Synthesis, crystal structure and magnetic properties of [Cr 2Cu2(bpy)4(OX)5]·2H 2O. An oxalato-bridged heterometallic tetramer

Article abstract of DOI:10.1016/S0277-5387(03)00462-5

A new heterometallic tetramer of formula [Cr₂Cu ₂(bpy)₄(ox)₅]·2H₂O (1) (bpy=2,2′-bipyridine; ox=oxalate dianion) has been prepared and characterised by single-crystal X-ray diffraction, magnetic susceptibility measurements and ESR spectroscopy. The tetranuclear unit in 1 can be viewed as the combination of two terminal [Cr(bpy)₂(ox)]^- units with a central oxalato-bridged copper(II) dimer. The chromium ions are in a distorted octahedral environment with metal-ligand distances ranging from 1.944(4) to 2.064(5) A?. The copper(II) centres lie in an axially distorted octahedron. The axial positions are occupied by one oxygen atom belonging to the central bridging oxalate anion [O(9)-Cu(1): 2.245(5) A?] and one oxygen atom coming from the [Cr(bpy)₂(ox)]^- moiety [O(7)-Cu(1): 2.357(5) A?]. The N₂O₂ equatorial environment is formed by a bpy ligand [Cu-N mean distance: 1.989(6) A?] and the remaining oxygen atoms [Cu-O mean distance: 2.162(5) A?]. The magnetic properties of 1 have been investigated in the 2-300 K temperature range. A ferromagnetic interaction between the copper(II) centres (J ₁=+2 cm^-1) is observed, whereas the interaction between the adjacent chromium(III) and copper(II) cations is weakly antiferromagnetic (J₂=-0.65 cm^-1).

Full text of DOI:10.1016/S0277-5387(03)00462-5

Polyhedron 22 (2003) 3115–3122
www.elsevier.com/locate/poly
Synthesis, crystal structure and magnetic properties
of [Cr2Cu2(bpy)4(ox)5] ꢀ 2H2O. An oxalato-bridged
heterometallic tetramer
*
ꢀ
ꢀ
Eugenio Coronado , Mari Carmen Gimenez, Carlos J. Gomez-Garcıa,
ꢀ
*
Francisco M. Romero
Institut de Cieꢁncia Molecular, Universitat de Valeꢁncia, Dr. Moliner 50, Burjassot 46100, Spain
Received 15 April 2003; accepted 1 July 2003
Abstract
A new heterometallic tetramer of formula [Cr2Cu2(bpy)4(ox)5] ꢀ 2H2O (1) (bpy ¼ 2,20-bipyridine; ox ¼ oxalate dianion) has been
prepared and characterised by single-crystal X-ray diffraction, magnetic susceptibility measurements and ESR spectroscopy. The
tetranuclear unit in 1 can be viewed as the combination of two terminal [Cr(bpy)2(ox)]ꢁ units with a central oxalato-bridged
copper(II) dimer. The chromium ions are in a distorted octahedral environment with metal–ligand distances ranging from 1.944(4)
ꢂ
to 2.064(5) A. The copper(II) centres lie in an axially distorted octahedron. The axial positions are occupied by one oxygen atom
ꢁ
ꢂ
belonging to the central bridging oxalate anion [O(9)–Cu(1): 2.245(5) A] and one oxygen atom coming from the [Cr(bpy)2(ox)]
ꢂ
moiety [O(7)–Cu(1): 2.357(5) A]. The N2O2 equatorial environment is formed by a bpy ligand [Cu–N mean distance: 1.989(6) A] and
ꢂ
ꢂ
the remaining oxygen atoms [Cu–O mean distance: 2.162(5) A]. The magnetic properties of 1 have been investigated in the 2–300 K
temperature range. A ferromagnetic interaction between the copper(II) centres (J1 ¼ þ2 cmꢁ1) is observed, whereas the interaction
between the adjacent chromium(III) and copper(II) cations is weakly antiferromagnetic (J2 ¼ ꢁ0:65 cmꢁ1).
Ó 2003 Elsevier Ltd. All rights reserved.
Keywords: Synthesis; Crystal structures; Magnetic properties; Oxalato complexes
1. Introduction
compound alternating layers that give rise to two differ-
ent collective properties. For instance, replacing the
quaternary onium cations by organic donors of the TTF
family affords materials showing electrical conductivity
and ferromagnetism [9]. 3D structures are more suitable
for the combination of ferromagnetism with a molecular
property, like luminescence [10]. In this context, the
family of compounds [Ru(bpy)3][MIIMIII(ox)3](ClO4)
has been studied, although its photophysical properties
have not yet been addressed in detail [11].
Recently, attention has also been paid to the use of
heteroleptic oxalato complexes of the type [Cr(L)(ox)2]ꢁ
(L ¼ a-diimine) in the synthesis of heterometallic com-
pounds [12–14]. As expected, the combination of the
[Cr(bpy)(ox)2]ꢁ building block [15] with divalent metal
ions affords the series [MIICr2(bpy)2(ox)4(H2O)x] with
1:2 M(II):Cr(III) stoichiometry [16–18]. We have been
interested in these compounds as starting materials for
Homoleptic oxalato complexes, [MIII(ox)3]3ꢁ
(ox ¼ oxalate dianion), have proved to be useful precur-
sors for the synthesis of anionic two- (2D) and three-
dimensional (3D) magnetic lattices [1–3]. The
dimensionality of these systems is directly related to the
presence of a given countercation, Aþ, which can be
viewed as a templating agent of the overall structure [4].
Quaternary onium cations [5,6] give rise to 2D structures
of the type A[MIIMIII(ox)3], while tris-chelated
[Z(bpy)3]2þ (bpy ¼ 2,20-bipyridine) species induce the
formation of chiral structures [7,8] of formulae
[MI2I(ox)3]ꢁ, [MIMIII(ox)3]ꢁ and [MIIMIII(ox)3]ꢁ. 2D
structures offer the possibility to combine in the same
* Corresponding authors.
E-mail address: fmrm@uv.es (F.M. Romero).
0277-5387/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0277-5387(03)00462-5

3116
E. Coronado et al. / Polyhedron 22 (2003) 3115–3122
requires C, 45.1; H, 2.7; N, 8.4%); mmax/cmꢁ1: 3488s,
ꢃ
the preparation of spinels with potential applications in
the field of ceramic pigments. However, a systematic
study of the reaction conditions (stoichiometry, con-
centration,. . .) permitted us to isolate the new tetranu-
clear complex [Cr2Cu2(bpy)4(ox)5] ꢀ 2H2O (1). We report
herein the crystal structure and the magnetic properties
of this novel Cr2Cu2 tetramer.
3117w, 3081w, 3030w, 2923w, 2853w, 1718vs, 1678vs,
1653vs, 1604s, 1498m, 1475m, 1448s, 1371w, 1315w,
1283w, 1237w, 1177w, 1160w, 1107w, 1061w, 1035w,
1023w, 980w, 893w, 807m, 779m, 731w, 667w, 652w,
552w, 481w; (Metal analysis (%): Cu, 49.4; Cr 50.6).
2.4. IR characterisation
2. Experimental
The heterometallic tetramer of formula [Cr2Cu2
(bpy)4(ox)5] ꢀ 2H2O shows four intense IR bands be-
tween 1718 and 1653 cmꢁ1 due to asymmetric mCO
stretching vibrations of the oxalate anions. The high
number of bands in this region suggests the existence of
two types of oxalate groups in the crystal lattice
(bridging and terminal). The bands between 1448–1315
and 807–779 cmꢁ1 are attributed to mCO and mCC oxalate
symmetric stretching vibrations, respectively. The lower
wavenumber band (552–481 cmꢁ1) can be assigned to
(–Cu–O–) and (–Cr–O–) deformation vibration. In the
3500 cmꢁ1 region, a strong and broad absorption is
observed, in agreement with the presence of uncoordi-
nated water molecules of crystallisation in the complex.
Other significant bands belong to the 2,20-bipyridine li-
gand.
2.1. Materials
Chromium(III) chloride hexahydrate, 2-20-bipyridine,
sodium oxalate, barium(II) chloride dihydrate and
copper(II) sulfate pentahydrate were purchased from
Fluka and all the manipulations were performed using
materials as received.
2.2. Preparation of Ba[Cr(bpy)2(ox)]2 ꢀ 3H2O precursor
[15]
An aqueous suspension (100 ml) containing stoichi-
ometric amounts of CrCl3 ꢀ 6H2O (53 mg, 2 mmol), 2,20-
bipyridine (32 mg, 2 mmol) and sodium oxalate (53 mg,
4 mmol) was refluxed (100 °C) during half-an-hour un-
der continuous stirring. The initially green solution
turned red-violet upon warming. After a few minutes, a
light-pink microcrystalline powder of [Cr(bpy)(ox)2]
[Cr(bpy)2(ox)] precipitates. The solid was immediately
filtered off. BaCl2 ꢀ 6H2O (24 mg, 1 mmol), dissolved in a
minimum amount of water (approximately 2.5 ml), was
added dropwise to the filtrate, under stirring at room
temperature. The resulting mixture was stirred for 1 h to
promote complete precipitation of a dark pink powder
of Ba[Cr(bpy)2(ox)]2 ꢀ 3H2O, which was removed by fil-
tration and washed with water. Red single crystals of
Ba[Cr(bpy)2(ox)]2 ꢀ 3H2O were obtained by slow evapo-
ꢃ
2.5. Physical techniques
Elemental analysis (C, H, N) and the measurement of
Cr:Cu ratios were performed by the SCSIE of the Uni-
versity of Valencia on a CE INSTRUMENTS EA 1110
CHNS elemental analyser and a Philips ESEM X230
scanning electron microscope equipped with an EDAX
DX-4 microsonde.
The infrared spectrum was recorded on a Nicolet 800
FTIR spectrophotometer as KBr pellets from 4000 to
400 cmꢁ1
.
Magnetic susceptibility measurements of a polycrys-
talline sample were carried out using a magnetometer
(Quantum Design MPMS-XL-5) equipped with a
SQUID sensor. The dc measurements were performed in
the temperature range 2–300 K at a magnetic field of 0.1
T (1000 Oe). The magnetisation was performed between
0 and 5 T at 2 K.
ration from the filtrate. (306 mg, 35%) mmax/cmꢁ1
:
3468m, 3075w, 2923w, 1718vs, 1687vs, 1655vs, 1498m,
1475s, 1447w, 1389m, 1320w, 1252w, 1162w, 1106w,
1063w, 1036w, 1023w, 897w, 809w, 769m, 730m, 654w,
548w, 476m, 450w; (metal analysis (%): Ba, 34.3; Cr,
65.7).
ESR measurements were performed on a grinded
polycrystalline sample at X-band in the T range 300–4.2
K with a BRUKER ELEXSYS E-500 spectrometer
equipped with a He-cryostat.
2.3. Preparation of [Cr2Cu2(bpy)4(ox)5] ꢀ 2H2O
A suspension of Ba[Cr(bpy)2(ox)]2 ꢀ 3H2O (26 mg,
0.25 mmol) in 25 ml of water was allowed to react with
CuSO4 ꢀ 5H2O (12 mg, 0.50 mmol) dissolved in 10 ml of
water. The resulting mixture was stirred for 3 h to pro-
mote complete precipitation of barium sulfate, which
was removed by filtration. After a few days, large brown
crystals were formed together with a light-blue precipi-
tate that analysed as copper(II) oxalate. (62 mg, 20%),
(Found: C, 45.0; H, 2.7; N, 8.4. C50H36Cr2Cu2N8O22
2.6. X-ray data collection and structure determination
Good-quality single crystals of [Cr2Cu2(bpy)4
(ox)5] ꢀ 2H2O were collected and one of them of dimen-
sions 0.31 ꢂ 0.17 ꢂ 0.16 mm was mounted on a CAD4
Enraf-Nonius X-ray diffractometer. Unit cell parame-
ters and orientation matrix were determined by least-

E. Coronado et al. / Polyhedron 22 (2003) 3115–3122
3117
Table 2
squares fit of 25 independent reflections. Data collection
was performed with the x-scan technique with three
standard reflections measured every 2 h which showed
no significant decay. Lorentz, polarisation, and empiric
absorption (W scan) corrections were carried out. The
structure was solved by direct methods using the SIR 97
program [19] and was refined on F2 using the SHELXL-
97 program [20].
ꢂ
Selected bond lengths (A) and angles (°) for 1
Chromium environment
Bond lengths
Cr(1)–N(1)
2.064(5)
2.044(5)
1.944(4)
1.946(4)
1.977(4)
2.004(4)
Cr(1)–N(2)
Cr(1)–O(1)
Cr(1)–O(2)
Cr(1)–O(5)
Cr(1)–O(6)
In the structural determination the heavy atoms were
found immediately and the other atoms were located
through successive Fourier differences. All non-hydrogen
atoms were refined anisotropically. One of the water
molecules O(12) was disordered in two alternative posi-
tions, O(12A) and O(12B) with occupancy factors of 0.25
and 0.75, respectively. The hydrogen atoms of the water
molecules were calculated on the basis of geometry and
force-field considerations [21]. The hydrogen atoms of the
2,20-bipyridine ligands were set in calculated positions
and refined as riding atoms with a common fixed isotropic
thermal parameter. Crystallographic data and refinement
parameters are summarised in Table 1. Table 2 gathers
selected bond lengths and angles.
Bond angles
O(1)–Cr(1)–O(2)
O(1)–Cr(1)–O(5)
O(2)–Cr(1)–O(5)
O(1)–Cr(1)–O(6)
O(2)–Cr(1)–O(6)
O(5)–Cr(1)–O(6)
O(1)–Cr(1)–N(2)
O(2)–Cr(1)–N(2)
O(5)–Cr(1)–N(2)
O(6)–Cr(1)–N(2)
O(1)–Cr(1)–N(1)
O(2)–Cr(1)–N(1)
O(5)–Cr(1)–N(1)
O(6)–Cr(1)–N(1)
N(2)–Cr(1)–N(1)
83.63(19)
92.4(2)
90.09(19)
171.42(18)
89.75(19)
82.19(19)
91.4(2)
95.6(2)
173.5(2)
94.7(2)
95.1(2)
174.4(2)
95.45(19)
92.0 (2)
78.9(2)
Copper environment
Bond lengths
Cu(1)–N(3)
3. Results and discussion
2.001(6)
1.977(5)
2.357(5)
2.080(5)
2.245(5)
1.964(5)
Cu(1)–N(4)
Cu(1)–O(7)
3.1. Crystal structure
Cu(1)–O(8)
Cu(1)–O(9)
Cu(1)–O(10)
The structure of compound 1 consists of neutral tet-
ranuclear complexes (Fig. 1) and water molecules of
crystallisation. Each tetranuclear complex is built from
two [Cr(bpy)(ox)2]ꢁ subunits acting as bidentate ligands
Bond angles
O(10)–Cu(1)–N(4)
O(10)–Cu(1)–N(3)
N(4)–Cu(1)–N(3)
O(10)–Cu(1)–O(8)
N(4)–Cu(1)–O(8)
N(3)–Cu(1)–O(8)
O(10)–Cu(1)–O(9)
N(4)–Cu(1)–O(9)
N(3)–Cu(1)–O(9)
O(8)–Cu(1)–O(9)
O(10)–Cu(1)–O(7)
N(4)–Cu(1)–O(7)
N(3)–Cu(1)–O(7)
O(8)–Cu(1)–O(7)
O(9)–Cu(1)–O(7)
175.8(2)
94.8(2)
81.4(2)
towards
a
central l-oxalatobis[(2,20-bipyridine)cop-
per(II)] fragment. An inversion centre is located in the
middle of the CAC bond of this central l-oxalato bridge
and the structure of the complex is centrosymmetric.
88.1(2)
95.9(2)
174.3(2)
79.65(18)
99.3 (2)
99.9(2)
Table 1
Crystal data and structure refinement data for 1
85.45(18)
91.66(18)
90.6(2)
Formula
FW
C25H18CrCuN4O11
665.98
triclinic
Crystal system
Space group
ꢃ
P1
98.97(19)
76.03(17)
159.83(16)
ꢂ
a (A)
9.6740(14)
10.9636(17)
14.543(5)
100.076(3)
94.492(4)
114.861(3)
1357.9(5)
2
ꢂ
b (A)
ꢂ
c (A)
a (°)
b (°)
c (°)
This is in contrast with the situation found in a similar
tetranuclear compound {[Cr(phen)(ox)2]2[Mn2(bpy)2
(H2O)2(ox)] } ꢀ 6H2O, where two [Cr(phen)(ox)2] anions
bind as unidentate ligands to a central l-oxalat-
obis[(2,20-bipyridine)manganese(II)] fragment, giving
rise to a non-centrosymmetric structure [14].
The chromium atoms are located in a slightly
distorted octahedral environment. The CrAO bond
distances for the terminal oxalato ligand (Cr(1)–
ꢂ
V (A)
Z
T (K)
ꢂ
293
k (A)
0.71073
1.673
0.0787
qcalc (g cm3)
a
R1
b
R2
a R1 ¼ ðFo ꢁ FcÞ= ðFoÞ.
0.1792
P
P
P
P
P
ꢂ
2
2
b R2 ¼ ½ ½xðFo2 ꢁ Fc2Þ ꢃ= ½xðFo2Þ ꢃꢃ1=2; x ¼ 1=½ 2ðFo2Þþ (0.1446P)2
O(1) ¼ Cr(1)–O(2): 1.946(4) A) are somewhat shorter
þ 4:8035Pꢃ, where P ¼ ðFo2 þ 2Fc2Þ=3.
than for the bridging ligand (Cr(1)–O(5): 1.977(4) and

3118
E. Coronado et al. / Polyhedron 22 (2003) 3115–3122
Fig. 1. ORTEP view of the [Cr2Cu2(bpy)4(ox)5] tetranuclear unit of 1. Thermal ellipsoids are plotted at the 50% probability level.
ꢂ
Cr(1)–O(6): 2.004(5) A). The chelate angle N(1)–Cr(1)–
N(2) (78.9(2)°) is smaller than the two oxalato chelate
angles (O(1)–Cr(1)–O(2): 83.63(19)°; O(5)–Cr(1)–O(6):
82.19(19)°). These structural features are in agreement
with other previously reported [Cr(bpy)(ox)2]ꢁ units
[14]. The copper atoms show the expected axially elon-
gated octahedral coordination (Jahn–Teller effect). The
resulting N2O4 environment is formed by two nitrogen
atoms from a bpy ligand, N(3) and N(4), two oxygen
atoms, O(7) and O(8), belonging to a [Cr(bpy)(ox)2]ꢁ
moiety, and two oxygen atoms, O(9) and O(10), from
the central l-oxalato bridging unit. As expected, the
bipyridine ligand is found in the equatorial plane
ꢂ
(Cu(1)–N(3): 2.001(6) A; Cu(1)–N(4): 1.977(5) A) with
ꢂ
ꢂ
Cu–O distances (Cu(1)–O(8): 2.080(5) A; Cu(1)–O(10):
ꢂ
1.964(5) A) shorter than in the axial direction (Cu(1)–
ꢂ
O(7): 2.357(5) A; Cu(1)–O(9): 2.245(5) A). The distance
ꢂ
between copper(II) ions is Cu(1)–Cu(1)(ꢁx; ꢁy; ꢁz):
ꢂ
5.438(2) A, while the chromium–copper separation
ꢂ
Cr(1)–Cu(1) equals 5.4473(17) A. The dihedral angles
between the N2O2Cu basal plane and the mean planes of
the bridging oxalate anions are 81.69(19)° (for the cen-
Fig. 2. Projection of the crystal structure of 1 onto the bc-plane showing the intricate hydrogen-bonded network. Plain octahedra denote metal
environments (black: Cu; white: Cr). Dashed lines refer to hydrogen bonds. Bipyridine carbon atoms are omitted.

E. Coronado et al. / Polyhedron 22 (2003) 3115–3122
3119
Fig. 3. Projection of the crystal structure of 1 onto the ac-plane showing the stacking of bpy ligands along the z-axis. Plain octahedra denote metal
environments (black: Cu; white: Cr).
tral bridging unit) and 86.02(16)° (for the Cr–Cu
bridge).
Two water molecules are present in the crystal
structure of 1. One molecule bridges via hydrogen bonds
two terminal oxalate anions corresponding to adjacent
ꢂ
Cr2Cu2 complexes (O(12B)–O(2): 2.982(13) A; O(12B)–
ꢂ
O(4)(ꢁx; ꢁy ꢁ 1; ꢁz þ 1): 3.067(15) A). This organises
ꢃ
chains of tetramers running along the [011] direction
(Fig. 2). The remaining water molecule is hydrogen-
bonded to a bridging oxalate of a [Cr(bpy)(ox)2]ꢁ unit
ꢂ
(O(11)–O(8): 3.011(13) A). Noteworthy is the presence
of p–p interactions (Fig. 3) between bpy ligands from
neighbouring chains of tetramers. This results in the
formation of one-dimensional stacks with AABB to-
pology. The bpy–bpy mean plane separations between
ꢂ
coplanar molecules are 3.67(5) A (AA dimer) and
ꢂ
3.68(4) A (BB dimer). AA and BB dimers are slightly
tilted (5.53(7)°), the closest distance between them being
3.497(11) A.
ꢂ
Fig. 4. Thermal variation of the vMT product of 1. The continuous line
represents the best-fit data to the linear tetrameric model described by
Eq. (1). The dashed line refers to the best-fit data assuming j ¼ 0.
3.2. Magnetic properties
The thermal variation of the vMT product (vM is the
K. These data are characteristic of a global antiferro-
magnetic behaviour, although a significant effect of the
axial zero-field splitting of the chromium ions cannot be
discarded. Further, the absence of a maximum in the
susceptibility curve is indicative of weak magnetic in-
teractions. The magnetisation versus field curve in-
creases continuously with increasing field in the [0–5]
molar magnetic susceptibility, T is the absolute tem-
perature) for compound 1 was measured in the 2–300 K
temperature range (Fig. 4). At room temperature, the
vMT value (4.46 emu K molꢁ1) is close to the one ex-
pected for the sum of two SCu ¼ 1/2 and two SCr ¼ 3/2
isolated spins. This value remains constant on cooling
the sample down to 20 K. Below this temperature, vMT
decreases smoothly and equals 2.26 emu K molꢁ1 at 2.0
Tesla range. The magnetisation value at
5
T

3120
E. Coronado et al. / Polyhedron 22 (2003) 3115–3122
through Eq. (2) affords the following set of parameters:
J ¼ ꢁ5 cmꢁ1, jDj ¼ 9:3 cmꢁ1 (the g values were fixed to
g ¼ 2:0). However, the correlation factor for this fit is
low (R ¼ 0:994) and the D value obtained is abnormally
high, as compared to those observed in similar systems
[12–18]. The introduction of an intermolecular interac-
tion term (mean-field approximation, Eq. (3)) does not
improve the quality of the fit, giving even higher values
of D
T
ðvMT Þ
¼ vMT ꢀ
:
ð3Þ
zj0ꢀvM
T
inter
T ꢁ
Ng2l2B
This clearly indicates that the magnetic model described
by Eq. (2) is not appropriate. This fact is corroborated
by the analysis of the field dependence of the magneti-
sation of the tetramer. Clearly, the calculated data using
the above-mentioned parameters (dashed line in Fig. 5)
deviate considerably downwards from the experimental
points at high field values. In a second step, the magnetic
data (thermal variation of vMT and field dependence of
the magnetisation) were simultaneously analysed
through the full spin Hamiltonian (1) [24]. A set of
valuable parameters for the fit of both experimental
curves was found for positive values of J and D. These
two parameters were strongly correlated and D was al-
lowed to vary in the range jDj < 5 cmꢁ1. In all cases, the
fitting procedure gave negative values of j and the best-
fit data (Fig. 4, solid line) afforded the following set of
parameters: J ¼ þ2 cmꢁ1, j ¼ ꢁ0:65 cmꢁ1, jDj ¼ 4:5
cmꢁ1. A mean value for the g factor ðgCu þ gCrÞ=2 was
used, with gCr ¼ 1:98 and gCu ¼ 2:167, as obtained from
the EPR spectrum (vide infra). A scaling factor C ¼ 0:93
was also introduced to account for possible impurities.
The D value is still high as compared with the literature
data and should be regarded as an upper limiting value
that takes also into account the intermolecular interac-
tions. Moreover, these parameters give a reasonably
good fit of the isothermal magnetisation performed at 2
K (solid line in Fig. 5).
The ferromagnetic nature of the weak interaction in
the central dicopper unit is not surprising, as the mag-
netic properties of oxalato-bridged dinuclear copper(II)
complexes have been analysed in detail [25]. Strong
antiferromagnetic interactions are expected when the
dx2ꢁy2 metal orbitals lie in the plane [26,27] of the oxa-
lato bridging group, whereas weak ferromagnetic inter-
actions are observed in a few cases where the two metal
orbitals lie in parallel planes that are perpendicular to
the oxalato bridge (the so-called parallel arrangement)
[28,29]. This particular requirement is satisfied in the
structure of 1 (Fig. 6). The two copper(II) cations are
related by an inversion centre located in the middle of
the oxalato bridging ligand, in such a way that their
corresponding N2O2 basal planes are strictly parallel.
This geometry is very similar to that previously observed
in the homometallic chain [Cu(bpy)(ox)] ꢀ 2H2O, where a
Fig. 5. Field dependence of the isothermal magnetisation of 1 at 2 K.
The continuous line represents the best-fit data to the linear tetrameric
model described by Eq. (1). The dashed line refers to the best-fit data
assuming j ¼ 0.
(M ¼ 5:8 lB) indicates that saturation is not reached at
the higher field of the experiment (Fig. 5). Taking into
account the crystal structure of 1 (Cr–Cu–Cu–Cr cen-
trosymmetric linear tetramer), the analysis of the mag-
netic data can be performed by using the following spin
Hamiltonian:
2
^
^
^
^ ^
^ ^
^
H ¼ ꢁ2JSCuSCu ꢁ 2jðSCrSCu þ SCrSCuÞ þ DðSz Þ
Cr
X
^
þ
gilBSiH;
ð1Þ
i
where J is the copper–copper exchange coupling pa-
rameter and j is the chromium–copper exchange cou-
pling parameter. In a first approach, the magnetic
interaction between chromium(III) and copper(II) cen-
tres was neglected (j ¼ 0). In fact, several reports on
chromium(III)–copper(II) dimers showed that this in-
teraction is zero or weakly antiferromagnetic [22]. In this
case, the magnetic behaviour of the compound is ap-
proximated by the sum of the contributions of a cop-
per(II) dimer (Bleaney–Bowers equation) [23] and two
isolated chromium(III) ions. This corresponds to the
following analytical expression for the vMT product:
vMT ¼ vB–BT þ 2vCrT;
gC2 u
4
3 expð2J=T Þ
vB–BT ¼
ꢀ
;
3 expð2J=T Þ þ 1
gx2 1 þ ð3T =4DÞ½1 ꢁ expðꢁ2D=TÞꢃ
ð2Þ
vCrT ¼
ꢀ
4
1 þ expðꢁ2D=T Þ
gz2 1 þ 9 expðꢁ2D=TÞ
þ
ꢀ
;
32 1 þ expðꢁ2D=T Þ
where D is the zero-field splitting parameter of the Cr3þ
ion. Least-squares fit (Fig. 4, dashed line) of vMT for 1

E. Coronado et al. / Polyhedron 22 (2003) 3115–3122
3121
Fig. 6. Orbital diagram showing the coparallel arrangement of the copper(II) dx2ꢁy2 orbitals.
ferromagnetic interaction (J ¼ þ1:22 cmꢁ1) was also
3.3. EPR spectra
observed [30]. The higher value of the interaction pa-
rameter (J ¼ þ2 cmꢁ1) observed in our case may be
attributed to a smaller axial distance (Cu(1)–O(9):
The EPR spectrum of 1 at room temperature (Fig. 7)
shows a broad feature centred at g ¼ 2:167 that can be
attributed to the copper(II) centres. The shape of this
EPR band does not change significantly with tempera-
ture and the position of the g-factor remains also un-
altered down to 4 K (see Supplementary Material).
ꢂ
ꢂ
2.242(5) A versus 2.313(2) A in [Cu(bpy)(ox)] ꢀ 2H2O).
The exchange coupling between adjacent octahedral
chromium(III) and copper(II) ions through an oxalate
anion in dimeric species has been reported to be weakly
ferromagnetic [31]. The observed behaviour was ex-
plained in terms of strict orthogonality between t2g and
eg magnetic orbitals of the chromium(III) and copper
(II) centres, respectively. This argument is valid for an
ideal coplanar arrangement of the axially distorted
copper(II) basal plane and the plane of the bridging li-
gand. In other cases, as in compound 1, the arrangement
of these planes is perpendicular and the elongated axis
of the copper(II) ion is extremely bent (O(7)–Cu–
O(9) ¼ 159.9°). Under these circumstances, the copper
magnetic orbital can have some dz2 character and it can
overlap with the chromium(III) orbitals of the same
symmetry, stabilising an antiferromagnetic interaction.
4. Conclusion
In this paper we have described the synthesis, struc-
ture and magnetic properties of the tetranuclear com-
pound [Cr2Cu2(bpy)4(ox)5] ꢀ 2H2O (1). Although the
magnetic properties of dinuclear and extended oxalato-
bridged compounds are known in detail, reports on ol-
igonuclear species are rather scarce. The analysis of the
structure and magnetism of the title compound has been
compared to previous work on dinuclear copper(II) and
chromium(III)–copper(II) complexes. Complex 1 be-
longs to a rare class of dicopper(II) l-oxalato-com-
pounds that exhibit a parallel arrangement of the
magnetic orbitals in a plane perpendicular to the oxalate
bridge. This arrangement hinders the appearance of
strong exchange coupling between the adjacent metal
centres, but stabilises a positive (ferromagnetic) inter-
action within the central dicopper subunit.
5. Supplementary material
Figure S1 showing the EPR spectra of 1 in the 300–
4.2 K range is available from the authors on request.
Crystallographic data for the structure analysis have
been deposited with the Cambridge Crystallographic
Data Center, CCDC No. 203684 for 1. Copies of this
information may be obtained free of charge from
CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK
(fax: +44-1223-336033; e-mail: deposit@ccdc.cam.acuk
or www: http://www.ccdc.cam.ac.uk).
Fig. 7. EPR spectrum of 1 at room temperature.

3122
E. Coronado et al. / Polyhedron 22 (2003) 3115–3122
~
[12] M.C. Munoz, M. Julve, F. Lloret, J. Faus, M. Andruh, J. Chem.
Soc. Dalton Trans. (1998) 3125.
Acknowledgements
€
[13] G. Marinescu, R. Lescouezec, D. Armentano, G. De Munno, M.
ꢀ
ꢀ
Dr. Gimenez-Saiz and Mr. Martınez-Agudo are ac-
knowledged for their continuous support in the mag-
netic and X-ray measurements. We also thank financial
support from Projects No. HPRN-CT-1999-00012
(EU), MAT2001-5408-E (MCYT) and Programa
Andruh, S. Uriel, R. Llusar, F. Lloret, M. Julve, Inorg. Chim.
Acta 336 (2002) 46.
€
~
[14] G. Marinescu, M. Andruh, R. Lescouezec, M.C. Munoz, J. Cano,
F. Lloret, M. Julve, New J. Chem. 24 (2000) 527.
[15] J.A. Broomhead, Aust. J. Chem. 15 (1962) 228.
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