave the best fit with spectroscopic splitting factors g\ = 2.065 and
g|| = 2.235. The value of gav = 2.122 obtained from the above split-
ting factors values is in good agreement with the g parameter ob-
tained from magnetic measurements. The value of parameter
G = 3.61 is slightly below the conventional borderline and suggests
that the local tetragonal axes are probably only slightly misaligned
[33]. The structural conclusions are supported by IR and electronic
spectrum of (3) (Table 2S).
Acknowledgements
We thank Scientific Grant Agency of the Ministry of Education
of Slovak Republic and the Slovak Academy of Sciences (1/0353/
08 and 1/0575/08), the Research and Development Agency of the
Slovak Republic (No. APVT-20-004504), the Polish Ministry of Sci-
ence and Computerization Grant No 1T09A 12430 for financial
support. We also thank the Structural Funds, Interreg IIIA, for
financial support in purchasing the diffractometer.
By comparing complexes (2) and (3), it should be stressed that
the g values obtained from above fit (g = 2.26 for complex (2) and
g = 2.13 for complex (3), respectively) are similar to those obtained
from EPR. Somewhat lower value of magnetic moment of complex
(3) corresponds well with the degree of tetragonal distortion
Appendix A. Supplementary data
CCDC 695712, 695713 and 695714 contains the supplementary
crystallographic data for [Cu(3-O2Nbz)2(nia)(H2O)2] (1), [Cu(4-
O2Nbz)2(nia)2(H2O)2] (2) and [Cu(4-O2Nbz)2(nia)2]ꢀ(4-O2NbzH)2
(3). 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 doi:10.1016/j.poly.2009.06.056.
around the Cu(II) (s = 0.807) – lower than a value for the complex
(2) ( = 0.818) [26,43]. Greater tetragonal distortion leads to a low-
s
er magnetic moment, too. However, the difference in composition
of coordination sphere of complex (3) (presence of chelating car-
boxylate group) leads to difference of EPR spectrum of (3) from
the one of (2), inspite of the similarities in molecular structure with
well separated paramagnetic centers.
The magnitude of the antiferromagnetic exchꢁa1nge between ions
obtained from these calculations zJ0 = ꢁ0.16 cm for complex (3)
is smaller than for complex (2) (zJ0 = ꢁ0.34 cmꢁ1). However, the
data in Table 6 show that practically no relation could be found be-
tween Cuꢀ ꢀ ꢀCu distance and zJ0 parameter. On the other hand, quite
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dimer.
The
antiferromagnetic
intradimer
interaction
(2J = ꢁ6.25 cmꢁ1) leads to magnetic phase transition magnitude
at 6.5 K and has its consequence in EPR spectrum. All together, it
could be taken as the experimental proof of our suggestion con-
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