Magnetic properties and crystal structure of a one-dimensional phase of tetrakis(μ2-benzoato-O,O′)-bis(dimethyl sulfoxide)dicopper(II)

Article abstract of DOI:10.1021/ja055316r

A new crystalline polymorphic phase of tetrakis(μ₂-benzoato- O,O′)-bis(dimethyl sulfoxide)dicopper(II) was obtained by direct synthesis, in space group P2₁/n. The copper coordination is in a slightly distorted square pyramidal geometry with an intramolecular Cu...Cu distance of 2.6494(8) A. The Cu-O distances of the two copper in a dimer are different, giving different chemical environments for each Cu ion. The crystal structure is built up of well-separated stacking columns oriented along the b-axis, with units uniformly spaced, producing a one-dimensional (1-D) zigzag chain through Cu(II)-S...S-Cu(II) interdimer interactions [S...S separation: 3.975(2) A]. Magnetization measurements in the range 2-300 K indicate two magnetic orderings, at low temperature (T < 10 K) a weak ferromagnetic ordering is observed, and above this temperature an antiferromagnetic behavior takes place. ESR spectra at 300 and 77 K of a polycrystalline sample show the characteristic signal of zero-field with D = 0.354 cm^-1, consistent with a ferromagnetic Cu...Cu exchange interaction at low temperature.

Full text of DOI:10.1021/ja055316r

Published on Web 11/01/2005
Magnetic Properties and Crystal Structure of a
One-Dimensional Phase of
Tetrakis(µ2-benzoato-O,O′)-bis(dimethyl sulfoxide)dicopper(II)
Yasmi Reyes-Ortega,*,† Jose´ L. Alca´ntara-Flores,† Mar´ıa C. Herna´ndez-Galindo,†
Daniel Ram´ırez-Rosales,‡ Sylvain Berne`s,† Jua´n C. Ram´ırez-Garc´ıa,§
Rafael Zamorano-Ulloa,‡ and Roberto Escudero
Contribution from the Centro de Qu´ımica, ICUAP. C. U. Puebla, Pue. 72570, Me´xico,
Departamento de F´ısica, ESFM, IPN, Me´xico, D.F. 07738, Me´xico, Facultad de Ciencias
Qu´ımicas. C. U. Puebla, Pue. 72570, Me´xico, and Instituto de InVestigaciones en Materiales,
UNAM. A. Postal 70-360. Me´xico, D.F. 04510 Me´xico
Received August 15, 2005; E-mail: yreyes@siu.buap.mx.
Abstract: A new crystalline polymorphic phase of tetrakis(µ2-benzoato-O,O′)-bis(dimethyl sulfoxide)dicopper-
(II) was obtained by direct synthesis, in space group P21/n. The copper coordination is in a slightly distorted
square pyramidal geometry with an intramolecular Cu‚‚‚Cu distance of 2.6494(8) Å. The Cu-O distances
of the two copper in a dimer are different, giving different chemical environments for each Cu ion. The
crystal structure is built up of well-separated stacking columns oriented along the b-axis, with units uniformly
spaced, producing a one-dimensional (1-D) zigzag chain through Cu(II)-S‚‚‚S-Cu(II) interdimer interactions
[S‚‚‚S separation: 3.975(2) Å]. Magnetization measurements in the range 2-300 K indicate two magnetic
orderings, at low temperature (T < 10 K) a weak ferromagnetic ordering is observed, and above this
temperature an antiferromagnetic behavior takes place. ESR spectra at 300 and 77 K of a polycrystalline
sample show the characteristic signal of zero-field with D ) 0.354 cm-1, consistent with a ferromagnetic
Cu‚‚‚Cu exchange interaction at low temperature.
1. Introduction
magnetic ordering is a difficult task from the chemical point of
view. From the physical side, it is important to mention that
one-dimensional solids also present distortions in the crystalline
state, due to effects related to Peierls transitions, as spin density
waves or charge density waves.10-15 These processes in some
compounds may be important if the magnetic ordering is related
to a distortion due to a spin density wave.
In the present work we report studies on the synthesis, crystal
structure, and magnetic characteristics of a new polymorph of
a well-known material, namely tetrakis(µ2-benzoato-O,O′)-bis-
(dimethyl sulfoxide)dicopper(II), 1a. The most interesting aspect
is that this phase exhibits a magnetic transition related to
ordering at low and at high temperatures. We propose a
mechanism for the magnetic ordering, based on magnetic
exchange of the copper ions bridged by sulfur atoms of
symmetry-related molecules, or by an antiparallel spins align-
Structural characteristics of binuclear copper complexes, such
as dimeric copper(II) species, have been reported in the past.
They proved difficult to obtain, particularly when unpaired
electrons should be located in orbitals with orthogonal
interactions.1-8 The magnetic behavior in these compounds is
strongly related to the Cu‚‚‚Cu distance, bond angles, neighbor-
ing atoms, local symmetry, and bridging atoms.1-8 Heintz et
al. found that the coupling leading to a parallel alignment of
spins is so weak that it hardly could persist at room tempera-
ture;9 this implies that the synthesis of materials with ferro-
† Centro de Qu´ımica, ICUAP.
‡ Departamento de F´ısica, ESFM.
§ Facultad de Ciencias Qu´ımicas.
Instituto de Investigaciones en Materiales, UNAM.
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J. AM. CHEM. SOC. 2005, 127, 16312-16317
10.1021/ja055316r CCC: $30.25 © 2005 American Chemical Society

Ferromagnetic Ordering in Binuclear Copper Complexes
A R T I C L E S
Table 1. Crystallographic Data of 1a
ment of the two copper(II)-homonuclear units through the
intramolecular eight oxygen bridging atoms belonging to the
four benzoato ligands.
formula
fw
crystal system
space group
Z, Z′
C32H32Cu2O10S2
767.78
monoclinic
P21/n
2. Experimental Methods
4, 1
2.1. Electronic spectra were measured with a Shimadzu UV-
3100S spectrophotometer on ca. 10-4 M chloroform solutions
at 298 K (λ ) 200-2200 nm). A Nicolet Magna-IR 750
spectrophotometer was employed to monitor the infrared spectra,
using KBr and polyethylene pellets. ESR spectra at X-band
frequency were obtained with a JEOL JES-RES 3X spectrom-
eter, from 300 to 77 K, on polycrystalline powder samples.
Magnetic measurements were performed in gelatin capsule using
a Quantum Design magnetometer. The calculated Pascal16
constant for compound 1a is about 264 × 10-6 emu and was
not taken into account, owing to its small value as compared
with the measured data. Measurements were performed in small
magnetic fields, from 20 to 180 G, and in the range 2-300 K,
in zero field-cooling (ZFC) and field-cooling (FC) modes.
2.2. The synthesis of 1a was carried out using the direct
synthesis method.17 Equimolar amounts (0.3143 mmol) of
copper(0), (-)-sparteine (sp), benzoyl bromide, and dimethyl
sulfoxide (DMSO) (0.85 mL) were placed in a flask, and the
mixture was stirred at 70 °C for 24 h. The mixture was then
filtered at 70 °C and the resulting solution cooled to room
temperature, allowing the separation of 1a as dark-green crystals
suitable for X-ray crystallography (yield: 5%), 1b, which was
identified as the phase previously reported for the same
compound (yield: 64%),2 and other products identified as:
[(Br)(PhCO2)(sp)]Cu(II),18 [Br2(sp)]Cu(II),19 [Br2(DMSO)2]Cu-
(II),20 and [(PhCO2)2(sp)]Cu(II).21 1a: mp 268-270 °C dec
UV-vis /CHCl3 λmax/ꢀ (nm/M-1 cm-1): 273/2479, 667/731;
IR/KBr, PET ν (cm-1): ν (COO) 1633 st. asymm., ν (COO)
1406 st. symm., ν(DMSO) 1024, ν (Cu-O) 495; ESR (powdered
sample) at 300 and 77 K: g1 ) 16.966, g2 ) 3.508, g3 ) 2.287,
g4 ) 1.953, g5 ) 1.417, g6 ) 1.385.
a, Å
b, Å
c, Å
12.0407(9)
17.0916(13)
17.2680(15)
103.062(6)
3461.7(5)
1.473
1.402
0.0540, 0.0840
1.016
â, deg
V, Å3
F
calc, g.cm-3
m, mm-1
a
R1, wR2
GOF a
a
∑||Fo| - |Fc||
∑|Fo|
∑w(Fo2 - Fc2)2
∑w(Fo2 - Fc2)2
R1 )
, wR2 )
, S )
2
∑w(Fo )2
x
m - n
x
to model S atoms with disordered sites, although slightly
improving R indices, were unsatisfactory, since they produced
prolate thermal ellipsoids and did not converge to a sensible
geometry for the minor parts of disordered sites. Highest
residuals probably reflect thermal motions for S atoms rather
than actual disorders.
3. Results and Discussion
3.1. Synthesis. The direct synthesis method, based on the
use of zerovalent metals as starting materials, provides a suitable
environment for the synthesis of different complexes of copper-
(II) as many ligands are present in the reaction medium. The
control of the reaction conditions (temperature, reaction time,
and solvent) allows increasing the yield of the complex of
interest without residual salts. The absence of residual salts
greatly favors the formation of pure phases, which may be easily
separated from the reaction medium.25 In the present work, this
methodology was essential for the isolation of two polymorphs
for the complex [(PhCO2)4(DMSO)2]Cu2, 1a and 1b, one of
which, 1a, is obtained with 5% yield.
3.2. Solid-State Structure. The asymmetric unit of 1a
consists of one binuclear Cu(II) complex in general position,
including four bridging benzoato ligands and two coordinated
DMSO molecules (Figure 1). The geometry around each Cu-
(II) ion in the dimer unit may be described by a slightly distorted
square pyramid, consistent with small geometric τ parameters:
2.3. X-ray diffraction data were collected at 296 K on a P4
diffractometer using Mo KR radiation (λ ) 0.71073 Å) and
following standard procedures.22 Relevant crystallographic data
are listed in Table 1. Data were collected at 0.84 Å resolution
and corrected for absorption effects on the basis of 20 ψ-scans
(transmission factors in the range 0.673-0.774). The structure
was solved and refined using routine methods.23,24 H atoms were
placed in idealized positions and constrained to ride on their
carrier C atoms. In the last difference map, two high residuals
are observed close to S atoms of DMSO molecules. Attempts
26
0.026 Å for Cu1 and 0.100 Å for Cu2. Four O atoms of the
four benzoato moieties conform the square base, with Cu-O
bond lengths spanning the small range 1.947(3)-1.984(3) Å
(Table 2). DMSO molecules are axially coordinated to the
metallic centers through their O atoms, at expected distances,
2.137(3) and 2.157(3) Å. The bridging networks Cu-O-C-
O-Cu are planar and give a Cu‚‚‚Cu separation of 2.6494(8)
Å. This is shorter than some distances previously reported1,8,15,27-29
but comparable to the mean value of 2.661 Å, computed from
(16) O’Connor, C. J. In Progress in Inorganic Chemistry; Lippard, S. J., Ed.;
Wiley: EUA, 1982; Vol. 29, p 203.
(17) Gutie´rrez, R.; Va´zquez, J.; Va´zquez, R. A.; Reyes, Y.; Toscano, R. A.;
Martinez, M.; AÄ lvarez, C. J. Coord. Chem. 2001, 54, 313-321.
(18) Reyes-Ortega, Y.; Alca´ntara-Flores, J.-L.; Herna´ndez-Galindo, M. C.;
Gutie´rrez-Pe´rez, R.; Ram´ırez-Rosales, D.; Berne`s, S.; Dura´n-Herna´ndez,
A.; Zamorano-Ulloa, R. J. Mol. Struct. Manuscript submitted
(19) Alca´ntara-Flores, J. L.; Ram´ırez-Rosales, D.; Berne`s, S.; Pe´rez-Ram´ırez
(Bokhimi), J. G.; Dura´n-Herna´ndez, A.; Gutie´rrez-Pe´rez, R.; Zamorano-
Ulloa, R.; Reyes-Ortega, Y. J. Mol. Struct. 2004, 693, 125-131.
(20) Willett, R. D.; Jardine, F. H.; Roberts, S. A. Inorg. Chim. Acta 1977, 25,
97-101.
(25) Sikorav, S.; Bkouche-Waksman, I.; Kahn, O. Inorg. Chem. 1984, 23, 490-
495.
(21) Reyes-Ortega, Y.; Alca´ntara-Flores, J.-L.; Herna´ndez-Galindo, Ma. del C.;
Gutie´rrez-Pe´rez, R.; Ram´ırez-Rosales, D.; Berne`s, S.; Cabrera-Vivas, B.
M.; Dura´n-Herna´ndez, A.; Zamorano-Ulloa, R. Manuscript to be submitted.
(22) XSCAnS Users Manual, release 2.21; Siemens Analytical X-ray Instruments
Inc.: Madison, WI, 1996.
(23) Sheldrick, G. M. SHELXTL-Plus, release 5.10; Siemens Analytical X-ray
Instruments Inc.: Madison, WI, 1998.
(24) Sheldrick, G. M. SHELX 97-2, Users Manual; University of Go¨ttingen:
Germany 1997.
(26) Addison, A. W.; Nageswara Rao, T.; Reedijk, J.; van Rijn, J.; Verschoor,
G. C. J. Chem. Soc., Dalton Trans. 1984, 1349-1356.
(27) McKee, V.; Dagdigian, J. V.; Bau, R.; Reed, C. A. J. Am. Chem. Soc.
1981, 103, 7000-7001.
(28) Rodriguez-Mart´ın, Y.; Ruiz-Pe´rez, C.; Sanchiz, J.; Lloret, F.; Julve, M.
Inorg. Chim. Acta 2001, 318, 159-165.
(29) Mallah, T.; Boillot, M.-L.; Kahn, O.; Gouteron, J.; Jeannin, S.; Jeannin,
Y. Inorg. Chem. 1986, 25, 3058-3065.
9
J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005 16313

A R T I C L E S
Reyes-Ortega et al.
Figure 1. Molecular structure and numbering scheme for the binuclear
unit complex in 1a. Thermal ellipsoids for non-H atoms are drawn at the
30% probability level.
Table 2. Selected Bond Distances and Angles for 1a (Å, θ deg)
Bond Distances
Figure 2. View of the infinite 1-D zigzag chains parallel to the
crystallographic b-axis. Dashed lines represent Cu‚‚‚Cu separations within
dimers and intermolecular S‚‚‚S magnetic interactions generating the
supramolecular structure. For the sake of clarity, H atoms have been omitted.
Cu1-O9
Cu1-O29
Cu2-O1
Cu2-O21
Cu2-O41
1.971(3)
1.985(3)
1.964(3)
1.947(3)
2.157(3)
Cu1-O19
Cu1-O39
Cu2-O11
Cu2-O31
Cu1-O51
1.972(3)
1.962(3)
1.949(3)
1.984(3)
2.137(3)
This free rotation for DMSO, along with a symmetry
modification, is reflected in essentially different crystal structures
for 1a and 1b. In the former case, dimer units are joined through
S‚‚‚S contacts and form zigzag chains oriented parallel to the
[010] axis. It is worth noting that a single S‚‚‚S separation is
observed in this one-dimensional (1-D) supramolecular structure,
characterized by a single distance, S‚‚‚Si ) 3.975(2) Å [sym-
Bond Angles
O9-Cu1-O19
O9-Cu1-O39
O19-Cu1-O29
O19-Cu1-O51
O29-Cu1-O51
O1-Cu2-O11
O1-Cu2-O31
O11-Cu2-O21
O11-Cu2-O41
O21-Cu2-O41
90.45(15)
89.16(15)
89.02(15)
94.25(15)
96.62(13)
90.75(16)
89.35(15)
88.36(16)
89.70(15)
96.47(15)
O9-Cu1-O29
O9-Cu1-O51
O19-Cu1-O39
O29-Cu1-O39
O39-Cu1-O51
O1-Cu2-O21
O1-Cu2-O41
O11-Cu2-O31
O21-Cu2-O31
O31-Cu2-O41
167.05(14)
96.32(13)
167.54(15)
88.59(15)
98.17(15)
168.34(14)
95.16(14)
167.75(14)
89.07(15)
102.49(14)
3
3
metry code: (i) /2 - x, -1/2 + y, /2 - z]. In the same way,
zigzag chains are built up on a single angle, for instance 54.39°,
the angle between consecutive Cu‚‚‚Cu vectors along the chain
(Figure 2). The steric demand of benzoato groups avoid
significant contacts between neighboring chains in the crystal,
such as hydrogen bonds. 1a may be thus considered, at least at
296 K, as an actual 1-D system of the type ‚‚‚A-A-A‚‚‚ where
A is the dimer unit. In the case of 1b, a short S‚‚‚S contact is
also observed between two symmetry-related molecules in the
unit cell (3.718 Å). However, the supramolecular character of
1b is limited to these dimers of dimers, since the conformation
of DMSO molecules and intermolecular distances do not allow
linking these units to form a polymeric structure. As a result,
the crystal structure of 1a makes this system potentially
accessible to a Peierls transition at low temperature, while such
a transition is very unlikely to occur for polymorph 1b.
3.3. UV-Vis and IR Spectroscopy. The UV/vis, NIR
spectrum shows π-π* transitions at 273 nm with ꢀ ) 2479
Torsion Angles
C43-S42-O41-Cu2
C53-S52-O51-Cu1 175.7(3) C54-S52-O51-Cu1 -80.0(4)
55.7(5)
C44-S42-O41-Cu2 -47.3(5)
432 entries found in the CSD including a tetrakis(µ2-benzoato-
O,O′)dicopper(II) fragment.
This geometry for the central core of 1a is very close to that
previously described for the polymorph 1b, crystallizing in space
group C2/c, with a Cu‚‚‚Cu separation of 2.627(3) Å.2 However,
a comparison of the molecular structures for both polymorphs
shows different conformations for one of the two DMSO
molecules with respect to the central core; this DMSO molecule
is rotated by ca. 180° around the formal σ bond Cu-O(DMSO).
In the case of 1a, C-S-O-Cu torsion angles describe +sc/-
sc and ap/-sc conformation for DMSO molecules coordinated
to Cu2 and Cu1, respectively (Table 2), while in the case of
1b, these descriptors show the DMSO molecules to be +sc/-
sc and ap/+sc (sc stands for syn-clinal or gauche, and ap stands
for anti-periplanar or trans). In other words, DMSO molecules
are arranged cis with respect to the Cu‚‚‚Cu line in 1a, and
almost trans in 1b.
M-1 cm-1 and at 667 nm with ꢀ ) 731 M-1 cm-1 30
It is
.
possible to observe the characteristic band assigned to d-d
transitions for distorted square-pyramidal copper(II) centers, in
agreement with the X-ray structure determination.2,31 The IR
(30) Du, M.; Guo, Y.-M.; Chen, S.-T.; Bu, X.-H.; Ribas, J. Inorg. Chim. Acta
2003, 346, 207-214.
9
16314 J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005

Ferromagnetic Ordering in Binuclear Copper Complexes
A R T I C L E S
Figure 3. Thermal dependence of the magnetization at different magnetic
field intensities H. Note the change of the magnetization from low to high
field. At about 80 G the behavior is clearly ferromagnetic. The inset shows
M-T curve measured from low to room temperature at 100 G in the FC
mode. It seems that the magnetization is increasing from about 60 K to
above, indicating a magnetic ordering persisting at room temperature, with
a maximum at ca. 260 K.
Figure 4. Isothermal magnetization measurements M-H from 2 to 15 K.
At 2 K and 3 kG the saturation magnetization value is Ms ≈ 0.012 mB.
spectrum shows the νCOO at 1633 cm-1, and a vibration
-
characteristic of DMSO2,31,32 at 1024 cm-1. Far-infrared spec-
trum shows the characteristic vibration νCu-O at 495 cm-1 33
.
3.4. Magnetic Measurements. Magnetization vs temperature
(M-T) measurements are presented in Figure 3, where M is
the molar magnetization of the binuclear unit in both ZFC and
FC measurements. For the sake of clarity, only five M-T curves
at different field strengths are depicted in Figure 3. It should
be noted that all the curves present two branches at low
temperature; at a given field, the low branch is the ZFC part,
whereas the upper branch corresponds to FC measurement. This
irreversibility decreases as the magnetic strength increases, and
at about 180 G the irreversibility almost disappears. As the
magnetic strength is increased, the maximum observed in ZFC
is displaced at lower temperatures, approximately in a linear
form. At low field, 20 G, the maximum is at about 8 K, and
decreases to 2.5 K with a field of about 180 G. This behavior
of the magnetization curves clearly indicates a weak ferromag-
netism.11,33 The inset in Figure 3 shows the magnetic ordering
at high temperature, starting from about 12 K to above. This
behavior may be related to an antiferromagnetic ordering due
to uncompensated spin alignment. The maximum on this curve
is found at T ) 260 ( 6 K, which means that using a Bonner
and Fisher fit would result in J < 0; the susceptibility passes
through a rounded maximum at a temperature Tmax defined by
Figure 5. Isothermal magnetization M-H data at 2 K, in the range (400
G. Note the coercive field with value of about 72 G.
at 2 K, with measurements performed up to 4000 G. However,
as the temperature is increased, the saturation is not longer
reached. At 15 K, a small hysteresis is observed in M-H curves,
but this is larger than the hysteresis at 10 and 12 K measure-
ments, indicating an antiferromagnetic behavior due to canted
and uncompensated spin alignment.
In Figure 5 for example, we present M-H data measured at
2 K. The curve shows the coercive field, and in Figure 6 we
present those values, extracted from data of Figure 4. Dots
represent the experimental values of the coercive field, and the
dashed line is a fit to the equation: HC(T) ) HC(0){1 - (T/
TC)1/2}, up to about 10 K; above this temperature, the coercive
field starts to increase again, and the fit no longer holds.
However, the fit gives the following parameters: Curie tem-
perature, TC ) 9.0 K; HC(0) ) 60 G. The fit looks adequate,
indicating the onset of different uncompensated spin ordering
above 10 K. At T > 10 K the signature of the spin-uncom-
pensated antiferromagnetism is observed, showing a rising small
coercive field.
kTmax/|J| ) 0.641, giving a value J ) -282 ( 6 cm-1 34, 35
.
To understand more about the magnetic characteristics of 1a,
we studied the M-H isothermal measurements in the range 1.7-
15 K (Figure 4). A magnetization close to saturation is observed
(31) Alca´ntara-Flores, J. L.; Va´zquez-Bravo, J. J.; Gutie´rrez-Pe´rez, R.; Ram´ırez-
Rosales, D.; Berne`s, S.; Ram´ırez Bokhimi, J. G.; Zamorano-Ulloa, R.;
Reyes-Ortega, Y. J. Mol. Struct. 2003, 657, 137-143.
(32) Nakamoto, K. Infrared, Raman Spectra of Inorganic and Coordination
Compounds, 4th ed.; Wiley: New York, 1986.
(33) Li, D.; Zheng, L.; Zhang, Y.; Huang, J.; Gao, S.; Tang, W. Inorg. Chem.
2003, 42, 6123-6129.
(34) Boillot, M.-L.; Journaux, Y.; Bencini, A.; Gatteschi, D.; Kahn, O. Inorg.
Chem. 1985, 24, 263-267.
Data of M-H measured at 2 K show that the magnetization
increases rapidly up to 1100 G, and reach saturation at Ms ≈
(35) Acevedo-Cha´vez, R.; Costas, M. E.; Escudero, R. J. Solid State Chem.
1997, 132, 24-32.
9
J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005 16315

A R T I C L E S
Reyes-Ortega et al.
Figure 7. ESR spectra at 77 and 300 K of a powdered sample of 1a.
Figure 6. Coercive field vs temperature. The fit to the equation HC(T) )
H(0){1 - (T/TC)1/2} gives values: HC(0) ) 60 G, and TC ) 9.0 K. Above
10 K the coercive field increases, indicating an uncompensated antiferro-
magnetic behavior. The bars indicate the uncertainty of the measurement.
binuclear copper(II) complexes. Hence, the Cu(II) ions should
be ferromagnetically coupled.
The spin Hamiltonian characterizing the triplet ground state,
0.012 mB, assuming a g Lande´ factor of 2. Using the saturation
1
2
with total spin S ) 1 is: H ) (âHgS) + D[Sz - /3S(S + 1)]
magnetization equation Ms ) |NgµâStotal|, we estimate Stotal
)
2
2
+ E(Sx - Sy ), where D is the crystal field terms with spin S
) 1 and with axial crystal field. The eigen-energies of this
subsystem under an axial field, D * 0, E ) 0,33 is adequate to
rationalize the experimental ESR signals. From the ESR spectra,
D is approximately 0.354 cm-1. In the ∆Ms ) (2 region, no
signals are detected. There is a poorly resolved signal at 2091.99
G (g ) 3.508). Other clear features are observed at 3300.00 G
(g ) 2.287); at 3375.82 G (g ) 1.953) two more features are
in the region of 4635.20 G (g ) 1.417) and at 4741.75 G (g )
1.385). Attempts for obtaining a more complete interpretation
about this magnetically complex system from the ESR spectra
were unsuccessful.
0.006.36,37
2
2
The orbital coupling due to Cu1 ion is mainly of a dx -y
type, extended along the coordination with π orbitals of the
oxygen benzoato atoms. On Cu2 ion, the orbital is mainly a dxy
type orbital, extended along the coordination with the π orbitals
of the other oxygen atoms in the same benzoato ligand. Each
benzoato group being coordinated to the two copper(II) ions in
each dimer unit, the whole structure is distorted and the Cu1-
O-C-O-Cu2 plane may be rotated in such a way that
magnetic orbitals are located in the xy-plane and that both are
accidentally orthogonal at low temperature.24,38 This structure
2
2
implies that magnetic orbitals of each copper ion, dx -y and
dxy, are parallel and rotated by about 45° to one another. The
magnetic orbital position is only slightly overlapped, resulting
in a weak ferromagnetic ordering. The effect of temperature is
an important factor that changes the interaction between orbitals;
at room temperature the overlap integral, S, is small, but at low
temperature the S value is close to zero.7 This value changes
because positive zones of the overlap density compensate for
the negative zones. Finally, it is important to mention that
another possibility for the magnetic interactions is through
dimeric unit interaction by means of axially coordinated DMSO
molecules.
4. Conclusions
From these magnetic studies reported here, we conclude that
the observed weak ferromagnetic behavior is related to a
ferromagnetic exchange interaction via Cu(II)-Cu(II) at low
temperatures. This is consistent with the presence of a ground
triplet state observed by the zero-field signal in the ESR spectra
at 300 and 77 K, although the signals of the ESR spectrum at
77 K are smaller than those at 300 K. It is important to mention
that the behavior at low temperatures may also be related to
uncompensated antiferromagnetic ordering due to canted spins.
In line with results described by Comarmond et al.,7 in a weak
interaction, the two spin levels are thermally populated at room
temperature in such a way that Jtotal will be the sum of the
negative antiferromagnetic JAF and positive ferromagnetic JF
contributions, Jtotal ) JAF + JF, with JAF ) -2∆S and JF ) 2C
(where S is the overlap integral between magnetic orbitals and
C is the two-electron exchange integral). An additional aspect
related to the influence of a Peierls distortion in this compound,
and which may be related to the observed magnetic effects in
this material, is that spin coupling influences the formation of
a spin density wave at about 9 K (see Figure 6). Thus, the
changes from antiferromagnetic behavior at high temperature
to ferromagnetism at low temperature could be related to that
distortion. The low value of the saturation magnetization, Ms,
3.5. ESR Studies. The powder X-band ESR spectra of 1a at
300 and 77 K show four transitions (Figure 7). Spectra are
typical of a triplet state which exhibits six features arising from
-1 f 0 and 0 f +1 transitions, each with x, y, and z
components.7,39-42 There is a feature at 395.85 G (g ) 16.966)
which corresponds to the zero field split for a triplet state in
(36) Bonner, J. C.; Fisher, M. E. Phys. ReV. A 1964, 135, 640-658.
(37) Mallah, T.; Thie´baut, S.; Verdaguer, M.; Veillet, P. Science 1993, 262,
1554-1557.
(38) Ferlay, S.; Mallah, T.; Ouahe`s, R.; Veillet, P.; Verdaguer, M. Nature 1995,
378, 701-703.
(39) Kahn, O.; Tola, P.; Galy, J.; Coudanne, H. J. Am. Chem. Soc. 1978, 100,
3931-3933.
(40) Tirado-Guerra, S.; Cuevas-Garibay, N. A.; Sosa-Torres, M. E.; Zamorano-
Ulloa, R. J. Chem Soc., Dalton Trans. 1998, 2431-2436.
(41) Sugiura, Y. Inorg. Chem. 1978, 17, 2176-2182.
(42) Marsh, W. E.; Patel, K. C.; Hatfield, W. E.; Hodgson, D. J. Inorg. Chem.
1983, 22, 511-515.
9
16316 J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005

Ferromagnetic Ordering in Binuclear Copper Complexes
A R T I C L E S
at 2 K, implies that the Cu(II) electrons have canted spins. At
higher fields, a parallel spin alignment would give rise to a
higher magnetization value; this assumption explains the M-T
behavior observed when the external field is increased, changing
the antiferromagnetic behavior to a ferromagnetic ordering in
the 1-D molecular polymer 1a. In conclusion, 1a is a molecular
solid featuring structural characteristics suitable for the coexist-
ence of both magnetic phases in the bulk material.
cio´n Superior and Vicerrector´ıa de Investigacio´n y Estudios de
Posgrado from BUAP, project No. II 163-04/NAT/G.
Supporting Information Available: X-ray crystallographic
file of 1a (CIF). This material is available free of charge via
the Internet at http://pubs.acs.org. The CIF file is also available
on application to the Cambridge Data Centre, 12 Union Road,
Cambridge CB21EZ, U.K. (Fax: (+44) 1223-336-033. E-
mail: deposit@ccdc.cam.ac.uk).
Acknowledgment. The present work has been supported by
Secretar´ıa de Educacio´n Pu´blica and Sub-Secretar´ıa de Educa-
JA055316R
9
J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005 16317