Inorganica Chimica Acta 357 (2004) 4220–4230

www.elsevier.com/locate/ica

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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