Synthesis, structure and magnetic properties of a novel Tb4 spin cluster and synthesis of a Tb chain

Article abstract of DOI:10.1016/j.poly.2007.02.003

The syntheses of two new polynuclear Tb^III compounds are reported. A tetranuclear complex [Tb₄(H₂L)₂(H₄L)₂(OAc)₈] (1), with H₄L = 2-((2-hydroxy-benzylidene)-amino)-2-hydroxy-methyl-propane-1,3-diol, has a butterfly-type structure, while [Tb(OAc)₃MeOH]_∞ (2) is a linear chain. A novel binding mode for H₂L^2- is observed in 1, with two alkoxides each bridging three Tb^III ions. The magnetic properties of 1 are determined by the superposition of the ligand field split ⁷F₆ states of Tb^III single-ions located on two crystallographically distinct sites. Weak antiferromagnetic exchange interactions possibly contribute to the lowering of the magnetic moment at the lowest temperatures.

Full text of DOI:10.1016/j.poly.2007.02.003

Polyhedron 26 (2007) 3023–3028
www.elsevier.com/locate/poly
Synthesis, structure and magnetic properties of a novel Tb4 spin
cluster and synthesis of a Tb chain
a
b
a,
b,
*
Roland Bircher , Brendan F. Abrahams , Hans U. Gudel *, Colette Boskovic
¨
a
Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3000 Bern 9, Switzerland
b
School of Chemistry, University of Melbourne, Vic. 3010, Australia
Received 14 December 2006; accepted 2 February 2007
Available online 13 February 2007
Abstract
The syntheses of two new polynuclear TbIII compounds are reported. A tetranuclear complex [Tb4(H2L)2(H4L)2(OAc)8] (1), with
H4L = 2-((2-hydroxy-benzylidene)-amino)-2-hydroxy-methyl-propane-1,3-diol, has a butterfly-type structure, while [Tb(OAc)3MeOH]
1
(2) is a linear chain. A novel binding mode for H2L2ꢀ is observed in 1, with two alkoxides each bridging three TbIII ions. The magnetic
properties of 1 are determined by the superposition of the ligand field split 7F6 states of TbIII single-ions located on two crystallograph-
ically distinct sites. Weak antiferromagnetic exchange interactions possibly contribute to the lowering of the magnetic moment at the
lowest temperatures.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Keywords: Lanthanide; Rare earth metal; Spin cluster; Magnetic measurements; Magnetic anisotropy
1. Introduction
despite the large number of new molecules, no significant
increase has been achieved in the blocking-temperature.
Very recently, the first SMMs containing rare earth ions
were reported [7,8]. Rare earth ions can provide up to seven
unpaired electrons and introduce a large magnetic anisot-
ropy. The most striking examples among this new class
of SMMs have been described by Ishikawa et al. [8]. Tb,
Dy or Ho ions sandwiched between two phthalocyanine
molecules were the first mononuclear SMMs, and slow
relaxation of the magnetization was observed at signifi-
cantly higher temperatures than for conventional transition
metal SMMs. These findings have triggered the exploration
of new molecular clusters containing rare earth ions.
We have turned our attention to the Schiff base proli-
gand 2-((2-hydroxy-benzylidene)-amino)-2-hydroxymethyl-
propane-1,3-diol (H4L, Chart 1). It has a potentially
pentadentate {O4N} donor set and possesses both chelat-
ing and bridging capacities. The large number of potential
binding sites is suitable for rare earth ions, which often
have coordination numbers of eight or nine. H4L is a mem-
ber of a family of Schiff base ligands that we have success-
fully employed in the synthesis of new transition metal spin
The discovery in the early 1990s of the first single-mole-
cule magnet (SMM), a dodecanuclear Mn complex (Mn12
acetate), has prompted considerable research in this area
[1]. SMMs are isolated high spin molecules that exhibit
magnetization hysteresis at low temperatures, which is
otherwise a property of bulk magnets. Large spin ground
states combined with predominantly uniaxial anisotropy
are responsible for this behavior. The quantum nature of
SMMs is manifested by quantum tunneling of the magne-
tization. In the last 15 years more than a hundred new
examples of SMMs have been prepared by synthetic chem-
ists, many of them containing Mn and belonging to the
same structural family as Mn12 acetate [2,3], or to a family
of Mn4 SMMs [4]. SMM synthesis has also been extended
to other transition metal ions such as Fe, Ni, V and Co
[5,6]. Below the so-called blocking temperature, SMMs dis-
play slow relaxation of the magnetization. However,
*
Corresponding authors. Tel.: +61 383444971; fax: +61 393475180.
E-mail address: c.boskovic@unimelb.edu.au (C. Boskovic).
0277-5387/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2007.02.003

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R. Bircher et al. / Polyhedron 26 (2007) 3023–3028
solution. A pure sample of 2 could not be obtained for
physical measurements, as the crystallization conditions
enhanced the simultaneous formation of 1 Æ 2H2O. A sam-
ple for crystallography was maintained in contact with
mother liquor to prevent loss of interstitial solvent.
2.2. Physical measurements
2.2.1. X-ray crystallography
The intensity data of compounds 1 Æ 2Et2O and 2 were
collected on a Bruker CCD diffractometer using graphite
˚
monochromated Mo Ka radiation (k = 0.71073 A). Crys-
tals were transferred directly from the mother liquor to a
protective oil, which was used to prevent solvent loss.
The structures were solved using direct methods and
refined using a full-matrix least squares procedure based
on F2 using all data [12]. Crystallographic data for
1 Æ 2Et2O and 2 are given in Table 1.
Chart 1. Proligand H4L.
clusters [9]. Several clusters incorporating derivatives of
H4L and Mn, Ni and Cu have been reported in the litera-
ture [10]. Although the ability of H4L to coordinate rare
earth ions has not previously been explored, it would
appear to be an excellent candidate as O and N are good
donor atoms for rare earth ions.
2.2.2. Magnetic measurements
Variable temperature magnetic measurements were per-
formed on a Quantum Design MPMS-XL SQUID magne-
tometer equipped with a 5 T magnet. Data were collected
on a randomly oriented polycrystalline sample of 1 Æ Et2O
restrained in eicosane to prevent torquing. Diamagnetic
corrections were made using Pascal’s constants and a value
of lDia = ꢀ1.3 · 10ꢀ8 emu/g for the sample and eicosane,
respectively. DC susceptibility measurements were per-
formed with measuring fields of 0.01, 0.1 and 3 T. AC sus-
ceptibility data were collected with an alternating field of
3 · 10ꢀ4 T and frequencies ranging between 10 and
103 Hz. Isothermal magnetization measurements were per-
formed at 2, 3, 4, 5, 10 and 20 K in a field range of 0–5 T.
2. Experimental
2.1. Syntheses
All manipulations were performed under aerobic condi-
tions, using materials as received. 2-((2-hydroxy-benzyli-
dene)-amino)-2-hydroxymethyl-propane-1,3-diol
was prepared as described [11].
(H4L)
2.1.1. [Tb4(H2L)2(H4L)2(OAc)8] (1)
Tb(OAc)3 Æ H2O (0.68 g, 2.0 mmol) and H4L (0.46 g,
2.0 mmol) were dissolved in MeOH (50 cm3), and the
resulting mixture was stirred overnight. The white precipi-
tate was removed by filtration and the filtrate was evapo-
rated to dryness and redissolved in a minimum amount
of MeOH. After two days, a mixture of crystals of 1 and
2 was filtered off and the remaining yellow solution was
layered with Et2O. After a further three days, pale yellow
plate shaped crystals of 1 Æ 2Et2O formed. The crystals were
isolated and washed with minimum MeOH and Et2O; 25%
yield. A sample for crystallography was maintained in con-
tact with mother liquor to prevent the loss of interstitial
solvent. Anal. Calc. for [Tb4(H2L)2(H4L)2(OAc)8] Æ Et2O,
C64H90N4O33Tb4: C, 36.97; H, 4.36; N, 2.69. Found: C,
37.20; H, 4.28; N, 2.36%. Selected IR data (cmꢀ1): 3401
(s), 2931 (w), 2874 (w), 1642 (s), 1631 (s), 1558 (s), 1493
(m), 1453 (m), 1345 (w), 1311 (w), 1242 (w), 1199 (w),
1154 (w), 1057 (m), 1019 (m), 946 (w), 763 (w), 668 (m),
615 (w), 574 (m), 473 (w).
Table 1
Crystallographic data for 1 Æ 2Et2O and 2
1 Æ 2Et2O
2
Formulaa
C68H100N4O34Tb4
2153.2
C7H13O7Tb
369.09
P21/c(14)
11.9785(10)
11.9409(10)
7.6080(6)
90
90.424(2)
90
1088.17(16)
4
130(1)
2.247
6.512
6713
2464
2223
0.0272
0.0517
Fw (g molꢀ1
Space group
)
ꢀ
P1 (2)
˚
a (A)
10.5188(5)
13.3615(6)
15.4902(7)
91.273(1)
109.422(1)
96.181(1)
2037.36(16)
1
130(1)
1.755
3.516
12826
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A )
Z
T (K)
qcalc (g cmꢀ3
)
l (mmꢀ1
)
Reflections measured
Unique reflections
8893
8210
0.0263
0.0729
Observed data [I > 2r(I)]
R1
b
c
2.1.2. [Tb(OAc)3MeOH] (2)
1
wR2
Under the same reaction conditions as for 1, needle
shaped crystals of 2 could be manually isolated from the
mixture of crystals formed from a concentrated methanolic
a
Including solvent molecules.
b
I > 2r(I).
c
F2, all data.

R. Bircher et al. / Polyhedron 26 (2007) 3023–3028
3025
2.2.3. Other measurements
Infrared spectra (KBr disk) were recorded on a Perkin–
Elmer Spectrum One FTIR spectrometer. Elemental anal-
´
yses were performed at the Ecole d’ingenieurs et architectes
de Fribourg, Switzerland.
3. Results and discussion
3.1. Syntheses
The overnight treatment of Tb(OAc)3 Æ nH2O with a
methanolic solution of 1–2 equiv. of H4L affords a yellow
solution and a white precipitate. Filtration followed by
evaporation of the solvent produces a yellow oil, re-disso-
lution of the residue in a minimum amount of MeOH
yields a mix of crystals of 1 Æ 2Et2O and 2 after two days.
Subsequent filtration and layering of the filtrate with
Et2O affords plate shaped crystals of 1 Æ 2Et2O after three
days in 25% yield. The reactions may be written as
4TbðOAcÞ3 ꢁ nH2O þ 4H4L
! ½Tb4ðH2LÞ2ðH4LÞ2ðOAcÞ8ꢂ þ 4HOAc þ 4nH2O ð1Þ
TbðOAcÞ3 ꢁ nH2O þ MeOH
! ½TbðOAcÞ3MeOHꢂ þ nH2O
ð2Þ
The formation of 1 involves the protonation of acetate li-
gands in conjunction with double deprotonation and coor-
dination of the H4L ligand. This is commonly observed
during the synthesis of polynuclear complexes. A likely
driving force is the chelate effect attributed to the multiden-
ticity of the ligand. The formation of 2 involves the substi-
tution of H2O ligands with MeOH as well as the change of
the binding modes of the acetate ligands. The ligand H4L is
not incorporated in the structure.
Fig. 1. (a) Ball and stick representation of complex 1 in 1 Æ 2Et2O.
Hydrogen atoms are omitted for clarity. (b) Coordination spheres of the
TbIII ions in 1 Æ 2Et2O. Tb purple; O, red; N, blue; C, gray. (For
interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)
rhomb (Fig. 1b), linked by four l3 alkoxide bridges from
two Schiff-base ligands and four l2-O atoms from acetate
ligands. The coordination sphere is completed by N-atoms
from the central Schiff-base ligands, acetates and two addi-
tional Schiff-base ligands. The structure comprises two
crystallographically distinct TbIII sites, both with C1 point
symmetry. Two different binding modes can be observed
for H4L and H2L2ꢀ (Fig. 2). The central doubly deproto-
nated ligands bind in a g1:g1:g2:g4:l4-fashion with four
out of five potential binding sites complexing Tb. The
peripheral H4L ligands bind to Tb only via the phenol
3.2. Structure descriptions
Compound 1 Æ 2Et2O crystallizes in the triclinic space
ꢀ
group P1 with one molecule per unit cell, together with dis-
ordered solvent. The molecule has Ci point symmetry. A
summary of interatomic distances and angles is given in
Table 2. A complete list can be obtained from the authors.
The structure of 1 in 1 Æ 2Et2O is shown in Fig. 1a.
Bond valence sum calculations [13] are in good agree-
ment with the expected oxidation state of 3+ for the Tb
ions in the cluster. The Tb centers are arranged in a planar
Table 2
˚
Selected bond distances (A) and angles (deg) for 1 Æ 2Et2O and 2
Parameter
1 Æ 2Et2O
2
Tb–Tb
3.3640(3)–3.7190(3)
2.335(2)–2.590(2)
85.79(7)–86.95(7)
96.32(8)–102.24(8)
2.466(3)–2.709(4)
4.0128(4)
2.367(2)–2.480(2)
109.97(9)–113.30(9)
Tb–Oa
Tb1–O–Tb1
Tb1–O–Tb2
Oꢁ ꢁ ꢁOb
a
l3-O and l2-O atoms.
Intramolecular hydrogen bonds.
b
Fig. 2. Binding modes of H2L2ꢀ and H4L in 1: g1:g1:g2:g4:l4 and g1.

3026
R. Bircher et al. / Polyhedron 26 (2007) 3023–3028
alcohol group. Two of the alcohol arms are linked by an
intramolecular hydrogen bond and bind to an acetate
group via a further intramolecular hydrogen bond (Table
2). Four of the acetates act as bridges between Tb ions in
a g1:g2:l2-fashion, two acetates act as g2 chelates and
two of the acetates bind as terminal ligands in syn g1-fash-
ion (Chart 2). An intramolecular hydrogen bond is formed
between the latter and the phenol alcohol of a H2L2ꢀ
ligand.
The Tb4 plane of 1 lies perpendicular to the crystallo-
graphic a-axis. Adjacent clusters in the a-direction are
linked by intermolecular hydrogen bonds between the
remaining alcohol arm of H2L2ꢀ (O7) and one of the ace-
tates (O16). In addition, one of the alcohol arms of H4L
(O2) is involved in hydrogen bonding with disordered
solvent.
Several tetranuclear terbium complexes are reported in
the literature [14], but the core structure of 1 is unique.
Similar core topologies have been described for transition
metal complexes, especially the so-called butterfly struc-
tures with a [M4O2] core [6,15]. The rhombic arrangement
of transition metal ions comprises only two l3 alkoxide
bridges, as the metal ions are six-coordinate. Owing to
the higher coordination number of rare earth ions (all TbIII
ions in 1 are nine-coordinate) additional bridging atoms
are necessary to obtain the same butterfly type arrange-
ment of the magnetic centers.
Compound 2 crystallizes in the monoclinic space group
P21/c. It comprises TbIII ions linked by acetate groups to
form chains (Fig. 3). The ligand H4L is not incorporated
in the structure of 2 [13]. Bond valence sum calculations
are consistent with the standard oxidation state of 3+ for
Tb. The structure contains only one crystallographic TbIII
site with C1 point symmetry. The asymmetric unit contains
two chain fragments, each consisting of two TbIII ions. The
TbIII centers are linked by three acetates, two bridging in
Fig. 3. Section of the chain structure of 2, showing the coordination
sphere of the TbIII ions. Hydrogen atoms are omitted for clarity. Tb,
purple; O, red; C, gray. (For interpretation of the references to color in
this figure legend, the reader is referred to the web version of this article.)
g1:g1:l2 fashion and one in a syn–syn g1:g1:l2-arrange-
ment. The remaining peripheral ligation is completed with
a MeOH molecule. A summary of interatomic distances
and angles is given in Table 2. It is worthwhile to compare
the structure of 2 with the crystal structure of the starting
material Tb(OAc)3 Æ nH2O [16], in which TbIII dimers are
linked by two acetates in g1:g2:l2-fashion, while the other
four acetates chelate a single Tb. The four remaining posi-
tions are occupied by H2O. In 2, the water molecules are
replaced by two MeOH ligands, the free coordination sites
are occupied by two g1:g2:l2 bridges and there are two ace-
tate links with a syn–syn g1:g1:l2-mode instead of four che-
lating acetates.
The chains in 2 are parallel to the crystallographic c-axis
of the monoclinic structure. They are well isolated from
each other, no hydrogen bonding pathway can be
observed. The shortest distance between TbIII ions of dif-
˚
ferent chains is 8.2 A. A Tb chain similar to 2 has been
reported in the literature, namely [Tb(O2CPh)3-
(MeOH)2(H2O)] [17]. Interestingly, it was also obtained
1
as a co-crystallized product from a reaction mixture con-
taining other ligands. This complex incorporates benzoate,
with binding modes different to those observed for acetate
in 2. The chains form hydrogen-bonded sheets. The fact
that the chains in 2 are well isolated makes the structure
unique.
3.3. Magnetic study of 1 Æ Et2O
Variable temperature AC and DC susceptibility mea-
surements were performed on a sample of randomly ori-
ented crystals of 1 Æ Et2O. The DC data measured at
0.1 T are plotted as vT versus T and v versus T in Fig. 4.
Between 300 and 100 K, vT stays nearly constant at the
room temperature value of 44.4 emu molꢀ1 K. Upon fur-
ther cooling, vT decreases first gradually and then steeply
below 5 K. Isothermal magnetization data collected at six
temperatures between 2 and 20 K are plotted in Fig. 5 as
M versus H/T. The isotherms do not superimpose and
the magnetization at 2 K does not completely saturate at
the high-field limit of our instrument; it reaches a value
M = 19.3 NlB at 5 T.
TbIII has a 4f8 electron configuration, and the 7F ground
7
Chart 2. Binding modes of the acetate groups in complexes 1 and 2.
term is split by spin–orbit coupling, leading to seven FJ

R. Bircher et al. / Polyhedron 26 (2007) 3023–3028
3027
eight-coordinate TbIII in a low symmetry ligand field.
The reduction from the free-ion value indicates that all
7
the crystal-field levels of F6 are not equally populated at
300 K. Upon cooling below 100 K, higher crystal field lev-
els are depopulated, leading to the observed decrease in vT.
The same behavior was found in the above mentioned
mononuclear and tetranuclear TbIII compounds [14a,20].
The superposition of the vT versus T curves measured with
H = 0.1 T and in AC mode with no applied DC field (inset
of Fig. 4) indicates that the low temperature decrease in vT
is not due to the effect of the applied field. In addition to
the dominant ligand field effects, a contribution of antifer-
romagnetic exchange interactions to the magnetic suscepti-
bility at the lowest temperatures cannot be excluded. These
interactions are generally very small between rare earth
ions, for example. J = ꢀ0.040 cmꢀ1 was determined for a
TbIII dinuclear complex by inelastic neutron scattering
[21]. The divergence of the isothermal magnetization curves
in the M versus H/T plot (Fig. 5) indicates that more than
one of the sublevels of 7F6 are populated in the temperature
range 2–20 K. The magnetization is not saturated at 5 T,
but the absolute value M = 19.3 NlB at 2 K and 5 T is
much lower than the maximum value M = 36 NlB for
7F6 [19]. This suggests that the wavefunctions of the lowest
energy substates are predominantly built up from basis
functions with small MJ values. A further interpretation
is difficult, especially because the measured properties are
a superposition of the properties of the two distinguishable
TbIII ions in the cluster. Site selective low-temperature
luminescence spectroscopy might be a way to disentangle
the two contributions and determine the individual energy
splittings.
Fig. 4. vT vs. T and v vs. T for 1 Æ Et2O, measured in an applied field of
0.1 T. The inset shows the low temperature regime of vT vs. T for 1 Æ Et2O.
Plus signs correspond to DC measurements with a measuring fields of
0.1 T, circles denote data from an AC measurement with a field of
3 · 10ꢀ4 T oscillating with a frequency of 60 Hz.
Sensitized by the phthalocyanine rare earth sandwich
SMMs [8], we performed AC magnetic measurements to
look for slow relaxation of the magnetization in 1 Æ Et2O.
No out-of-phase signal was detected in the full temperature
range down to 1.8 K. In the TbIII sandwich complex
reported by Ishikawa et al. the ligand field of the two pht-
halocyanines generates a fortunate situation with the two
lowest substates separated by an energy gap of more than
400 cmꢀ1 from the next excited substates [8]. In the excep-
tional case of the sandwich compound SMM behavior can
be attributed to the quasi-axial symmetry of the ligand
field. A considerable mixing of basis functions with differ-
ent MJ values and a more complex energy level scheme
are expected for TbIII ions with C1 point symmetry.
Fig. 5. Isothermal magnetization measurements on a sample of randomly
oriented crystals of 1 Æ Et2O, measured in a field range of 0–5 T at six
temperatures between 2 and 20 K.
states. The ground state is 7F6, which is separated by
7
approximately 2000 cmꢀ1 from the first excited F5 state.
7F6 is further split into a maximum of 13 sublevels by the
low symmetry ligand field. The total splitting is expected
to be of the order of 300–600 cmꢀ1 [18]. A superposition
of the splittings of the two crystallographically distinct
TbIII ions determines the magnetic properties of 1 Æ Et2O.
We restrict ourselves to a mainly qualitative discussion,
as a more quantitative approach would be pointless in
the absence of suitable information on the anisotropy of
the magnetic properties of 1 Æ Et2O.
4. Conclusions
The room temperature value of vT = 44.4 emu molꢀ1 K
is lower than vT = 47.3 emu molꢀ1 K calculated for four
Complex 1 was successfully prepared by incorporating
the proligand H4L in two different binding modes in its
neutral and double deprotonated state. The displayed che-
lating and bridging properties of this ligand make it an
ideal candidate for the synthesis of spin clusters. In addi-
tion to 1, compound 2 was also isolated from the reaction
mixture, exhibiting a novel Tb-chain structure. Cluster 1
did not show SMM behavior. This is mainly attributed to
uncoupled TbIII ions in a free-ion approximation with an
7
´
isolated and degenerate F6 ground state and a Lande-fac-
tor gJ = 3/2 [19]. An almost identical room temperature
value vT = 44.3 emu molꢀ1 K was measured for a Tb4 cube
[14a], as well as for a mononuclear TbIII complex (vT =
11.1 emu molꢀ1 K for one TbIII) [20], both containing

3028
R. Bircher et al. / Polyhedron 26 (2007) 3023–3028
(c) C.M. Zaleski, E.C. Depperman, J.W. Kampf, M.L. Kirk, V.L.
Pecoraro, Angew. Chem., Int. Ed. 43 (2004) 3912.
[8] (a) N. Ishikawa, M. Sugita, T. Ishikawa, S.-Y. Koshihara, Y. Kaizu,
J. Am. Chem. Soc. 125 (2003) 8694;
the low symmetry of the ligand field, as compared to the
quasi-axial symmetry of the ligand field in the TbIII sand-
wich complex [8]. In order to impose axial-type coordina-
tion of the rare earth centers it seems necessary to use
more rigid ligands than H4L.
(b) N. Ishikawa, M. Sugita, T. Ishikawa, S. Koshihara, Y. Kaizu, J.
Phys. Chem. B 108 (2004) 11265;
(c) N. Ishikawa, M. Sugita, W. Wernsdorfer, Angew. Chem., Int. Ed.
44 (2005) 2931.
[9] (a) C. Boskovic, E. Rusanov, H. Stoeckli-Evans, H.U. Gudel, Inorg.
¨
Acknowledgement
Chem. Commun. 5 (2002) 881;
(b) C. Boskovic, R. Bircher, P.L.W. Tregenna-Piggott, H.U. Gudel,
¨
C. Paulsen, W. Wernsdorfer, A.-L. Barra, E. Khatsko, A. Neels, H.
Stoeckli-Evans, J. Am. Chem. Soc. 125 (2003) 14046;
This work was supported by the Swiss National Science
Foundation and the Australian Research Council.
(c) C. Boskovic, A. Sieber, G. Chaboussant, H.U. Gudel, J. Ensling,
¨
W. Wernsdorfer, A. Neels, G. Labat, H. Stoeckli-Evans, S. Janssen,
Inorg. Chem. 43 (2004) 5053;
Appendix A. Supplementary material
CCDC 630776 and 630777 contain the supplementary
crystallographic data for this paper. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: (+44) 1223-336-033; or e-mail: depos-
it@ccdc.cam.ac.uk. Supplementary data associated with
this article can be found, in the online version, at
doi:10.1016/j.poly.2007.02.003.
(d) A. Sieber, C. Boskovic, R. Bircher, O. Waldmann, S.T. Ochsen-
bein, H.U. Gudel, N. Kirchner, J. van Slageren, W. Wernsdorfer, A.
¨
Neels, H. Stoeckli-Evans, S. Janssen, F. Juranyi, H. Mutka, Inorg.
Chem. 44 (2005) 4315.
[10] (a) D.P. Kessissoglou, C.P. Raptopoulou, E.G. Bakalbassis, A.
Terzis, J. Mrozinski, Inorg. Chem. 31 (1992) 4339;
(b) D.P. Kessissoglou, M.L. Kirk, M.S. Lah, X. Li, C. Raptopoulou,
W.E. Hatfield, V.L. Pecoraro, Inorg. Chem. 31 (1992) 5424;
(c) M. Dey, C.P. Rao, P.K. Saarenketo, K. Rissanen, Inorg. Chem.
Commun. 5 (2002) 924;
(d) M. Dey, C.P. Rao, P.K. Saarenketo, K. Rissanen, Inorg. Chem.
Commun. 5 (2002) 380.
[11] M. Dey, C.P. Rao, P.K. Saarenketo, K. Rissanen, E. Kolehmainen,
P. Guionneau, Polyhedron 22 (2003) 3515.
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