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Inorganica Chimica Acta 357 (2004) 4220–4230
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Phase transitions and antiferromagnetism in copper(II) hexanoates:a new tetranuclear type of copper carboxylate
paddle-wheel association
Bojan Kozlev�car a,*, Ivan Leban a, Marko Petri�c b, Sa�sa Petri�cek a, Olivier Roubeau c,Jan Reedijk c, Primo�z �Segedin a
a Department of Inorganic Chemistry, Faculty of Chemistry and Chemical Technology, University of Ljubljana, A�sker�ceva 5, P.O. Box 537,
1001 Ljubljana, Sloveniab Department of Wood Science and Technology, Biotechnical Faculty, University of Ljubljana, Ro�zna dolina, C VIII/34, 1000 Ljubljana, Slovenia
c Gorlaeus Laboratories, Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands
Received 17 March 2004; accepted 2 June 2004
Abstract
A series of compounds of formula [{Cu2(OOCCmH2mþ 1)4(urea)}2] ðm ¼ 5–11Þ have been characterized. X-ray structure analysis
for the hexanoate compound reveals a new type of tetranuclear dicopper(II) tetracarboxylate, where the central coordination sphere
in [{Cu2(OOCC5H11)4(urea)}2] is composed of two dinuclear dicopper tetracarboxylates, connected via two inter-dinuclear Cu–O
coordination bonds at a distance 2.222(2) �A through the apical positions of two dimers. Urea molecules (Cu–O 2.114(2) �A) occupy
both outside apical positions of the resulting tetranuclear units. A strong antiferromagnetic behaviour has been shown for
[{Cu2(OOCC5H11)4(urea)}2] (�2J ¼ 261:4ð4Þ cm�1), and compared with related isolated dinuclear and polymeric hexanoate
compounds [Cu2(OOCC5H11)4(urea)2], [Cu2(OOCC5H11)4]n, respectively. Only small differences in the magnetic susceptibility have
been found, while EPR spectroscopy showed significantly different results for all three hexanoate compounds, also with the dicopper
tetracarboxylate central core and square-pyramidal CuO4O chromophores. A solid-to-solid phase transition for [{Cu2(OOCC5H11)4(urea)}2] was observed by magnetic measurement and analysed for the whole series [{Cu2(OOCCmH2mþ 1)4(urea)}2]
by TG, DTA, and variable temperature XRD studies.
� 2004 Elsevier B.V. All rights reserved.
Keywords: Copper; Hexanoates; Urea; Structure; Dinuclear; EPR
1. Introduction
Copper(II) carboxylates comprise a large and diversegroup of coordination compounds, which are interesting
due to their possible application in many areas [1–4].
Such diversity of properties is often connected to the
coordinated ligands, although special attention is also
devoted to structures of the compounds, which may
differ significantly. Sometimes even small changes in the
structure may be a reason for significant changes in
* Corresponding author. Tel.: +386-1-2419127; fax: +386-1-2419220.
E-mail address: bojan.kozlevcar@uni-lj.si (B. Kozlev�car).
0020-1693/$ - see front matter � 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2004.06.012
physico-chemical properties. Up till now, we have been
interested in copper carboxylates as wood-protecting
agents and also as candidates where liquid-crystallinemesophases may be found [5–10]. In both cases, dinu-
clear compounds have shown promising preliminary
results. Dinuclear copper(II) carboxylates differ signifi-
cantly from the other polymeric and monomeric cop-
per(II) complexes by their lower magnetic moment.
They show antiferromagnetic coupling, observed al-
ready at room temperature by magnetic susceptibility
measurements and EPR spectroscopy. The largest sub-group in the dinuclear family of compounds are isolated
dinuclear compounds, where additional ligands on the
apical positions of the central dicopper tetracarboxylate
Scheme 1. Structural types in dinuclear copper(II) aliphatic complexes with and without apical ligands (L): (a) an isolated dinuclear species
[Cu2(OOCCmH2mþ 1)4L2]; (b) a polymer of the dinuclear units [Cu2(OOCCmH2mþ 1)4]n; and (c) a dimer of the dinuclear units
[{Cu2(OOCC5H11)4L}2] (�tetramer�).
B. Kozlev�car et al. / Inorganica Chimica Acta 357 (2004) 4220–4230 4221
core are coordinated. Inter-dinuclear connection is then
possible only by H-bonding or weaker association
(Scheme 1(a)). Also a small group of polymeric dimers is
known, where direct Cu–O(carboxylate) coordination
bonds enable an infinite chain of the dinuclear units
(Scheme 1(b)). Both groups show very different results
of thermal analysis [6].
Based on the predicted dicopper tetracarboxylatestructures (with a leff � 1:4 BM at room temperature),
long aliphatic chains of the carboxylates and known
structures of related isolated dinuclear [Cu2(OO CC5H11)4(urea)2] [11] and polymeric [Cu2(OOCCm H2mþ 1)4]ncomplexes [12–15], a series of fatty acid copper(II)
carboxylates with urea [{Cu2(OOCCm H2mþ 1)4(urea)}2]
are especially interesting for thermal, magnetic and
structural investigation. The present paper reports thefirst of such a study. The main goal of this work has
been an investigation of influence of this type of hybrid
complex to the phase transitions and temperature de-
pendence of magnetic properties as vM or EPR signals.
Indeed, the analysis revealed solid-to-solid phase tran-
sitions and temperature depending EPR signals not
found in [Cu2(OOCCH3)4(H2O)2] type, both originating
in inter-dinuclear connection.
2. Experimental
2.1. Materials
The complexes [Cu2(OOCC5H11)4]n, [Cu2(OOCC5H11)4(urea)2] and a series [{Cu2(OOCCmH2mþ 1)4(urea)}2]ðm ¼ 5–11Þ, were prepared as reported earlier [1,11].
Single crystals of X-ray quality of [{Cu2(OOCC5 H11)4-
(urea)}2] were obtained by a modified procedure. Cop-
per(II) hexanoate [Cu2(OOCC5H11)4]n was dissolved in
acetonitrile acidified with hexanoic acid and added to
urea in molar ratio [Cu2(OOCC5H11)4]:urea¼ 1:2. The
crystals were grown during the slow evaporation of the
solvent on air. Measured d-spacings and their relative
intensities for [{Cu2(OOCC5H11)4(urea)}2] are in agree-
ment with the calculated values [16], obtained from the
crystal structure analysis.
2.2. X-ray crystallography
The crystal data of bis-(tetrakis(l-hexanoato-O,O0)(urea-O)dicopper(II)) were collected on a Kappa
CCD Nonius diffractometer with graphite monochro-
mated Mo Ka radiation. The structure was solved by
direct methods [17], and the figures were drawn using
ORTEP-II [18,19] and PLATONPLATON [16]. Refinements were
based on F 2 values and done by full-matrix least-squares[20] with all non-H atoms anisotropic. Hydrogen atoms
were located from a DF synthesis and included in the
refinement at calculated positions and with isotropic
displacement parameters of 1.2 times the Ueq value of
their respective attached heavy atom, 1.5 times for the
methyl hydrogens. Crystal data for C50H96Cu4N4O18:
Fr ¼ 1295:47, monoclinic, a ¼ 16:4321ð15Þ �A,
b ¼ 9:4576ð10Þ �A, c ¼ 20:3914ð18Þ �A, b ¼ 99:215ð4Þ�,V ¼ 3128:1ð5Þ �A3, T ¼ 200 K, space group P21=c (No.
14), Z ¼ 2, l (Mo Ka)¼ 1.408 mm�1, 53 387 reflections
measured, 4724 unique ðRint ¼ 0:0468Þ which were
used in all calculations. The final wRðF 2Þ was 0.0907 (all
data).
2.3. TG and DTA
Simultaneous measurements were made on a Mettler
TA 2000 under 99.999% pure argon with the flow rate of
35 ml/min. The reference material, a-Al2O3, was em-
ployed in all experiments. The first measurements for
each compound were done at a heating rate of 2 K/min
4222 B. Kozlev�car et al. / Inorganica Chimica Acta 357 (2004) 4220–4230
up to 773 K. The subsequent measurements were per-
formed at a heating rate of 1 K/min, some of them with
consequent cooling and repeated heating. The temper-
ature and enthalpy calibration were done by melting of
indium (429.6 K) and zinc (692.6 K). Repeated mea-surements have shown uncertainties in the temperature
range of ±1 K. The noise in the DTA curve is negligible
in the applied temperature range and is less than 2% of
the observed peaks.
2.4. X-ray powder diffraction
The patterns were collected on a Siemens D-5000diffractometer, with a HTK-16 high-temperature
chamber using Cu Ka radiation. The compounds were
heated in air at 10 K/min to the starting temperature,
which was than held until the end of scanning. The
samples were scanned in the 2h range between 4� and
20� in steps of 0.026� 2h and at integration time of 4 s/
step. Room temperature measurements were recorded
also on a Huber Guinier camera.
2.5. EPR
Spectra of the powdered samples were recorded by a
Bruker ESP-300 spectrometer, operating at X-band
(9.59 GHz) at variable temperatures. The values of pa-
rameters gk, g?ðgx; gyÞ, D, E and 2J were calculated di-
rectly from the signal positions Hz1, H?ðHx;HyÞ and Hz2
in the spectra [1,21–23].
Fig. 1. The tetranuclear unit in [{Cu2(OOCC5H11)4(urea)}2]. Hydro-
gen atoms have been omitted for clarity.
2.6. Bulk magnetization measurements
The data of smoothly powdered polycrystalline
samples were obtained using either a Quantum Design
MPMS-5S or MPMS-XL squid magnetometer, with an
applied field of 0.1 T. Contribution from the sampleholder was determined experimentally and corrected for,
and corrections for the diamagnetic portions of the
samples were applied as determined by use of Pascal’s
tables [24].
Table 1
Selected bond distances (�A) and angles (�) in the structure of [{Cu2(OOCC5
Cu1–O31 1.932(2)
Cu1–O41 1.938(2)
Cu1–O21 1.954(2)
Cu1–O11 1.990(2)
Cu1–O11a 2.222(2)
Cu2–O32 1.959(2)
Cu2–O42 1.969(3)
Cu2–O22 1.990(2)
Cu2–O12 1.996(2)
Cu2–O 2.114(2)a From the neighbouring dinuclear unit.
3. Results and discussion
3.1. Crystal structure
The compounds of a series [{Cu2(OOCCmH2mþ 1)4(urea)}2] were already presented and partly characterized
[11,25]. They were described also as hemiurea complex
Cu(OOCCmH2mþ 1)2(urea)0:5, due to their stoichiometry.
These compounds are related to monourea series
Cu(OOCCmH2mþ 1)2(urea), known also as isolated di-
mers of [Cu2(OOCCmH2mþ 1)4(urea)2]. We succeeded to
obtain single crystals of [{Cu2(OOCC5H11)4(urea)}2] and
X-ray structure analysis showed a new type of paddle-wheel-based complex structure. Two dicopper tetra-
carboxylate units are connected by two inter-dinuclear
coordination bonds through the apical positions by the
copper atom from one dinuclear unit to the carboxylate
oxygen atom of the adjacent dinuclear unit and vice versa
Cu1–O11i, Cu1i–O11 2.222(2) �A (Fig. 1), thus forming a
tetranuclear unit (Table 1). Both dimers are crystallo-
graphically related by the center of symmetry. The apical
H11)4(urea)}2]
O31–Cu1–O11 171.52(10)
O41–Cu1–O11 91.21(10)
O21–Cu1–O11 88.46(10)
O32–Cu2–O12 166.59(10)
O42–Cu2–O12 90.59(11)
O22–Cu2–O12 87.68(10)
O11a–Cu1–Cu2 164.24(6)
O–Cu2–Cu1 177.02(7)
B. Kozlev�car et al. / Inorganica Chimica Acta 357 (2004) 4220–4230 4223
position on the other side of the tetracarboxylate is not
occupied by another dinuclear unit, as observed in the
related polymeric compounds [Cu2(OOCCmH2mþ 1)4]n,
but by a urea molecule (Cu–O 2.114(2) �A) similarly as in
isolated dinuclear complexes [Cu2(OOCCmH2mþ 1)4(urea)2]. Therefore, the new structure is a kind of hybrid
of these two families of tetracarboxylates as schemati-
cally presented in Scheme 1(c). Such types of tetracarb-
oxylates have been reported earlier, but only for rhodium
complexes [26], while with copper a similar tetranuclear
central core analogue in a N–C–O bridged (instead of
carboxylate O–C–O) compound has been observed [27].
Five oxygen atoms are coordinated around each copperatom in the square-pyramidal arrangement, thus forming
a CuO4O chromophore in all cases. In the polymeric
hexanoate complex [12], the distance of Cu atom to O4
plane is 0.174(1) �A, while in isolated hexanoate dimers
[11], analogous is 0.216(1) �A. The distance of copper
atom out of the basal O4 plane in CuO4O is different in
[{Cu2(OOCC5H11)4(urea)}2]; 0.230(1) �A for CuO4O
(urea) and 0.138(1) �A for CuO4O(carb.). The Cu–Cudistance in dimers in the tetranuclear unit is 2.6061(6) �A,
that is close to average values in polymers 2.5791(5) �Aand in isolated dimers 2.6439(12) �A.
In the structure of tetranuclear [{Cu2(OOCC5H11)4(urea)}2], the hydrogen bonds N(urea)–H� � �O(carb.) are
present as in related [Cu2(OOCC5H11)4(urea)2] [11].
There are two intermolecular (N1–H1A� � �O22 3.019(4)�A, N1–H1B� � �O21 3.022(4) �A) and one intramolecularhydrogen bond (N2–H2A� � �O12 2.987(4) �A) per urea
molecule (Fig. 2). Both, tetranuclear and isolated dinu-
clear complexes crystallize in the same monoclinic P21=c(No. 14) space group. One carboxylate group in each
tetracarboxylate moiety is involved in an intramolecular
hydrogen bond (N2–H2A� � �O12) and in an inter-dimer
association (Cu1i–O11). At the same time, only one
carboxylate group perpendicular to the previous one,forms two intermolecular hydrogen-bonds connecting
the neighbouring tetranuclear molecules to an extended
network.
Fig. 2. Hydrogen bonding network (N–H� � �O) in [{Cu2(OOCC5H11)4(urea)}2] forming a layered lattice structure. The alkyl chains are
omitted for clarity.
The other factor that influences the crystal packing is
a paraffin chain orientation. Two hexanoate chains in
each tetracarboxylate group are completely elongated in
a fully extended zig-zag trans arrangement, while the
other pair aligns parallel to them in the gauche confor-mation at the C12–C13 bond (Fig. 3). Such orientation
enables a layered scheme of non-polar paraffin chains
and polar tetracarboxylate–urea moieties. Gauche ori-
entation and alternate polar–non-polar layers were
found also in polymeric [Cu2(OOCC5H11)4]n [12,28], but
only layers without gauche-oriented chains in isolated
urea hexanoate [Cu2(OOCC5H11)4(urea)2] [11,28].
It was observed that [{Cu2(OOCCnH2nþ 1)4(urea)}2]can be isolated for pentanoate and longer chain ana-
logues and [Cu2(OOCCnH2nþ 1)4(urea)2] only for hexa-
noate and shorter chain analogues [11,25]. Longer alkyl
chains therefore probably enable additional stabilization
of the tetramer structure, which favours its precipitation
against the isolated dimer. In addition to this, every urea
molecule forms three hydrogen bonds in tetranuclear
and isolated dinuclear complexes. Therefore, isolateddimers are favoured in the case of shorter alkyl chains,
possibly due to higher number of hydrogen bonds per
dinuclear unit.
3.2. Thermal analysis and X-ray powder diffraction
[{Cu2(OOCCmH2mþ 1)4(urea)}2] ðm ¼ 5–11Þ decom-
pose in two well-separated steps. The decompositionbegins in the tetranuclear complexes above 410 K with
the loss of urea (Table 2), followed by melting and
further decomposition above 510 K as characteristic for
the polymeric copper(II) carboxylates (identified by
powder XRD). On the contrary, loss of urea and fol-
lowing decomposition of copper(II) carboxylate can
practically not be separated in [Cu2(OOCC5H11)4(urea)2] (Fig. 4). Repeated heating–cooling cycles weredone for [{Cu2(OOCCmH2mþ 1)4(urea)}2] (m¼ 7, 8, 10).
Urea is removed during the first heating up to 463 K.
The formed [Cu2(OOCCmH2mþ 1)4]n (m¼ 7, 8, 10) un-
dergo during the second heating solid-to-liquid crystal
phase transitions at temperatures, which are typical for
the polymeric copper(II) carboxylates [10]. A similar
formation of copper(II) carboxylates during decompo-
sition of complex was observed also for complexes withpyridine [Cu2(OOCCmH2mþ 1)4(py)2] ðm ¼ 6–11Þ [6].
An endothermic peak prior to the first decomposition
step at Ttrans was observed for the tetranuclear series
(Fig. 5), with an exception for the dodecanoate complex
(Table 2). Repeated heating up to 405 K and subsequent
cooling was performed for compounds [{Cu2(OOCCm
H2mþ 1)4(urea)}2] ðm ¼ 5–9Þ to check the enantiotropic
nature of the phase transitions. DTA curves obtainedduring subsequent heating were practically unchanged.
The transition temperatures increase with longer
paraffin chains of the fatty acids and get closer to the
Fig. 3. Parallel orientation of the hexanoate chains in molecular packing of [{Cu2(OOCC5H11)4(urea)}2]. The paraffin non-polar spacings between
polar tetracarboxylate–urea moieties. Hydrogen atoms have been omitted for clarity.
4224 B. Kozlev�car et al. / Inorganica Chimica Acta 357 (2004) 4220–4230
temperatures of the decomposition. Only decompositionand no transition was observed in the complexes of
higher fatty acids like in [{Cu2(OOCC11H23)4(urea)}2].
The results of variable temperature X-ray powder dif-
fraction studies proved a solid-to-solid phase transition
in [{Cu2(OOCCmH2mþ 1)4(urea)}2] ðm ¼ 6–9Þ prior to
loss of urea. Significant inter-dinuclear association
(Scheme 1(b)) through copper–oxygen bonds between
[Cu2(OOCCmH2mþ 1)4] units that forms chains, is obvi-
Table 2
The temperature of solid-to-solid phase transition in [{Cu2(OOC-
C5H11)4(urea)}2] ðTtransÞ, the first step of decomposition – loss of urea
ðTdecompÞ and the enthalpy of solid-to-solid phase transition ðDHtransÞdetermined from DTA curves
m Ttrans (K)a Tdecomp (K)a DHtrans (kJ/mol)b
5 361 410 8.0
6 370 411 12
7 397 412 13
8 398 415 19
9 398, 403 416 23
10 413 418 29
11 418a ±1 K.b ±10%.
ously decisive for the formation of liquid crystallinephase in the polymeric copper(II) tetracarboxylates.
These chains are interrupted in [{Cu2(OOCCmH2mþ 1)4(urea)}2] by urea and only pairs of dimers are connected
by Cu–O bonds, which may prevent formation of liquid
crystalline phase. A solid-to-solid phase transition was
observed by a comparison of X-ray powder diffraction
patterns of [{Cu2(OOCC7H15)4(urea)}2] at room tem-
perature (solid phase 1) and at 388 K (solid phase 2)(Fig. 6(a) and (b)). A weak peak at 2h ¼ 6.18� indicating
-100
-80
-60
-40
-20
0
400 500 600 700
DTA 6/2
DTA 6/1
5 Vµ
∆m (%)
T (K)
Fig. 4. TG and DTA curves for [{Cu2(OOCC5H11)4(urea)}2] (DTA
6/1) and [Cu2(OOCC5H11)4(urea)2] (DTA 6/2).
B. Kozlev�car et al. / Inorganica Chimica Acta 357 (2004) 4220–4230 4225
traces of liquid crystalline phase of [Cu2(OOCC7H15)4]n,
appears already at 398 K beside the characteristic peaks
of prevalent solid phase 2 (Fig. 6(c)). After heating to
423 K, the tetranuclear complex almost com-
pletely decomposes to liquid crystalline phase of[Cu2-(OOCC7H15)4]n (Fig. 6(d)). The observed solid-to-
solid phase transitions could be attributed to changes in
the orientation of paraffin chains in some tetranuclear
complexes.
Fig. 6. X-ray powder diffraction patterns of tetranuclear [{Cu2(OO
CC7H15)4(urea)}2]: (a) solid phase 1 at room temperature; (b) solid
phase 2 at 388 K; (c) solid phase 2+ traces of liquid crystalline phase of
[Cu2(OOCC7H15)4]n at 398 K; and (d) liquid crystalline phase of
[Cu2(OOCC7H15)4]n and traces of solid phase 2 at 423 K.
300 350 400 450
(c)
S-->S
(b)
S-->LC
S-->S
LC-->S
1 Vµ
(a)
T / K
Fig. 5. DTA curve for a repeated heating and cooling of the
[{Cu2(OOCC8H17)4(urea)}2]. The first heating (a) with a solid-to-solid
phase transition at 398 K (SfiS in urea complex), followed by a loss
of urea (b) and formation of liquid crystalline phase of [Cu2(OOCC8H17)4]n above 430 K. The formed [Cu2(OOCC8H17)4]n un-
dergoes a typical liquid crystalline phase transition during the second
heating cycle at 377 K (SfiLC) (c).
A mixture of copper(I) and copper(II) oxides and
copper is a result of decomposition of all studied urea
complexes at 773 K, similar as in other copper(II)
carboxylates and their complexes.
3.3. EPR and magnetic measurements
EPR spectra of the three structurally different cop-
per(II) hexanoates (isolated dinuclear [Cu2(OOCC5H11)4(urea)2], polymer [Cu2(OOCC5H11)4]n and tetranuclear
[{Cu2(OOCC5H11)4(urea)}2]) were recorded at different
temperatures (Figs. 7–9) and analysed.
The ambient temperature spectrum of the isolateddinuclear compound [Cu2(OOCC5H11)4(urea)2] shows
classical triplet state feature of copper(II) acetate hy-
drate type, where three resonance lines (Hz1, H? and Hz2)
are observed (Fig. 7), that may be described by the spin
Hamiltonian (Eq. (1)) [1,21–23]
H ¼ DS2z þ EðS2
x � S2y ÞgzHzSz þ gxHxSx þ gyHySy ; ð1Þ
D and E values are the tetragonal and rhombic zero-field
splitting parameters and S ¼ 1. x, y and z are the prin-
cipal coordinate axes, where z-axis is taken parallel to
the Cu–Cu direction inside the dimer. The other symbols
have their usual meaning. By lowering of the tempera-
ture to 95 K, Hz1 signal in the spectrum of[Cu2-(OOCC5H11)4(urea)2] splits, due to hyperfine in-
teraction of the unpaired electron with two equivalent
copper nuclei within the dinuclear unit (A ¼ 6:2 mT).
In the room temperature spectrum of the polymeric
hexanoate [Cu2(OOCC5H11)4]n, broad and poorly re-
solved bands are found (Fig. 8). The strongest signal is
positioned between 300 and 430 mT (H* in Table 3),
Fig. 7. Copper(II) acetate hydrate type EPR spectra in isolated dinu-
clear compound [Cu2(OOCC5H11)4(urea)2].
Fig. 8. EPR spectra of the polymeric [Cu2(OOCC5H11)4]n, measured at
different temperatures.
0 200 400 600
H (mT)
I
291 K
200 K
95 K
Fig. 9. EPR spectra of the tetranuclear [{Cu2(OOCC5H11)4(urea)}2],
measured at different temperatures.
4226 B. Kozlev�car et al. / Inorganica Chimica Acta 357 (2004) 4220–4230
while the other two signals are placed close to 0 mT and
at 450–500 mT. Those are the regions, where Hz1 and H?signals are observed in spectra of the copper acetate
hydrate type of compounds. Hz2 resonance line is not
noticed, possibly due to overlap with the broad H?signal nearby. The intensity of the dominant signal de-
creases at lower temperature (200 K), while further
lowering of the temperature results in the disappearanceof this signal in the spectrum measured at 95 K (Fig. 8).
In the same region some weak signals are noticed in-
stead, which are assigned to mononuclear impurities.
Simultaneously, by lowering of the temperature, the Hz1
and H? signals become narrower and the Hz2 signal at
600 mT emerge. The H? signal splits (95 K) into two
signals (Hx 456 mT, Hy 496 mT), what is attributed to
the rhombic deviation from the axial symmetry (E is not
negligible). Similar results were reported for hexanoate
analogues with longer paraffin chains [Cu2(OOCCm
H2mþ 1)4]n ðm ¼ 6–11Þ [29,30].The tetranuclear [{Cu2(OOCC5H11)4(urea)}2] room
temperature EPR spectrum differs significantly from the
spectra of isolated dinuclear or polymer compounds.
The signals corresponding to Hz1, H? and Hz2 are found
as expected for dinuclear Cu tetracarboxylates (Fig. 9),
however, in the regions among them, additional set of
signals was observed (H* in Table 3). The intensity of
these additional resonance lines is decreasing with low-
ering of the temperature and they disappear completelyin the spectrum, measured at 95 K (Fig. 9). Simulta-
neously, the linewidths for the signals corresponding to
Hz1, H? and Hz2 become narrower and additional signals
appear at 330–340 mT in the 95 K spectrum. Although
the region of 330–340 mT is very narrow, these bands
are probably due to mononuclear impurities. The
structural origin of the ‘new’ set of the signals is prob-
ably related to the differencies in the tetranuclearstructure with respect to the isolated dinuclear com-
pounds. As noticed for [{Cu2(OOCC5H11)4(urea)}2] and
[Cu2(OOCC5H11)4]n (and its analogues with longer
paraffin chains) [29,30], similar extra set of signals was
found in another complex, where the dicoppertetra-
carboxylate moiety connects two tetranuclear
Cu4 (OCH3)4(OOCCH3)4 units [31].
The absence of the hyperfine splitting of Hz1 signal inthe tetranuclear spectrum, measured at 95 K, is in
agreement with non-equivalent copper nuclei in each
dimer. Similar absence of splitting for Hz1 was noticed
also for the polymeric [Cu2(OOCC5H11)4]n, however, its
structure reveals (as well as for the polymeric analogues
with longer fatty chains) [12–15,29] equivalency of both
copper atoms, therefore additional explanation is nee-
ded. It is worth to mention, that intensities of the signalsin the room temperature spectrum of [Cu2(OOCC5H11)4]nare not comparable with intensities of the signals in the
spectra of [{Cu2(OOCC5H11)4(urea)}2] and [Cu2(OOC-
C5H11)4 (urea)2] complexes, if the spectra are measured
at the same parameters. The polymer signals have much
lower intensity, therefore the hyperfine structure might
not be observable due to lower resolution (all spectra
presented in this paper are normalized, therefore theintensities are not absolutely comparable).
In all herein presented spectra, the resonance lines
Hz1, H? and Hz2, as observed in copper(II) acetate hy-
drate type of complexes, are attributed to a triplet state
ðS ¼ 1Þ, with axial symmetry (D � 0:35 cm�1) [1], while
E is not negligible only in the spectrum of [Cu2(OOCC5H11)4]n, measured at 95 K (Fig. 8). The origin
of the additional set of signals has been given in theliterature by several ways, that are all based on obser-
vations, that for copper(II) complexes, where the inter-
dinuclear interactions play an important role, the
Table 3
Selected EPR parameters for different types of hexanoate complexes, measured at different temperatures
[Cu2(OOCCmH2mþ 1)4L2]
isolated dimmer
[Cu2(OOCCmH2mþ 1)4]npolymer
[{Cu2(OOCC5H11)4(urea)}2]
tetramer
T (K) 291 200 95 291 200 95 291 200 95
Hz1 (mT) 32.5 30.2 28.7 �0 �0 13.3 22.8 19.4 18.8
H? (mT) 476 476 476 �480 �485 456 ðHxÞ,496 ðHyÞ
472 473 474
Hz2 (mT) 601 604 604 �590 600 594 598 598
H* (mT) �370 �370 252, 391 247, 391
g? ðgx; gyÞ 2.09 2.09 2.09 2.04 ðgxÞa 2.06 ðgxÞb 2.06c 2.09 2.08 2.08
2.09 ðgyÞa 2.07 ðgyÞbgk ðgzÞ 2.41 2.39 2.38 2.35a 2.35b 2.33c 2.40 2.37 2.37
D (cm�1) 0.356 0.353 0.35-
2
0.340a 0.338 b 0.334c 0.345 0.341 0.341
E (cm�1) 0.0138a 0.0114b
j2J j (cm�1) 215 240 244 257a 264b 279c 228 258 253
H values were obtained from the spectra, while the other parameters were calculated from equations as described in the last paragraph of the EPR
section [1,21–23].H* – the signals, not characteristic for X-band EPR spectra of copper(II) acetate hydrate type complexes, but noticed in the spectra
of polymeric and tetrameric hexanoates.a;bThe results of two iterative calculations, considering gx < gy ðHx < HyÞ and 2:00 < gx, gy < 2:10cThe approximate calculations i.e., E � 0.
B. Kozlev�car et al. / Inorganica Chimica Acta 357 (2004) 4220–4230 4227
number of signals, their intensity and line widths in the
100–500 mT region of X-band EPR spectra are chang-
ing with variation of the temperature. Broadening of the
signals in anhydrous complexes (e.g., [Cu2(OOCCm
H2mþ 1)4]n), might be due to chemical equilibrium of the
monomeric Cu(II) species ðS ¼ 1=2Þ and dinuclear
Cu(II) species ðS ¼ 1Þ [29]. Among all the spectra foundin the literature, the room temperature spectrum of the
polymeric complex composed of dinuclear and tetra-
nuclear units [31] is most similar to the room tempera-
ture spectrum of [{Cu2(OOCC5H11)4(urea)}2]. A set of
the signals in the range 100–450 mT was assigned to the
higher states ðSP 2Þ of the tetranuclear units, populatedat room temperature. Higher states were included for
the detailed explanation of EPR spectra also for theother type of tetranuclear copper cluster, with distorted
square-pyramidal co-ordination geometry around cop-
per, where non-typical bands were found in the same
region [32]. Next to these two theories, another one has
been presented [33–35] that might give a complete
overview also for our investigations. Such spectra could
be explained by the presence of intermolecular exchange
interactions between excited triplet states. The proba-bility for them to occur depend on the effective con-
centration of magnetic species (i.e., the triplet-state
population), that is temperature depending. As a model,
triplet excitons were proposed, which can migrate
through the crystal lattice (i.e., from one molecule to
another). That can lead to a removal of the hyperfine
structure of an EPR spectrum, because of the randomly
changing nuclear-spin environment. A collision of theexcitons, leading to the change of the spin orientation is
possible and the frequency of these collisions may av-
erage not only the hyperfine but also the fine structure of
the spectrum [33].
The last theory may provide an explanation for a
presence of the additional set of resonance lines, their
width and intensities, together with their correlation to
the varying temperature. Extremely broad signals and
absence of the hyperfine structure in the polymeric
complexes can be interpreted as well. Very similar anal-
ysis for the EPR spectra of dinuclear and tetranuclearcopper N–C–O bridged paddle-wheel analogues have
been presented by others [27]. Three types of resonance
(due to mononuclear and dinuclear Cu(II) species and
intermolecular interactions between dinuclear species)
were noticed with distinct temperature dependence.
The magnetic properties of the three hexanoate
compounds corresponding to the three structural types
shown in Scheme 1, i.e., the dinuclear [Cu2(OOCC5
H11)4(urea)2], the tetranuclear [{Cu2(OOCC5H11)4(urea)}2] and the polymeric [Cu2(OOCC5H11)4]n, are
given in Fig. 10. Numerical values of vM at 395 K are as
expected for uncoupled Cu(II) S ¼ 1=2 spin systems,
considering the molecular formula corresponding to
each structural type. Upon lowering the temperature, in
all three cases, vM first increases very smoothly and then
decreases rapidly, with a broad maximum around 280,310 and 270 K, respectively, for the dinuclear, tetranu-
clear and alternating chain structures. This behaviour is
characteristic for strong antiferromagnetic coupling
within the Cu2 pairs forming these three compounds. At
temperatures below 60 K, vM increases again to different
extents depending on the compound. These residual
paramagnetic tails can be attributed to small amounts of
monomeric impurity, often found in Cu(II) compoundsof this type. The tetranuclear compound presents an
additional curiosity: a step is observed in the vM versus
T curve at 360 K. This is probably related to a structural
solid-to-solid phase transition observed at that temper-
Fig. 10. The full lines represent fit to the appropriate theoretical ex-
pression of the susceptibility (see text), with dominant Cu–Cu anti-
ferromagnetic interaction of 2J¼)238.3(6) cm�1, 2J ¼ �261:3ð6Þcm�1 and 2J ¼ �232ð2Þ cm�1 for compounds [Cu2(OOCC5H11)4(urea)2] ð�Þ, [fCu2(OOCC5H11)4(urea)g2] (s) and [Cu2(OOCC5H11)4]n(h), respectively.
4228 B. Kozlev�car et al. / Inorganica Chimica Acta 357 (2004) 4220–4230
ature, and described in X-ray powder diffraction and
thermal analysis part. Therefore, in the fitting procedure
described below only the data below 360 K were
considered.
The experimental data for the dinuclear compound
can be well reproduced by considering the expression ofvM (Eq. (2)) derived from the isotropic spin Hamiltonian
(Eq. (3))
vM ¼ 2NAg2b2
kBT3
�þ exp
�� 2JkBT
���1
; ð2Þ
H ¼ �2J S1 � S2: ð3Þ
The best-fit parameters give a measure of the gyromag-
netic constant g ¼ 2:29ð1Þ, the magnetic interaction
2J ¼ �238:3ð6Þ cm�1, a paramagnetic impurity p of
0.3(1)% and a temperature independent paramagnetism
(TIP) of 1.0(4) · 10�5 cm3 mol�1. The same expression
also yields a perfect fit of the experimental data
of the tetranuclear compound, with g ¼ 2:04ð1Þ,2J ¼ �261:4ð4Þ cm�1, p ¼ 0:68ð2Þ% and TIP¼ 3.54(4) ·10�4 cm3 mol�1. This indicates that the coupling within
the Cu2 pairs is strong enough to result in a diamagnetic
state at temperatures where inter-dinuclear interaction
start to be effective. Indeed, using an expression [36] for a
tetranuclear structure considering only nearest-neigh-
bour interactions (J1 and J2, setting J3 and J4 to 0) yields
good fits with almost the same intra-dinuclear interaction2J1 ¼ �261:3ð6Þ cm�1 but with any value of inter-dinu-
clear interaction 2J2 between 40 and )40 cm�1. Although
the expression for a simple dinuclear structure also re-
produces correctly the temperature dependence of vM of
the chain compound, a better fit is obtained by using the
expression developed by Hatfield and co-workers [37] for
alternating chain compounds, which gives g ¼ 2:08ð1Þ,2J ¼ �232ð2Þ cm�1, an alternating factor a ¼ 0:014ð5Þand p ¼ 1:1ð1Þ% (the TIP was here fixed to 6 · 10�5 cm3
mol�1). Therefore, in the chain compound too, the strong
antiferromagnetic coupling within the tetracarboxylate-
bridged pairs of Cu(II) yields a diamagnetic state at
temperatures where the other exchange interactionwithin
the chain is not yet effective. These results are in agree-ment with the many reports of coupling of Cu(II) ions
through a tetracarboxylate bridge [36]. In the case of the
tetranuclear and chain compounds, the additional bridge
corresponds to the apical coordination sites of the Cu(II)
ions, where the spin density is expected to be negligible in
any case, since the structure indicates that the magnetic
orbital is dx2�y2 . On the contrary, the carboxylate bridges
are coordinated in equatorial positions, in a syn–syn
fashion, yielding a strong overlap and therefore a strong
antiferromagnetic coupling. It is thus not surprising that
the three compounds studied here behave as dinuclear
compounds, from the magnetic viewpoint.
The 2J value described in the EPR section as singlet–
triplet energy gap or magnetic interaction in magnetic
susceptibility measurements, should correspond to each.
The EPR path for the copper acetate hydrate type hasbeen shown in the literature [1,21–23]. Six transitions for
Dms ¼ 1 are allowed (Eqs. (4)–(9): (H0 ¼ hm=geb,D0 ¼ D=geb, E0 ¼ E=geb)), however, energy of the axial
zero-field splitting parameter D exceeds microwave
quantum applied in X-band EPR spectroscopy resulting
in four signals [34,35]. Four signals Hz1, Hx, Hy and Hz2
enable four Eqs. (5), (7)–(9), but five unknowns need to
be extracted gx, gy , gz D, E. By known J from vM, we getEq. (10) (g? ¼ ðgx þ gyÞ=2, gk ¼ gz):
H 2x1 ¼
gegx
� �2
ðH0 � D0 þ E0ÞðH0 þ 2E0Þ; ð4Þ
H 2x2 ¼
gegx
� �2
ðH0 � D0 þ E0ÞðH0 þ 2E0Þ; ð5Þ
H 2y1 ¼
gegy
� �2
ðH0 � D0 þ E0ÞðH0 þ 2E0Þ; ð6Þ
H 2y2 ¼
gegy
� �2
ðH0 � D0 þ E0ÞðH0 þ 2E0Þ; ð7Þ
H 2z1 ¼
gegz
� �2
ðH0
�� D0Þ � ðE0Þ2
�; ð8Þ
H 2z2 ¼
gegz
� �2
ðH0
�� D0Þ � ðE0Þ2
�; ð9Þ
D ¼ � J8
1
4ðgk
�� 2Þ2 � ðg? � 2Þ2
�� g2k
�þ 1
2g2?
�b2
r3:
ð10ÞThe iterative calculations did not give an exact solution
(D(start)¼D(final)), where gx < gy ðHx < HyÞ and
2:00 < gx, gy < 2:10, therefore, an as small as possible
B. Kozlev�car et al. / Inorganica Chimica Acta 357 (2004) 4220–4230 4229
shift for the J value was applied. These results give
value j2J j � 260 cm�1 that is much closer to the j2J jvalue from the susceptibility measurements (232 cm�1)
than the value obtained from the approximate calcu-
lations (279 cm�1), where E � 0, as shown in Table 3.The difference 2J(EPR, E not negligible)) 2JðvMÞ � 30
cm�1 is not very small, however, similar deviations may
also be noticed for here described (Table 3, 291 K
spectra) isolated dinuclear and the tetranuclear com-
plex. A possible reason for this is lower accuracy of
the measurements where broader EPR signals were
obtained.
Future investigations on such polynuclear copper(II)compounds might be more effective when Q-band EPR
spectroscopy would be applied [35].
4. Concluding remarks
The structure of tetranuclear [{Cu2(OOCC5H11)4(urea)}2] complex differs significantly from the otherknown copper(II) paddle-wheel compounds, and pre-
sents a hybrid example of both, the isolated dinuclear
and the polymer of the dinuclear type of dicopper tet-
racarboxylates. Due to almost identical results of other
characterization methods [11,25], at least a very similar
structure for the whole series [{Cu2(OOCCmH2mþ 1)4(urea)}2] ðm ¼ 5–11Þ is expected. Similar analogy is very
probable also for EPR spectra and magnetic suscepti-bility, although only the hexanoate complexes have been
described in detail.
In both types of urea complexes (tetranuclear and
isolated dinuclear), a transition to liquid crystalline
phase does not occur. A solid-to-liquid crystal phase
transition was observed only after thermal decomposi-
tion of tetranuclear copper(II) carboxylates to polymeric
copper(II) carboxylates. A solid-to-solid phase transi-tion was observed in complexes of urea [{Cu2(OOCCmH2mþ 1)4(urea)}2] depending on the length of
paraffin chains. In the urea complexes the transition
temperatures rise with an increasing number of carbon
atoms in aliphatic chains and get closer to the temper-
atures of decomposition.
In the ambient temperature EPR spectrum of tetra-
nuclear [{Cu2(OOCC5H11)4(urea)}2], several additionalsignals were noticed beside three characteristic triplet
signals of copper(II) acetate hydrate type. The region,
where these additional bands were found, their disap-
pearance from the spectra by lowering of the tempera-
ture to 95 K and the simultaneous narrowing of the
copper(II) acetate type signals, is in close correlation
with the observations for the polynuclear [Cu2(OOCC5H11)4]n. From this point of view, the Cu–Ointer-dinuclear interactions, that are present in both, the
tetranuclear and polymeric complex and not in isolated
dinuclear copper(II) units in [Cu2(OOCC5H11)4(urea)2],
are a suitable explanation for the origin of these addi-
tional signals.
A more detailed and complete explanation for the
present EPR spectra, not only for the signals described
by the usual S ¼ 1 spin Hamiltonian, has been obtainedtaking temperature dependent intermolecular exchange
into account. The observed signals were discussed as-
suming that the triplet exciton of copper(II) pair is able
to migrate through the crystal lattice [33–35]. Never-
theless, at least partial description is appropriate also by
considering the chemical equilibrium of different copper
species in the sample [29] and/or higher spin states
ðSP 2Þ of the complexes [31,32]. Namely, equilibria andpopulation of the spin states are strongly temperature
dependant.
All three copper(II) hexanoate complexes studied
here, behave as isolated dinuclear compounds with a
strong antiferromagnetic coupling present within Cu2pairs. The structures indicate that the uncoupled copper
electron is in the orbital oriented towards carboxylate
oxygen atoms, enabling superexchange mechanism,while the orbital oriented towards axial ligand is filled
by two electrons and therefore not magnetic, so the in-
ter-dinuclear association is negligible in magnetic sus-
ceptibility properties of all herein presented, structurally
different Cu(II) hexanoates.
5. Supporting information
CCDC-217657 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained
free of charge at www.ccdc.cam.ac.uk/conts/retriev-
ing.html [or from the Cambridge Crystallographic Data
Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax:
(internat.) +44-1223-336-033; e-mail: deposit@ccdc.cam.
ac.uk].
Acknowledgements
The work described in the present paper has been
financially supported by the Ministry of Education,
Science and Sport, Republic of Slovenia, through
Grants P0-511-103-00, X-2000 and by the Leiden Uni-versity Study Group WFMO (Werkgroep Fundamen-
teel Materialen Onderzoek). We thank Prof. V. Kau�ci�cfrom the ‘National Institute of Chemistry’ in Ljubljana
for X-ray powder diffraction data and Dr. M. �Sentjurcfrom the EPR center at ‘Jo�zef Stefan’ Institute in
Ljubljana for EPR spectra.
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