One-dimensional copper(II) compound with a double out-of-plane carboxylato-bridge - Another polymorphic form of Cu(pyridine-2-carboxylate)2

Article abstract of DOI:10.1016/j.poly.2006.10.019

The crystal and molecular structure of a blue-violet copper(II) compound of the formula Cu(2-pic)₂ (2-pic = 2-picolinate or pyridine-2-carboxylate ion) was determined. Its structure consists of a copper atom lying in the centre of symmetry, trans square-planar coordinated via two pyridine nitrogens and the carboxylate oxygen atoms of the two picolinate anions. The bonding interaction between the copper and the oxygen of carboxylate atoms of neighboring planes completes the square-planar coordination to a strongly tetragonally distorted octahedral stereochemistry (CuN₂ O₂ O₂^′ chromophore), when long axial Cu-O bond distances are included. This intermolecular interaction, with the Cu?Cu distance of 5.163 ?, is weak, but permits to define the structure as a one-dimensional chain with a double out-of-plane carboxylate bridge. The magnetic behavior of the compound can be explained as a result of magnetic exchange within the chain through long Cu-O axial bonds (J = -0.73 cm^-1) and a superexchange pathway through the hydrogen bond network between adjacent molecules (zJ′ = -0.06 cm^-1). The magnitude and the nature of the exchange coupling are explained on the basis of the structural data results.

Full text of DOI:10.1016/j.poly.2006.10.019

Polyhedron 26 (2007) 1251–1258
www.elsevier.com/locate/poly
One-dimensional copper(II) compound with a double out-of-plane
carboxylato-bridge – Another polymorphic form of
Cu(pyridine-2-carboxylate)2
_
*
´
Bogumiła Zurowska, Jerzy Mrozinski , Zbigniew Ciunik
Faculty of Chemistry, University of Wroclaw, F. Joliot-Curie 14, 50-383 Wroclaw, Poland
Received 6 June 2006; accepted 13 October 2006
Available online 19 October 2006
Abstract
The crystal and molecular structure of a blue-violet copper(II) compound of the formula Cu(2-pic)2 (2-pic = 2-picolinate or pyridine-
2-carboxylate ion) was determined. Its structure consists of a copper atom lying in the centre of symmetry, trans square-planar coordi-
nated via two pyridine nitrogens and the carboxylate oxygen atoms of the two picolinate anions. The bonding interaction between the
copper and the oxygen of carboxylate atoms of neighboring planes completes the square-planar coordination to a strongly tetragonally
distorted octahedral stereochemistry ðCuN2O2O0 chromophore), when long axial Cu–O bond distances are included. This intermolecular
2
˚
interaction, with the Cuꢀ ꢀ ꢀCu distance of 5.163 A, is weak, but permits to define the structure as a one-dimensional chain with a double
out-of-plane carboxylate bridge. The magnetic behavior of the compound can be explained as a result of magnetic exchange within the
chain through long Cu–O axial bonds (J = ꢁ0.73 cmꢁ1) and a superexchange pathway through the hydrogen bond network between
adjacent molecules (zJ0 = ꢁ0.06 cmꢁ1). The magnitude and the nature of the exchange coupling are explained on the basis of the struc-
tural data results.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Keywords: Copper(II); Pyridinecarboxylate; Structure; Magnetism
1. Introduction
The ligand used in this study, pyridine-2-carboxylate
(ligand abbreviated here as 2-pic), is particularly attractive
as it possesses a carboxylate group. In our recent publica-
tion [4a], the crystal structure and the spectroscopic, and
magnetic properties of a violet form of pyridine-2-carbox-
ylate of the formula Cu(2-pca)2 (ligand abbreviated there
as 2-pca) with a CuN2O2 chromophore were presented.
The Cu(2-pca)2 is an example of a square-planar copper(II)
compound in which structure is achieved by important p–p
stacking intermolecular interaction. It is interesting from
the point of view of synthesis that the violet form of
Cu(pyridine-2-carboxylate)2 was obtained in an oxidative
P-dealkylation/dephosphorylation reaction, which was
presented by us earlier [4a]. It is important to note that
low molecular weight coordination compounds are often
used to study correlations between structure, magnetism
and spectroscopy [4,5].
The structure and bond properties of copper(II) com-
pounds are of continuous interest in inorganic chemistry
[1]. Copper(II) carboxylates, especially those with nitrogen
donor ligands, have been the subject of numerous investi-
gations [2]. Some carboxylic acids and their derivatives also
play an important role in biological processes [3]. Different
coordination modes of carboxylate groups lead to the for-
mation of mononuclear and polynuclear structures [4]. In
recent years, great attention has been devoted to mag-
neto-structural studies of polynuclear complexes of Cu(II)
with carboxylate bridges which act as a superexchange
pathway [4].
*
Corresponding author. Tel./fax: +48 71 3757307.
´
E-mail address: jmroz@wchuwr.chem.uni.wroc.pl (J. Mrozinski).
0277-5387/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2006.10.019

_
B. Zurowska et al. / Polyhedron 26 (2007) 1251–1258
1252
Our interest is continuation of investigating coordinat-
nal and estimating the contribution vD from Pascal
constants [8]. The effective magnetic moments were calcu-
lated from leff = 2.83(vMT)1/2 using temperature-indepen-
dent paramagnetism of 60 · 10ꢁ6 cm3 molꢁ1 for Cu(II).
ing properties of Cu(II) with pyridine-2-carboxylate ligand.
Until now, aside from the violet form of the formula Cu-
(2-pca)2 obtained by us earlier [4a], only a few 1:1 mono-
nuclear adducts of bis(pyridine-2-carboxylate)copper(II)
[6] have been reported for which structural data are
available.
2.2. X-ray crystallography
In this work, we present a synthesis, the spectral and
magnetic properties, and the X-ray single-crystal analysis
of a blue-violet form of Cu(2-pic)2, where 2-pic = pyri-
dine-2-carboxylate. We would like to emphasize that the
compound studied is another new complex of copper(II)
with pyridine-2-carboxylate structurally and magnetically
characterized by us. The anhydrous Cu(pyridine-2-carbox-
ylate)2 prepared directly from picolinic acid was previously
obtained by others [7] and the square-planar environment
was proposed, but the crystal structure was not deter-
mined. This compound was obtained by us using a proce-
dure slightly modified with respect to those described in
the literature.
All measurements of crystal were performed on a Kuma
KM4CCD j-axis diffractometer [9a] with graphite-mono-
chromated Mo Ka radiation. The crystal was positioned
at 65 mm from the KM4CCD camera. Frames (612) were
measured at 0.75ꢁ intervals with a counting time of 45 s.
No absorption correction was applied. Data reduction
and analysis were carried out with KUMA DIFFRACTION
(Wrocław) programs. The structure was solved by direct
methods (program SHELXS97 [9b]) and refined by the full-
matrix least-squares method on all F2 data using the
SHELXL97 [9c] programs. Non-hydrogen atoms were refined
with anisotropic displacement parameters; hydrogen atoms
were included from geometry and were fixed.
In view of the structural data, it should be expected that
exchange interactions between copper(II) ions occur
through double out-of-plane carboxylate bridges. Hence,
the object of our interest was an investigation of the mag-
netic properties of this compound and an estimation of the
value of the exchange interaction.
Here, we describe the results of spectroscopic and mag-
netic studies of blue-violet form of Cu(pyridine-2-carboxyl-
ate)2 in the light of structure description.
2.3. Synthesis
This compound was obtained by us with a procedure
similar to those described in literature [7] but modified by
us. Copper(II) acetate monohydrate (1 mmol) was dis-
solved in DMF (40 cm3) and tetrahydrofuran (THF,
5 cm3) was added to a solution of picolinic acid (2 mmol)
in DMF (20 cm3). After ca. 30 min of stirring the blue solu-
tion was filtered. Single blue crystals suitable for X-ray
analysis were obtained by slow evaporation of the solvent.
Anal. Calc. for [C12H8N2O4Cu]: C, 46.84; H, 2.62; N,
9.10; Cu 20.65. Found: C, 46.32; H, 2.58; N, 8.95; Cu,
20.79%.
2. Experimental
2.1. Reagents and physical measurements
Starting materials and solvents for the synthesis were
obtained commercially and used as received. Metal content
was determined using a Carl-Zeiss-Jena atomic absorption
spectrophotometer. Elemental analyses were carried out
using a Perkin–Elmer elemental analyzer 2400CHN.
Infrared spectra (100–4000 cmꢁ1) were recorded on a
Bruker IFS 113v spectrophotometer using KBr pellets,
and solid-state electronic spectra (28000–4000 cmꢁ1) were
performed on a Cary 500 spectrophotometer.
The EPR spectra of powder samples were recorded (at
295 and 77 K) on a Bruker ESP 300E spectrometer operat-
ing at X-band and equipped with a Bruker NMR gauss-
meter ER 035 M and a Hewlett–Packard microwave
frequency counter HP 5350B. The EPR parameters were
calculated by computer simulation of the experimental
spectra of the powder samples using Bruker’s WIN-EPR
SimFonia Version 1.25.
3. Results and discussion
3.1. Description of the structure
Crystallographic parameters are summarized in Table 1
and selected bond distances and angles are listed in Table 2.
The structure of the Cu(2-pic)2 compound with crystal-
lographic numbering is depicted in Fig. 1.
The Cu(2-pic)2 molecule, as a whole, is nearly planar
and forms obliquely stacked planes. The copper(II) ion
has nearly square-planar coordination with trans
positioned oxygen and nitrogen donor atoms of two N,O-
chelating picolinate ligands (2-pic = pyridine-2-carboxyl-
ate) to form a complex involving a five-membered stable
chelate ring. The copper(II) ion is slightly deviating from
˚
the basal plane (0.02 A). No unusual bond angles and bond
lengths are observed for the coordinated picolinate anion.
Both axial and planar Cu–O bond lengths are identical
within experimental error (3r). The Cu–O and Cu–N dis-
tances in the basal plane agree well with those found in
other square-planar copper(II) complexes [10]. Addition-
ally, there are weak axial interactions between copper
Magnetic measurements of polycrystalline sample were
carried out with a Quantum Design SQUID magnetometer
(MPMSXL-5 type). Measurements were recorded at mag-
netic field of 0.5 T over the temperature range 1.8–300 K.
The correction for diamagnetism of the experimental sus-
ceptibility was based on subtracting the sample-holder sig-

_
B. Zurowska et al. / Polyhedron 26 (2007) 1251–1258
1253
Table 1
Crystal data and structure refinement of Cu(2-pic)2
Empirical formula
Formula weight
T (K)
C12H8N2O4Cu
307.74
100(1)
0.71073
monoclinic
P21/c
˚
k (A)
Crystal system
Space group
Unit cell dimensions
˚
a (A)
5.163(2)
24.658(6)
8.452(2)
˚
b (A)
˚
c (A)
b (ꢁ)
V (A )
92.22(3)
1075.2(6)
4
1.901
2.043
3
˚
Z
Dcalc (Mg mꢁ3
l (mmꢁ1
F(000)
)
)
620
Fig. 1. One-dimensional structure of Cu(2-pic)2 with crystallographic
numbering. Cuꢀ ꢀ ꢀCu distances are 5.163 A.
Crystal size (mm)
Diffractometer
h Range for data collection (ꢁ)
h,k,l Ranges
0.10 · 0.10 · 0.04
Kuma KM4CCD
3.46 ꢁ28.40
ꢁ6 ! 5, ꢁ28 ! 29,
ꢁ10 ! 10
6192
1881 (0.1248)
1881/172
1.247
˚
are sequentially doubly bridged by carboxylate groups to
form an infinite linear one-dimensional polymeric chain
extended along the crystallographic x-axis (Fig. 1). The
crystal packing is shown in Fig. 2.
Reflections collected
Independent reflections (Rint
Data/parameters
Goodness-of-fit (F2)
)
Each polymeric chain is linked to other ones by very
weak hydrogen bonds C(3)–H(3)ꢀ ꢀ ꢀO(2) (2 ꢁ x, 1 ꢁ y,ꢁz)
Final R1/wR2 indices (I > 2rI)
0.1227/0.1994
1.601 and ꢁ0.722
Largest difference in peak and hole
˚
˚
[C(3)ꢀ ꢀ ꢀO(2) 3.277(13) A; H(3)ꢀ ꢀ ꢀO(2) 2.49 A; \C(3)–
H(3)ꢀ ꢀ ꢀO(2) 140ꢁ] and C(15)–H(15)ꢀ ꢀ ꢀO(12) (1 + x, 1/
ꢁ3
˚
(e A
)
˚
2 ꢁ y,ꢁ1/2 + z) [C(15)ꢀ ꢀ ꢀO(12) 3.162(13) A; H(15)ꢀ ꢀ ꢀ
˚
O(12) 2.40 A; \C(15)–H(15)ꢀ ꢀ ꢀO–(12) 137ꢁ]. These bonds
Table 2
Selected bond lengths (A) and angles (ꢁ) of Cu(2-pic)2
˚
Cu(1)–O(1)
Cu(1)–N(2)
Cu(1)–N(1)
Cu(1)–O(11)
Cu(1)–O(2A)
1.944(7)
1.949(9)
1.960(9)
1.960(7)
2.745(8)
N(2)–Cu(1)–O(11)
N(1)–Cu(1)–O(11)
O(1)–Cu(1)–O(2A)
N(1)–Cu(1)–O(2A)
N(2)–Cu(1)–O(2A)
O(11)–Cu(1)–O(2A)
O(1)–Cu(1)–O(12B)
N(1)–Cu(1)–O(12B)
N(2)–Cu(1)–O(12B)
O(11)–Cu(1)–O(12B)
83.6(3)
95.6(4)
97.9(3)
84.8(3)
98.0(3)
81.8(3)
82.2(3)
95.0(3)
82.2(3)
98.0(3)
Cu(1)–O(12B)
2.770(8)
96.5(3)
84.4(3)
176.9(4)
179.7(4)
O(1)–Cu(1)–N(2)
O(1)–Cu(1)–N(1)
N(2)–Cu(1)–N(1)
O(1)–Cu(1)–O(11)
atoms of one plane and carboxylic oxygens of neighboring
planes (Cuꢀ ꢀ ꢀO(2A) = 2.745(8), Cuꢀ ꢀ ꢀO(12B) = 2.770(8)
˚
A). Because axial bonding interactions to the copper(II)
˚
ion are reported up to 3.3 A [11] and the angle between
the Cu–O(2A) vector and the normal to the equatorial
plane, which is the measure of overlap of metal–ligand
orbitals, is 0ꢁ, the axial interaction must be considered as
bonding [12], though the interaction is assumed to be very
weak. The spectroscopic evidence supports the axial bond-
ing (see Section 3.2). Therefore, the polyhedron can be
described as a tetragonally distorted (CuN2O2O0 chromo-
2
phore) octahedron, elongated along in the axial direction
with a long Cu–O distance, as is expected for apical bond
lengths in six-coordinated, strongly tetragonally distorted
copper(II) complexes [13]. The most important features
of the structure result from the fact that the copper(II) ions
Fig. 2. Crystal packing of Cu(2-pic)2 showing hydrogen bonds (dashed
lines).

_
B. Zurowska et al. / Polyhedron 26 (2007) 1251–1258
1254
arise from interaction of a CH from pyridine and an O
atom from the carboxylate group. These hydrogen bond
interactions, linked in a zigzag chain arrangement, lead
to the formation of a 2D sheet structure (Fig. 3a,b). The
shortest interchain Cuꢀ ꢀ ꢀCu separations are 7.43 and
the COO stretching frequencies should be expected
[15a]. In other words, large Dm values (>200 cmꢁ1) indi-
cate highly asymmetrical bridging, i.e. a ‘‘pseudo-mono-
dentate’’ coordination, which may be approaching a
monodentate coordination. So, the large Dm value
observed in our compound can be viewed as a pseudo-
monodentate coordination, which is consistent with the
structure. Generally, the number, shape and position of
the corresponding absorption bands are strongly affected
by some hydrogen bonding interactions of the carboxyl
group of pyridine-carboxylate [16]. So, the shoulder
observed on the broad band of mas(COO) stretch about
1650 cmꢁ1, overlapping with the stretching vibration of
m(C@N) of the pyridine ring, may be responsible for the
presence of hydrogen bonding as confirmed by the crystal
structure (Fig. 3).
The characteristic band attributed to the out-of-plane
ring deformation vibration of the pyridine ring d(C@N),
which occurs for a free ligand at 407 cmꢁ1, is shifted in
the complex to a higher frequency (419 cmꢁ1), indicating
bond formation between the copper ion and the nitrogen
atom of the heterocyclic ring. The bands associated with
Cu–N and Cu–O vibrations at 257 and 340 cmꢁ1, respec-
tively, were tentatively assigned on the basis of previous
studies of pyridines as N-donor ligands, carboxylate com-
pounds and by comparison with free ligands [17]. For
Cu(2-pic)2, where CuN2O2 chromophore has trans
square-planar geometry, only a single band corresponding
to Cu–O and Cu–N stretches is observed. However, for the
cis-isomer two absorption bands would be expected. This is
consistent with the criteria published by Herlinger et al. [18]
and with the reported crystal structure.
˚
8.09 A.
As previously noted, each carboxylate group bridges
two copper(II) ions through the O(2A) and O(12B) oxygen
atoms of the equatorial and the axial position on the cop-
per(II) coordination polyhedron, respectively. This coordi-
nation behavior causes the non-planarity of the Cu–O–C–
O–Cu0 fragment and gives rise to the formation of a chair-
like eight-membered ring Cu–O–C–O–Cu0–O–C–O–Cu
˚
with the CuÆCu distance of 5.163 A. This distance is near
˚
the range of reported values of ca. 5 A observed for poly-
atomic bridges [14]. The bridging pathway Cu–O–C–O–
Cu0 is asymmetric not only in its conformation but also
with respect to Cu–O bond lengths. The intermolecular
˚
Cuꢀ ꢀ ꢀCu distance (5.163 A) suggests magnetic interactions
between the copper(II) centers (see Section 3.3).
3.2. Spectroscopic properties
The IR spectrum of the Cu(2-pic)2 compound shows
asymmetric and symmetric stretching vibrations of the
coordinated carboxylate groups, mas(COO) and ms(COO),
at 1649 and 1351 cmꢁ1, respectively. The difference
between the two bands, Dm, which gives information on
the bonding mode of the carboxylate group [15], is equal
for the complex to 298 cmꢁ1, and may suggest a monod-
entate coordination. Owing to the non-symmetric bonding
mode of the carboxylate group, a large splitting (Dm) of
Fig. 3. (a, b) Two views of the 2D sheet structure of Cu(2-pic)B2 showing the sites of hydrogen bonding between the molecules (dashed lines).

_
B. Zurowska et al. / Polyhedron 26 (2007) 1251–1258
1255
The X-band EPR spectrum of a polycrystalline sample
of Cu(2-pic)2 (1) is of axial type with no hyperfine struc-
ture. As a result of a computer simulation procedure
(powder spectrum simulation) the following values of the
spectroscopic splitting factors were obtained: gi = 2.228,
g^ = 2.052 (Fig. 4). The spectra are independent of temper-
ature. The g-values obtained from the simulations indicate
basically a d(x2 ꢁ y2) ground state for Cu(II) as observed
for other copper–aminoacid complexes [19].
susceptibility results for polycrystalline sample obey the
Curie–Weiss law between 25 and 300 K, v = C/(T ꢁ h),
where v is the magnetic susceptibility per copper atom,
the Curie constant C = Ng2b2S(S + 1)/3k and S = 1/2.
The best fit values of the Curie (C) and Weiss (h) constants
are equal to 0.42 cm3 K molꢁ1 and ꢁ1.9 K, respectively.
Negative Weiss constant value suggests a weak antiferro-
magnetic interaction between the copper(II) ions. The tem-
perature variation of vT is depicted in Fig. 5.
The hyperfine structure expected for Cu(II) ions is not
observed, suggesting the existence of exchange interactions
coupling the paramagnetic ions [20]. The gi value is in
agreement with that observed for CuN2O2 chromophores
with strong planar and weaker axial bonds [21].
The magnetic moment at room temperature (1.83 lB) is
typical of the normal value for an uncoupled copper(II)
system and might suggest the absence of spin exchange,
but the vT value decreases slowly with temperature,
lowering from 0.418 cm3 K molꢁ1 at 300 K to 0.291 cm3
K molꢁ1 at 1.8 K. This suggests the existence of a weak
antiferromagnetic exchange between the copper(II) ions
at low temperatures.
The ligand-field spectrum shows one broad asymmetric
band centered at 16 670 cmꢁ1, characteristic of a tetrago-
nally distorted octahedral with a CuN2O2O0 chromophore
2
[22], consistent with the structural analysis.
Taking into account the crystal structure of the com-
pound, the magnetic properties could be explained by the
Heisenberg linear-chain model of magnetic interaction aris-
ing from the isotropic model S = 1/2 described by the
Hamiltonian H ¼ ꢁ2J i¼1Si ꢀ Siþ1. The magnetic data
were analyzed using the Bonner–Fisher expression [25a],
assuming a first-order molecular-field correction to account
for the interchain interaction, Eq. (1) [25b]:
3.3. Magnetic properties
P
n
Because the results of structural determination of Cu-
(2-pic)2 clearly indicate that in this complex there are car-
boxylate bridging groups through which an exchange inter-
action may be mediated, we performed a magnetic study of
them. In general, the ability of the carboxylate group to
propagate exchange interactions between two copper(II)
vCu
coor
Cu
v
¼
ð1Þ
2ZJ0
1 ꢁ
vCu
Nb2g2
˚
ions separated by more than 5 A has been well demon-
strated [23]. Additionally, because the interchain Cuꢀ ꢀ ꢀCu
where vCu is the susceptibility of copper(II) centers in an
isolated Heisenberg chain, J0is the interchain exchange,
and z is the number of nearest neighbors chains. In the
frame of this model, the best-fit parameters obtained by
˚
separations of 7.43 and 8.09 A are in the range, up to
˚
11 A, in which magnetic exchange has been observed [24],
a weak exchange interaction mediated by an H-bond sys-
tem is also expected.
The magnetic properties of the Cu(2-pic)2 were investi-
gated in the temperature range of 1.8–300 K. The magnetic
the least-squares fit are the following: J = ꢁ0.73 cmꢁ1
,
zJ0 = ꢁ0.06 cmꢁ1, g = 2.08 with the value of R=3.93 ·
10ꢁ5, where R is the agreement factor defined as
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
P
P
2
1
1
R ¼
2 ðvei xp ꢁ vci alcÞ =
ðvei xp
Þ
ðvei xp
Þ
2
Fig. 5. Plot of vCuT (O) vs. T for Cu(2-pic)2. The solid line represents the
best fit of the experimental data to the Bonner–Fisher chain model with
the parameters reported in the text.
Fig. 4. The experimental EPR powder spectrum of Cu(2-pic)2 obtained at
9.5 GHz (exp) and computer simulation (sim).

_
B. Zurowska et al. / Polyhedron 26 (2007) 1251–1258
1256
The temperature variation of experimental and calcu-
lated values of vMT of the Cu(2-pic)2 is shown in Fig. 5.
near-orthogonality of magnetic orbitals, as it has been
shown for chloro- [28d,29], carbonato- [30] and oximato-
[31] bridged compounds. In the case of interactions in api-
cal-equatorial bond modes, the orthogonality of magnetic
orbitals leads to a coupling which must be very weakly
antiferromagnetic, zero or weakly ferromagnetic.
3.4. Magneto-structural correlations
Different coordination modes of carboxylato groups
lead to the formation of mononuclear and polynuclear
structures. From the magnetic point of view, the range of
magnetic behaviour as a function of different coordination
modes was pointed out, from strongly coupled to moderate
or weak antiferromagnetic compounds and even ferromag-
netic ones.
It is worthy of note that the weak interaction between
copper(II) centers has been previously observed for some
carboxylato-bridged complexes (J from ꢂ0.5 to ꢂꢁ1
cmꢁ1) in which the carboxylate group bridges from the
basal to the axial position with a long Cu–O distance
˚
(2.28–2.68 A) [5b]. The value of J is small as expected for
For copper(II) complexes, the syn–syn conformation
mediates strong antiferromagnetic interactions (singlet–
triplet separation value ꢂꢁ300 cmꢁ1). Weak antiferro- or
ferromagnetic interactions are observed in carboxylato-
bridged copper(II) complexes in which the carboxylate
group adopts the syn–anti conformation [4b]. In an anti–
anti carboxylate bridge, a weak to moderate antiferromag-
netic coupling is observed [26].
It is worth noticing that a large number of dinuclear and
polynuclear copper(II) geometries, planar or parallel pla-
nar, especially those bridged by hydroxide [27] and chloride
[28] ions, have been reported. These compounds are often
divided into two types (Fig. 6): (i) parallel plane bridging
in which the bridging ligands are equatorially bound to
both metal ion and (ii) perpendicular plane bridging in
which the bridging ligands are axially bound to one metal
ion and equatorially bound to the other, characteristic
for hydroxo- and chloro-bridged systems, respectively.
The carboxylato-bridged compound studied in this work
belong to (ii) type, i.e. with a double out-of-plane bridging
framework and parallel planar geometry of copper(II) ion.
Generally, magnetic coupling for all known out-of-plane
polynuclear compounds is relatively weak, because of the
this kind of compounds exhibiting an out-of-plane bridging
framework.
In the case of the Cu(2-pic)2, the very weak antiferro-
magnetic coupling observed between the copper atoms is
expected in the light of the following electronic and struc-
tural features: (i) relatively long Cu–O axial distance, (ii)
non planarity of the Cu–O–C–O–Cu0 fragment (an out-
of-phase exchange pathway), and (iii) each carboxylate
bridge involves an axial position on a copper(II) ion, where
the spin density of the unpaired electron is very small, in
square-based geometry the unpaired electron is in
d(x2 ꢁ y2). In other words, the bridge links the axial-
equatorial coordination sites of the square-planar neigh-
boring Cu(II) centers via both superexchange pathways,
and the delocalization of unpaired electron density along
the axial coordination site should be very poor, and mag-
netic orbitals are almost orthogonal. All these features
are unfavorable to the propagation of strong interactions.
The magnitude of the interaction in Cu(2-pic)2 is much
less than in the di-chlorobridged series [28d,29], because
of the greater distance through which exchange effects have
to be transmitted in the carboxylate group. The results
indicate that the carboxylate group is less effective in allow-
ing for electron spin delocalization than the pathways
between ligands and Cu(II) ions involving a chloro-bridge.
In summary, a comparison between the values of J for
Cu(2-pic)2 analyzed here and those for out-of-plane single
carboxylato-bridged compounds [5b] reveals that the
monocarboxylato-bridged series exhibit weaker antiferro-
magnetic coupling. This is consistent with the simple orbi-
tal model proposed by Kahn et al. [32], in which magnetic
coupling is proportional to the square of the overlapping
integral. So, for mono-out-of-plane bridged copper(II)
compounds, the values of coupling constant are smaller
due to a decreasing number of the bridging groups.
Finally, although the distance between the donor and
the acceptor of C–Hꢀ ꢀ ꢀO hydrogen bonds is relatively
short, only very weak antiferromagnetic interactions are
possible due to large Cuꢀ ꢀ ꢀCu separations. It is worth of
mentioning, that despite large axial Cu–O bond distances,
a covalent carboxylate bridge is more effective in allowing
for electron spin delocalization than pathways involving
non-covalent hydrogen bonds. These results are important
in view of the fact that those chemical pathways are present
in most metalloproteins.
Fig. 6. Relative orientation of the coordination planes in the copper(II)
compounds of in the plane (I) and out-of-plane (II) bridging modes type.

_
B. Zurowska et al. / Polyhedron 26 (2007) 1251–1258
1257
[6] (a) F.S. Stephens, J. Chem. Soc. A (1970) 2377;
4. Concluding remarks
(b) A. Takenaka, H. Utsumi, T. Yamamoto, A. Furusaki, I. Nitta, J.
Chem. Soc. Jpn. Pure Chem. 91 (1970) 928.
[7] (a) R.D. Gillard, S.H. Laurie, F.S. Stephens, J. Chem. Soc. A (1968)
2588;
In summary, a new example of copper(II) compound
(blue-violet form) with the pyridine-2-carboxylate ligand
of the formula Cu(2-pic)2 is reported in this paper. We
would like to emphasize that the compound here studied
and a violet form of Cu(2-pca)2 obtained by us earlier [4a]
are the polymorphic forms of Cu(pyridine-2-carboxylate)2.
Two key factors control the crystal structure of Cu-
(2-pic)2 and provide superexchange paths for magnetic
interactions between the copper ions, the bridging coordi-
nation and the interchain hydrogen bonds. The hydrogen
bonds generate a supramolecular structure and governing
stability of the complexes, and have an important role in
the magnetism by a further increase in the dimensionality
of the overall structure.
(b) A. Kleinstein, G.A. Webb, J. Inorg. Nucl. Chem. 33 (1971) 405;
(c) M. Inoue, M. Kubo, J. Magn. Reson. 4 (1971) 175;
(d) M. Herna´ndez-Molina, J. Gonza´lez-Platas, C. Ruiz-Pe´rez, F.
Lloret, M. Julve, Inorg. Chim. Acta 284 (1999) 258, and references
therein.
[8] E. Ko¨nig, Magnetic Properties of Coordination and Organometallic
Transition Metal Compounds, Springer, Berlin, 1966.
[9] (a) Oxford Diffraction, Poland Sp. z o.o., CRYSALIS CCD and CRYSALIS
RED Programs, Wrocław, 2002.;
(b) G.M. Sheldrick, SHELXS97, Program for Solution of Crystal
Structures, University of Go¨ttingen, 1997;
(c) G.M. Sheldrick, SHELXL97, Program for Crystal Structure Refine-
ment, University of Go¨ttingen, 1997.
[10] (a) M. Melnik, M. Kabesˇova´, M. Dunaj-Jurcˇo, C.E. Holloway, J.
Coord. Chem. 41 (1997) 35;
Finally, it was interesting to design a new polynuclear
copper(II) compound with parallel planar geometry and
with an out-of-plane framework of carboxylate bridges.
(b) M.E. Bluhm, M. Ciesielski, H. Gorls, O. Walter, M. Doring,
Inorg. Chem. 42 (2003) 8878;
(c) H. Masuda, A. Odani, O. Yamauchi, Inorg. Chem. 28 (1989) 624.
[11] A. Walsh, B.J. Hathaway, J. Chem. Soc., Dalton Trans. (1984) 15.
[12] L.P. Battaglia, A.B. Corradi, L. Menabue, Inorg. Chem. 22 (1983)
3251.
Acknowledgements
[13] G. Davey, F.S. Stephens, J. Chem. Soc. A (1971) 103.
[14] E. Colacio, J.P. Costes, R. Kiveka¨s, J.P. Laurent, J. Ruiz, M.
Sundberg, Inorg. Chem. 30 (1991) 1475.
[15] (a) G.B. Deacon, R.J. Philips, Coord. Chem. Rev. 33 (1980) 227;
(b) K. Nakamoto, Infrared and Ramann Spectra of Inorganic and
Coordination Compounds, Wiley, New York, 1997.
[16] G. Marcotrigiano, L. Menabue, G.C. Pellacani, J. Inorg. Nucl. Chem.
40 (1978) 165.
This work was supported by the Polish Ministry of Sci-
ence and Higher Education (Grant No. 1T09A 12430). We
thank Professor Julia Jezierska (Faculty of Chemistry,
University of Wrocław, Wrocław, Poland) for the simula-
tion of EPR spectra and helpful discussions.
Appendix A. Supplementary material
[17] (a) M. Cadene, A.M. Vergnoux, Spectrochim. Acta, Part A 28 (1972)
1663;
(b) C.W. Frank, L.B. Rogers, Inorg. Chem. 5 (1966) 619.
[18] A.W. Herlinger, S.L. Wenhold, T.V. Long II., J. Am. Chem. Soc. 92
(1970) 6474.
[19] O.R. Nascimento, A.J. Costa-Filho, D.I. De Morais, J. Ellena, L.F.
Delboni, Inorg. Chim. Acta 312 (2001) 133.
[20] D. Gatteschi, A. Bencini, Electron Paramagnetic Resonance of
Exchange Coupled Systems, Springer-Verlag, Berlin, 1990.
[21] A.C. Fabretti, G. Franchini, P. Zannini, M. Divaira, Inorg. Chim.
Acta 105 (1985) 8.
[22] L. Abuhijleh, C. Woods, Y. Amed, Inorg. Chim. Acta 130 (1991)
11.
CCDC 255026 contains the supplementary crystallo-
graphic data for the structures reported in this paper. 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: depos-
it@ccdc.cam.ac.uk. Supplementary data associated with
this article can be found, in the online version, at
doi:10.1016/j.poly.2006.10.019.
[23] E. Colacio, J.M. Dominguez-Vera, R. Kiveka¨s, J. Ruiz, Inorg. Chim.
Acta 218 (1994) 109.
References
[24] (a) B.J. Hathaway, Struct. Bond. 14 (1973) 49;
(b) B.J. Hathaway, D.E. Billing, Coord. Chem. Rev 5 (1970) 143;
(c) T.D. Smith, J.R. Pilbrow, Coord. Chem. Rev. 13 (1974) 73.
[25] (a) J.C. Bonner, M.E. Fisher, Phys. Rev., Sect. A 135 (1964) 640;
(b) J.W. Stout, R.C. Chisholm, J. Chem. Phys. 36 (1962) 979.
[26] (a) A. Rodriguez-Fortea, P. Alemany, S. Alvarez, E. Ruiz, Chem.
Eur. J. 7 (2001) 627;
[1] (a) B.P. Murphy, B.J. Hathaway, Coord. Chem. Rev. 124 (1993) 63;
(b) B.J. Hathaway, in: G. Wilkinson, R.D. Gillard, J.A. McCleverty
(Eds.), Comprehensive Coordination Chemistry, vol. 5, Pergamon
Press, Oxford, 1987, p. 553.
´
[2] (a) M. Melnık, Coord. Chem. Rev. 42 (1982) 259;
(b) M. Kato, Y. Muto, Coord. Chem. Rev. 92 (1988) 45.
[3] (a) B. Kozlevcˇar, N. Lah, I. Leban, Croat. Chim. Acta 73 (2000) 733;
(b) S. Youngme, C. Pakawatchai, W. Somjitsripunya, K. Chinnakali,
H.-K. Fun, Inorg. Chim. Acta 303 (2000) 181;
´
´
´
(b) M. Melnık, M. Koman, D. Hudecova, J. Moncol, B. Dudova, T.
´
(c) E. Colacio, J.M. Domınguez-Vera, M. Ghazi, R. Kiveka¨s, M.
´
Głowiak, J. Mrozinski, C. Holloway, Inorg. Chim. Acta 308 (2000) 1;
Klinga, J.M. Moreno, Eur. J. Inorg. Chem. (1999) 441;
(d) M. Inoue, M. Kubo, Inorg. Chem. 9 (1970) 2310;
(c) J. Moncol’, M. Palicova´, P. Segl’a, M. Koman, M. Meln´ık, M.
Valko, T. Głowiak, Polyhedron 21 (2002) 365.
´
(e) E. Colacio, J.M. Domınguez-Vera, R. Kiveka¨s, J.M. Moreno, A.
_
´
[4] (a) B. Zurowska, J. Ochocki, Z. Ciunik, J. Mrozinski, J. Reedijk,
Inorg. Chim. Acta 357 (2004) 755;
Romerosa, J. Ruiz, Inorg. Chim. Acta 212 (1993) 115.
[27] (a) P. Hay, J. Thibeault, R. Hoffmann, J. Am. Chem. Soc. 97 (1975)
4884;
_
´
(b) B. Zurowska, J. Mrozinski, Inorg. Chim. Acta 342 (2003) 23, and
references therein.
(b) D.J. Hodgson, Prog. Inorg. Chem. 19 (1975) 173;
(c) E. Ruiz, P. Alemany, S. Alvarez, J. Cano, J. Am. Chem. Soc. 119
(1997) 1297;
[5] (a) C.J. O’Connor, E.E. Eduok, F.R. Fronczek, O. Kahn, Inorg.
Chim. Acta 105 (1985) 107;
(b) P.R. Levstein, R. Calvo, Inorg. Chem. 29 (1990) 1581.

_
B. Zurowska et al. / Polyhedron 26 (2007) 1251–1258
1258
´
(d) E. Ruiz, P. Alemany, S. Alvarez, J. Cano, Inorg. Chem. 36 (1997)
3683;
[29] (a) J.V. Folgado, E. Coronado, D. Beltran, J. Chem. Soc., Dalton
Trans. (1986) 1061;
(e) V.H. Crawford, H.W. Richardson, J.R. Wasson, D.J. Hodgson,
W.E. Hatfield, Inorg. Chem. 15 (1976) 2107.
(b) K. Hara, M. Inoue, S. Emori, M. Kuto, J. Magn. Reson. 4 (1971)
337.
[28] (a) S.G.N. Roundhill, D.M. Roundhill, D.R. Bloomquist, C. Landee,
R.D. Willet, D.M. Dooley, H.B. Gray, Inorg. Chem. 18 (1979) 831;
[30] S. Youngme, N. Chaichit, P. Kongsaeree, G.A. van Albada, J.
Reedijk, Inorg. Chim. Acta 324 (2001) 232.
´
(b) J.V. Folgado, E. Coronado, D. Beltran, J. Chem. Soc., Dalton
[31] B. Cervera, R. Ruiz, F. Lloret, M. Julve, J. Cano, J. Faus, C. Bois, J.
Mrozin´ski, J. Chem. Soc., Dalton Trans. (1997) 1061.
[32] (a) O. Kahn, M.F. Charlot, Nouv. J. Chim. 4 (1980) 567;
(b) J.J. Girerd, M.F. Charlot, O. Kahn, Mol. Phys. 34 (1977)
1063;
Trans. (1986) 1061, and references therein;
(c) R. Faure, H. Loiseleur, G. Thomas-David, Acta Crystallogr., Sect.
B 29 (1973) 1890;
ˇ
(d) P. Segl’a, M. Jamnicky´, M. Koman, J. Sima, T. Głowiak,
Polyhedron 17 (1998) 4525.
(c) O. Kahn, B. Briat, J. Chem. Soc., Faraday Trans. 2 (1976) 268.