A hexanuclear copper(II) Schiff base complex incorporating rare bicapped cubane core: Structural aspects, magnetic properties and EPR study

Article abstract of DOI:10.1016/j.poly.2012.07.016

The reaction of Cu₂(CH₃COO)₄· 2H₂O with the Schiff base ligand, H₄L [(OH) ₂C₆H₃CHN-C(CH₃)(CH ₂OH)₂], yields a novel hexanuclear copper (II) cluster {[Cu₆(HL)₄]·CH₃OH·0.5CH ₃CN·6.5H₂O} (1) with a rare bicapped cubane core. Title complex has been characterized by different physico-chemical techniques. Single-crystal X-ray structure reveals that the bicapped cubane core was formed by a cubane-like Cu ₄O₄ subcore capped by two copper(II) atoms over opposite faces. The cubane copper(II) atoms are in distorted square pyramidal geometry and connected each other via μ₃-alkoxo oxygens while two apical copper(II) atoms are in distorted square planar environments and connected to the main cubane subcore by doubly phenoxo bridged oxygen atoms. EPR spectra of the complex reveals overall strong antiferromagnetic behavior, well supported by cryomagnetic susceptibility measurements in the range 2-300 K under the magnetic field of 0.1 T leading different coupling constants: J₁ = +60 cm^-1, J₂ = -45.5 ± 1 cm^-1; J ₃ = -140 ± 1 cm^-1, using the general Hamiltonian H = -J(S_i·S_j). A plausible magneto-structural correlation has also been established.

Full text of DOI:10.1016/j.poly.2012.07.016

Polyhedron 52 (2013) 963–969
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
A hexanuclear copper(II) Schiff base complex incorporating rare ‘‘bicapped
cubane’’ core: Structural aspects, magnetic properties and EPR study
Shyamapada Shit a, Madhusudan Nandy a, Georgina Rosair b, M. Salah El Fallah c, Joan Ribas c,
Eugenio Garribba d, Samiran Mitra a,
⇑
a Department of Chemistry, Jadavpur University, Kolkata 700 032, India
b Department of Chemistry, School of Engineering & Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK
c Departament de Química Inorgànica, Facultat de Química, Universitat de Barcelona, Martí Franquès 1-11, 08028 Barcelona, Spain
d Dipartimento di Chimica e Farmacia and Centro Interdisciplinare per lo Sviluppo della Ricerca Biotecnologica e per lo Studio della Biodiversità della Sardegna, Università di
Sassari, Via Vienna 2, I-07100 Sassari, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 20 July 2012
The reaction of Cu2(CH3COO)4Á2H2O with the Schiff base ligand, H4L [(OH)2C6H3CH@N–C(CH3)(CH2OH)2],
yields a novel hexanuclear copper (II) cluster {[Cu6(HL)4]ÁCH3OHÁ0.5CH3CNÁ6.5H2O} (1) with a rare
‘‘bicapped cubane’’ core. Title complex has been characterized by different physico-chemical techniques.
Single-crystal X-ray structure reveals that the ‘‘bicapped cubane’’ core was formed by a cubane-like
Cu4O4 subcore capped by two copper(II) atoms over opposite faces. The cubane copper(II) atoms are in
Dedicated to Alfred Werner on the 100th
anniversary of his Nobel Prize in Chemistry
in 1913
distorted square pyramidal geometry and connected each other via l3-alkoxo oxygens while two apical
copper(II) atoms are in distorted square planar environments and connected to the main cubane subcore
by doubly phenoxo bridged oxygen atoms. EPR spectra of the complex reveals overall strong antiferro-
magnetic behavior, well supported by cryomagnetic susceptibility measurements in the range 2–300 K
Keywords:
Hexanuclear copper(II) complex
Schiff base
Bicapped cubane core
X-ray structure
Antiferromagnetic coupling
EPR spectra
under the magnetic field of 0.1 T leading different coupling constants: J1 = +60 cmÀ1, J2 = À45.5 1 cmÀ1
;
J3 = À140 1 cmÀ1, using the general Hamiltonian H = ÀJ(SiÁSj). A plausible magneto-structural correla-
tion has also been established.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
On the other hand, multinuclear copper(II) complexes have
been receiving much attention which particularly stems from their
Coordination compounds developed as a new direction in inor-
ganic chemistry with Alfred Werner’s work. This has now become a
multidisciplinary area having a wide range of applications based
on structural variety covering simple compounds, clusters, and
polymers etc. High-nuclearity clusters are a field of growing inter-
est due to their novel structural characteristics and potential appli-
cations in magnetic, optical, electronic, and catalytic aspects [1–4].
Few synthetic strategies have been successfully employed to de-
sign the nuclearity as well as the dimensionality of such complexes
and their synthesis and design is still a great challenge [5,6]. Poly-
dentate ligands, due to their simultaneous activities as chelator
and bridge, have proven to be useful, as they often utilize their
non-coordinating groups to connect other metal centers resulting
in more complicated structures. In this regard, polydentate hydro-
xyl-rich ligands containing amino groups have added importance
in their potential applications as biomimetic model systems of
metalloproteins and metalloenzymes [5].
interesting magneto-structural correlations, in addition to being
considered as potential biomimic models for a number of key bio-
systems [6–10]. With their synthetic flexibilities, structural varie-
ties, and potential applications [11–21], tri- and tetradentate
Schiff base ligands are often used for the synthesis of such multinu-
clear copper(II) complexes [22–24]. Although many multinuclear
copper(II) species with both simple and multifarious polydentate
ligands are known, some nuclearities remain comparatively scarce,
particularly clusters involving more than four metal centers.
Working on copper(II) Schiff base complexes [25–37], we have
recently explored the possibility of a hydroxyl-rich Schiff base li-
gand in the formation of high nuclear copper(II) cluster [38]. We
explored herein another hydroxyl-rich polydentate Schiff base li-
gand, H4L (Scheme 1), obtained from an N-substituted amino-diols
(2-amino-2-methyl-1,3-propanediol) and hydroxyl-rich aldehyde
(2,3-dihydroxybenzaldehyde), to obtain a high nuclear copper
cluster. We observed that with a slight modification of the alde-
hyde component of a ligand used earlier [38], a rare hexanuclear
copper(II) cluster complex incorporating a ‘‘bicapped cubane’’ core
was obtained. This was systematically characterized by elemental
analysis, Fourier transform IR, UV–Vis, and electron paramagnetic
⇑
Corresponding author. Tel.: +91 33 2414 6666x2779; fax: +91 33 2414 6414.
E-mail address: smitra_2002@yahoo.com (S. Mitra).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.07.016

964
S. Shit et al. / Polyhedron 52 (2013) 963–969
H9
OH
H3C
H9
H
8 H3C
H6
N
OH1
H9
H7
CHO
OH
OH
OH
OH
H3C
H2N
H5
+
N
H9
OH
OH1
H4L
OH
H4
OH2
OH
-3H+
OH3
Scheme 2. Proton numbering scheme of Schiff base (H4L).
OH
H3C
H
N
room temperature. Diffraction quality brown block shaped single
crystals of 1 were separated out from the filtrate after two days.
Yield: 70%. Anal. Calc. for C46H66.5Cu6N4.5O23.5: C, 38.34; H, 4.61;
N, 4.37; Cu, 26.46. Found: C, 38.33; H, 4.59; N, 4.35; Cu, 26.45%.
O-
O-
Characteristic IR absorption bands (KBr, cmÀ1): 1598 (
mC=N), 418
HL3-
O-
(mCu–N), 375 (
mCu–O). UV–Vis (k, nm): 625 (dÀd). ESI-MS, m/z:
[C44H48Cu6N4O16+Na+] = 1293.
Scheme 1. Synthetic scheme of the Schiff base (H4L).
2.3. Physical measurements
resonance (EPR) spectroscopy. Low temperature magnetic suscep-
tibility measurements reveal the antiferromagnetic behavior of the
complex. Concerning the homonuclear hexameric copper(II) clus-
ters, several representatives are known in the literature, but only
one of ‘‘bicapped cubane’’ type, derived from an N-substituted ami-
noalcohol, has been reported [39].
Elemental analyses (C, H, and N) were carried out using a Perkin
Elmer 2400 II elemental analyzer. Copper content of 1 was esti-
mated quantitatively by standard iodometric procedure. The Fou-
rier transform IR (FT-IR) spectra of H4L and its copper(II)
complex were recorded on a Perkin Elmer Spectrum RX I FT-IR sys-
tem with a KBr disk in the range 4000–200 cmÀ1. The electronic
spectrum was recorded on a Perkin Elmer Lambda 40 UV–Vis spec-
trometer using high-performance liquid chromatography grade
acetonitrile in the range 200–800 nm. 1H NMR spectra of H4L
was recorded on a Bruker 300 MHz FT-NMR spectrometer using
trimethylsilane as internal standard in DMSO-d6. The positive ion
ESI-MS of ligand and complex were performed in a QTOF micro
mass spectrometer using methanol and acetonitrile, respectively.
EPR spectra of 1 were recorded on polycrystalline samples or in
CH3CN and N,N-dimethylformamide (DMF) solutions with an X-
band Bruker EMX spectrometer at 298 and 120 K and were simu-
lated with the computer program Bruker WinEPR SimFonia. Vari-
2. Experimental
2.1. Materials
All of the chemicals and solvents used for the syntheses were of
high-purity analytical grade. Cu2(CH3COO)4Á2H2O, 2,3-dihydroxy-
benzaldehyde, 2-amino-2-methyl-1,3-propanediol, and triethyl-
amine were purchased from Aldrich Chemical Co. Inc. and used
without any further purification.
2.2. Synthesis of ligand and complex
able temperature magnetic susceptibility measurements of
1
2.2.1. Synthesis of Schiff base ligand (H4L)
were carried out on polycrystalline samples (30 mg) with a Quan-
tum Design SQUID MPMS-XL susceptometer operating in the range
2–300 K under the magnetic field of 0.1 T. The diamagnetic correc-
tions were evaluated from Pascal’s constants.
(OH)2C6H3CH@N-C(CH3)(CH2OH)2. The Schiff base ligand was
prepared by the reflux condensation of 2,3-dihydroxybenzalde-
hyde (0.138 g, 1 mmol) and 2-amino-2-methyl-1,3-propanediol
(0.105 g, 1 mmol) in 20 mL methanol for 1 h. The resulting solution
yielded shiny yellow crystals of ligand upon slow evaporation.
They were dried and stored in vacuo over CaCl2 for subsequent
2.4. X-ray crystallography
use. Yield: 0.185 g (82%). Anal. Calc. for
C11H15O4N
A diffraction quality brown block-shaped single crystal of 1 was
mounted on a Bruker APEX2 CCD area diffractometer. Intensity data
were collected using graphite monochromated molybdenum radia-
(M = 225.24 g molÀ1): C, 58.66; H, 6.71; N, 6.22. Found: C, 58.65;
H, 6.70; N, 6.20%. Characteristic IR absorption bands (KBr, cmÀ1):
1625 (mC=N), 3032 (mAr–OH), 3342 (mR–OH). UV–Vis (k, nm): 245,
tion from a sealed tube (kMo K = 0.71073 Å). Crystal data for 1 were
collected using Bruker APEX2 [40] software at a temperature of
a
373. 1H NMR (d, 300 MHz): 1.21 (s, 3H8), 2.49 (s, 4H9), 3.44–3.52
(m, 2H1 H2, H3), 6.39 (t, J = 7.7 Hz, 1H5), 6.69 (d, J = 7.5 Hz, 1H6),
100(2) K using
u and x scans. Multiscan absorption corrections
,
6.79 (d, J = 7.9 Hz, 1H4), 8.41 (s, 1H7), ppm. (Scheme 2). ESI-MS,
were applied to the intensity values (Tmax = 0.7575, Tmin = 0.5077)
empirically using TWINABS [41]. The crystal was a two component
non-merohedral twin. The structure of 1 was solved by Direct
Methods using the program SHELXS-97 [42] and refined with full-ma-
trix least-squares based on F2 using program SHELXL-97 [42]. All non-
hydrogen atoms were refined anisotropically. Solvent water and
disordered alkoxo oxygen atoms had their anisotropy restrained
with ISOR. Phenyl hydrogen atoms were constrained to idealized
geometry and treated as a riding model. The solvent was modeled
and approximated as 1.0CH3OHÁ0.5CH3CNÁ6.5H2O. Hydrogen
atoms for one water molecule and all CH3OH and CH3CN are not
m/z: [M+H+] = 226, [M+Na+] = 248.
2.2.2. Synthesis of [Cu6(HL)4]ꢀCH3OHꢀ0.5CH3CNꢀ6.5H2O (1)
Appropriate quantity of the solid Schiff base ligand H4L (0.225 g,
1 mmol) was dissolved in 20 mL mixture of methanol and
acetonitrile (1:1 v/v). A 10 mL warm methanolic solution of
Cu2(CH3COO)4Á2H2O (0.199 g, 1 mmol) was added dropwise fol-
lowed by triethylamine (0.202 g, 2 mmol). The resultant mixture
was first stirred for 30 min. and then refluxed for 1 h. The
resulting brown solution was filtered and kept undisturbed at

S. Shit et al. / Polyhedron 52 (2013) 963–969
965
imine group and
p ?
p⁄ intra-ligand transition are observed in the
Table 1
Crystallographic data and structure refinement parameters for 1.
region of 310–455 and 220–295 nm, respectively [52,53]. A strong
intense absorption band at 310–380 nm, corresponding to the li-
gand to metal charge transfer (LMCT) transition from a coordinated
unsaturated ligand to metal ion, is also observed [53,54]. The posi-
tive ion ESI-MS of micromolar solution of 1 shows a peak around
1293 m/z matching well with the isotropic distribution expected
for [Cu6(HL)4+Na]+, thus confirming that the cluster remains intact
in solution.
Empirical formula
Formula weight
T (K)
C46H66.5Cu6N4.5O23.5
1439.78
100(2) K
0.71073
monoclinic
P2(1)/n
11.6884(15)
13.1625(16)
34.853(4)
98.324(6)
5305.5(11)
4
1.803
2.450
2944
0.32 Â 0.20 Â 0.12
2.34–28.36
180038
kMo K (Å)
a
Crystal system
Space group
a (Å)
b (Å)
c (Å)
b (°)
V (Å3)
3.3. Crystal structure description
Z
Dcalc (gcmÀ3
)
l
(mmÀ1
F (000)
)
A perspective view of 1 with atom numbering scheme is shown
in Fig. 1. Selected bond lengths, angles of the complex are listed in
Table 2. The complex consists of a neutral [Cu6(HL)4] unit. The neu-
tral complex unit is a ‘‘bicapped cubane’’ formed by six copper(II)
atoms and four tri-anionic Schiff base ligands HL3À. The cubane
core is composed of four Cu centers alternating with four oxygen
atoms from pendant deprotonated hydroxyl groups from the Schiff
Crystal dimension (mm3)
h range (°)
Total data
Unique data
21954
Observed data [I > 2
r
(I)]
11336
0.0631
0.1423
0.94
Ra
b
RW
base in a l3-alkoxo bridging mode. Each Schiff base has two alkoxo
Goodness-of-fit on F2, S
groups which give a total of eight and of these four are parts of the
cubane core. Two of the remaining four alkoxo O atoms make long
contacts (>2.5 Å) to copper centers Cu3 and Cu4 whilst the other
two are non-coordinating. The two exo-copper centers (Cu5 and
Cu6) are linked to the cubane core by phenoxo bridges. Within
the cubane core, all four copper centers have varying degrees of
distortion from square pyramidal coordination geometry. Each
cubane copper is bound to one nitrogen atom and four oxygen
atoms, although two Cu centers have an additional sixth long con-
tact to a protonated alkoxo oxygen atom. The two exo-Cu atoms
have square planar geometry. The deviation of the Cu5 and Cu6
atoms from the mean square coordination plane is 0.012(2) and
0.051(2) Å, respectively. In the cubane core, around Cu1, the basal
plane of the square pyramid is made up of O1, N3, O10, and O52,
with O28 at the apex. The Cu1–(basal atom) bond lengths are in
the range 1.919(3)–1.975(3) Å, and the apical Cu1–O28 distance
is 2.204(3) Å. The deviation of the Cu1 atom from the basal plane
towards the apical O28 is 0.167(2) Å. The distortion is slightly
Rint
0.0809
1.572
À1.141
D
D
qmax (eÅÀ3
)
qmin (eÅÀ3
)
a
R =
Rw = {
R
(|Fo À Fc|)/
R|Fo|.
b
½
R
[w(|Fo À Fc|)2]/
R .
[w|Fo|2]}
included. Hydrogen atoms bound to O7W are fixed for the final
refinement cycles. Water hydrogen atoms located with CALC-OH
[43]. The highest residual peak is associated with disordered sol-
vent. No suitable hydrogen atom positions were found for O5W.
The molecular graphics and crystallographic illustrations for 1 were
prepared using Bruker SHELXTL [44] program. All the relevant crystal-
lographic data and structure refinement parameters of the complex
are summarized in Table 1.
3. Results and discussion
higher than that of the earlier reported system [39] and the
of 0.348 indicates the significant distortion from square pyramidal
s value
3.1. Fourier transform infrared spectra
geometry of Cu1 in complex 1 [55].
The Fourier transform infrared spectrum of 1 has been analyzed
in comparison with that of the free Schiff base ligand H4L in the re-
gion 4000–200 cmÀ1. Strong sharp band at 1625 cmÀ1 correspond-
ing to azomethine stretching frequency (–CH@N–) of free ligand, is
shifted to 1598 cmÀ1 in 1, indicating the coordination of the azo-
methine nitrogen to the metal center [45,46]. The band corre-
sponds to phenolic –OH of free ligand also disappeared in the
infrared spectrum of 1, indicates its deprotonation followed by
complexation. Another strong band at 1205 cmÀ1, assignable to
phenolic mAr–OH of the free ligand, is shifted to the lower frequency
region (1185–1190 cmÀ1), providing additional evidence for coor-
dination of deprotonated phenolic oxygen atoms [47,48]. Coordi-
nation of the ligand to metal in 1 is further supported by bands
at 418 and 375 cmÀ1 attributable to
respectively [49,50].
m(Cu–N) and m(Cu–O) bonds,
3.2. UV–Vis spectrum
The electronic spectral data resulting from six interacting cop-
per(II) ions of complex 1 are not straightforward to interpret and
here the main features of the spectra are presented on a prelimin-
ary basis. The electronic spectrum of 1 shows a low intensity broad
band centered around 625 nm, attributed to d–d transition for cop-
per(II) center with distorted square-pyramidal geometries [51]. In
the complex, intense bands assignable to n ? p⁄ transition for the
Fig. 1. An ORTEP view of 1 with atom numbering scheme (thermal ellipsoids are
drawn at 40% probability level). Hydrogen atoms and solvent molecules are omitted
for clarity.

966
S. Shit et al. / Polyhedron 52 (2013) 963–969
Table 2
Selected bond lengths (Å) and bond angles (°) for 1.
Bond lengths (Å)
Cu1–O10
Cu1–O52
Cu2–O14
Cu2–O28
Cu3–N31
Cu3–O28
Cu4–N49
Cu4–O1
1.919(3)
1.975(3)
1.919(3)
1.987(4)
1.932(4)
1.997(3)
1.949(4)
2.399(3)
1.985(3)
1.918(4)
Cu1–N3
1.929(4)
2.204(3)
1.931(4)
2.251(3)
1.952(3)
2.248(4)
1.952(3)
1.888(4)
1.997(4)
1.969(3)
Cu1–O1
1.958(3)
3.0157(9)
1.940(3)
3.0151(10)
1.967(3)
1.920(3)
1.960(3)
1.914(4)
1.902(3)
2.016(3)
Cu1–O28
Cu2–N21
Cu2–O52
Cu3–O38
Cu3–O24
Cu4–O52
Cu5–O15
Cu5–O14
Cu6–O42
Cu1–Cu3
Cu2–O24
Cu2–Cu4
Cu3–O1
Cu4–O42
Cu4–O24
Cu5–O9
Cu5–O10
Cu6–O43
Cu6–O37
Cu6–O38
Bond angles (°)
O10–Cu1–N3
O10–Cu1–O52
O10–Cu1–O28
O52–Cu1–O28
O1–Cu1–Cu3
O14–Cu2–N21
O14–Cu2–O28
O14–Cu2–O52
O28–Cu2–O52
O24–Cu2–Cu4
N31–Cu3–O38
N31–Cu3–O28
N31–Cu3–O24
O28–Cu3–O24
O1–Cu3–Cu1
O42–Cu4–N49
O42–Cu4–O24
O42–Cu4–O1
O24–Cu4–O1
O52–Cu4–Cu2
O15–Cu5–O9
O15–Cu5–O14
O37–Cu6–O43
O37–Cu6–O38
95.99(16)
92.58(14)
93.67(13)
85.24(13)
39.91(10)
97.12(17)
97.69(14)
90.57(14)
83.72(13)
39.62(10)
97.63(16)
83.62(16)
115.22(16)
77.52(13)
39.69(10)
95.28(16)
96.25(14)
90.09(13)
80.91(13)
48.29(10)
91.51(16)
84.13(15)
89.31(15)
84.11(14)
O10–Cu1–O1
N3–Cu1–O52
N3–Cu1–O28
O10–Cu1–Cu3
O52–Cu1–Cu3
O14–Cu2–O24
N21–Cu2–O28
N21–Cu2–O52
O14–Cu2–Cu4
O28–Cu2–Cu4
N31–Cu3–O1
O38–Cu3–O28
O38–Cu3–O24
N31–Cu3–Cu1
O28–Cu3–Cu1
O42–Cu4–O52
N49–Cu4–O24
N49–Cu4–O1
O42–Cu4–Cu2
O24–Cu4–Cu2
O15–Cu5–O10
O9–Cu5–O14
O37–Cu6–O42
O43–Cu6–O38
174.62(15)
153.69(16)
118.83(16)
134.79(11)
89.19(10)
168.82(15)
152.65(16)
119.00(15)
129.42(11)
89.44(10)
154.92(17)
163.70(15)
87.36(13)
124.42(13)
46.93(10)
166.68(15)
164.38(16)
109.59(16)
135.37(10)
39.14(10)
166.30(15)
167.67(15)
172.86(14)
169.69(16)
N3–Cu1–O1
85.34(16)
88.44(14)
81.14(13)
101.99(13)
41.46(8)
84.44(17)
85.46(14)
79.08(13)
98.69(13)
40.34(8)
98.37(14)
86.38(14)
84.72(13)
137.24(10)
81.75(9)
84.75(16)
86.44(14)
77.42(13)
128.69(13)
82.55(8)
O1–Cu1–O52
O1–Cu1–O28
N3–Cu1–Cu3
O28–Cu1–Cu3
N21–Cu2–O24
O24–Cu2–O28
O24–Cu2–O52
N21–Cu2–Cu4
O52–Cu2–Cu4
O38–Cu3–O1
O1–Cu3–O28
O1–Cu3–O24
O38–Cu3–Cu1
O24–Cu3–Cu1
N49–Cu4–O52
O52–Cu4–O24
O52–Cu4–O1
N49–Cu4–Cu2
O1–Cu4–Cu2
O9–Cu5–O10
O10–Cu5–O14
O43–Cu6–O42
O42–Cu6–O38
84.14(15)
102.63(14)
83.96(15)
102.89(14)
Table 3
Hydrogen bonding interactions (Å, °) for 1.
In Cu2, the basal plane of the square pyramid is made up of O14,
N21, O24, and O28, and O52 is at the apex. The Cu2–(basal atom)
bond lengths vary in the range 1.919(3)–1.987(3) Å, and the apical
Cu2–O52 distance is 2.251(3) Å with Cu2 deviating 0.110(2) Å from
the square plane towards the apical O52 atom. The distortion from
D–H. . .A
d(D–H)
d(H. . .A)
d(D. . .A)
<(DHA)
O13–H13. . .O6Wi
0.84
0.84
0.84
0.84
2.08
2.07
2.03
2.34
2.48
2.70
2.01
1.89(2)
2.16(5)
2.04(3)
2.35(8)
2.37(4)
2.116(16)
2.00(4)
1.98(3)
2.01(5)
2.85(3)
1.34
2.648(10)
2.66(3)
124.2
126.5
166.6
135.2
106.6
131.9
162.2
177(7)
144(7)
161(6)
113(6)
117(3)
161(3)
150(6)
168(6)
146(8)
127.8(12)
152.2
O13A–H13A. . .O55Aii
O26–H26. . .O3 W
2.859(6)
2.993(8)
2.833(14)
3.322(11)
2.820(8)
2.767(5)
2.917(6)
2.911(7)
2.836(6)
2.905(6)
2.959(9)
2.832(5)
2.876(6)
2.822(11)
3.483(16)
2.224(14)
2.073(11)
3.064(10)
O41–H41. . .O5Wi
regular geometry in Cu2 is less than that of Cu1, evident from the
s
O41A–H41A. . .N31
O41A–H41A. . .O24
O55–H55. . .O2 W
0.84
0.84
0.84
value of 0.269. Square pyramidal coordination around Cu3 is com-
posed of four atoms in the basal plane [O38, N31, O28, and O1] and
one atom in the apical position [O24] with Cu3 being 0.039(2) Å off
the plane. However there is a long sixth Cu–O contact with O13 of
2.655(5) Å, trans to O24 [O–Cu–O = 157.99(15)°]. This is from one
of the protonated alkoxo oxygen atoms. A similar feature is found
at Cu4 where a long Cu–O contact [2.516(4) Å] with alkoxo O26 is
trans to the apical atom O1 [O–Cu–O angle is 159.8(15)°]. O42,
O1 W–H21 W. . .O15
O1 W–H11 W. . .O4Wiii
O2 W–H12 W. . .O1Wiv
O2 W–H22 W. . .O9iv
O3 W–H23 W. . .O4 W
O3 W–H13 W. . .O7Wv
O4 W–H14 W. . .O37vi
O4 W–H24 W. . .O5Wv
O6 W–H26 W. . .O9
O6 W–H16 W. . .O13Avi
O7 W–H17 W. . .N1Svii
O7 W–H27 W. . .O1S
O7 W–H27 W. . .O5Wviii
0.877(19)
0.875(19)
0.905(19)
0.920(19)
0.908(19)
0.877(19)
0.911(19)
0.907(19)
0.93(2)
0.917(19)
0.95
N49, O52, and O24 form the basal plane. The
s values of 0.146
and 0.038 for Cu3 and Cu4 respectively, suggest that distortion in
the geometry of Cu3 and Cu4 from square pyramidal geometry is
less than that of Cu1 and Cu2. The deviation of Cu3 and Cu4 from
their mean basal planes towards the apical oxygen atoms O24 and
O1 is 0.039(2) and 0.012(2) Å, respectively.
In the cubane core the Cu1–Cu4, Cu2–Cu3, Cu2–Cu4 and Cu1–
Cu3 separations are 3.0942(9), 3.0889(9), 3.0151(10), and
3.0157(9) Å, respectively whilst the O1–O52, O24–O28, O24–O52,
O1–O28 diagonal separations of these respective faces are
2.743(5), 2.665(5), 2.679(5), and 2.714(5) Å, respectively. These
measurements indicate the distortion of the cubane core.
0.93
0.93
1.15
2.61
170.7
110.4
Symmetry transformations used to generate equivalent atoms:
i
xÀ1,y,z.
ii
Àx+1/2,y+1/2,Àz+1/2.
iii
x,y+1,z.
iv
Àx+3/2,yÀ1/2,Àz+1/2.
v
x,yÀ1,z.
vi
x+1,y,z.
vii
Àx,Ày+2,Àz.
viii
The complex possesses abundant classical hydrogen bonding
interactions (intra- and intermolecular) which lead to a three
dimensional supramolecular structure (Table 3). These involve
the solvent in the lattice which has been modellled as CH3OH,
Àx+1,Ày+2,Àz.
0.5CH3CN and 6.5H2O in the asymmetric unit which link the com-
plexes via phenoxo and alkoxo O and H atoms.

S. Shit et al. / Polyhedron 52 (2013) 963–969
967
3.4. EPR spectra
ions) shown in Fig. 4. The value of v K
MT at 300 K is 1.7 cm3 molÀ1
which is lower than that expected for six magnetically isolated
EPR spectra recorded at room or liquid nitrogen temperature on
the polycrystalline powder of 1 were characterized by the absence
of a spectral signal, indicating strong antiferromagnetic coupling
between copper(II) ions (Fig. 2). Such behavior can be easily ex-
spin doublet. Indeed, assuming g = 2.00 (the lower limit)
v
antiferromagnetic coupling the starting value (6 Cu(II) ions) would
MT = 0.375 cm3 molÀ1 K for each copper(II) ion. Thus, without
be 6 Â 0.375 = 2.25 cm3 molÀ1 K). From room temperature,
vMT
plained by an examination of the structure of 1. For
l-alkoxo and
values decrease in a monotonous way from 1.7 cm3 molÀ1 K at
300 K to 0.0 cm3 molÀ1 K at 2 K, with a small plateau between 20
and 2 K. This feature is characteristic of strong dominant antiferro-
magnetic interaction. Complex 1 is actually a complicated hexanu-
clear copper(II) entity. The scheme of the possible J values and the
type of interactions on the nature of the bonds is summarized in
Table 4.
l-phenoxo bridged copper complexes, the angle for change from
ferromagnetic and antiferromagnetic coupling is ꢁ97.5° and
ꢁ77.1°, respectively [56,57]; while the angles Cu–O–Cu, in which
alkoxo oxygen atoms behave as
l3-bridges, vary between 89.9°
and 104.0°, with those Cu–O–Cu, corresponding to
l2-phenoxo
bridges, are in the range 127.4–128.9°. Therefore, on the basis of
the solid state structure of 1, alkoxo bridges should propagate only
a moderate ferro- or antiferromagnetic interaction, whilst the con-
tribution of phenoxo bridges should result in a strong antiferro-
magnetic contribution and determine the overall magnetic
behavior of 1. This is confirmed by variable temperature magnetic
susceptibility measurements (see below).
EPR spectra of 1 dissolved in DMF and CH3CN, in which it is sol-
uble, are very weak, suggesting that polynuclear arrangement is
retained in such solvents. However, the weak resonances
superimposed to the baseline, can be attributed to the formation
of a small amount of a mononuclear copper(II) species with
g|| = 2.251 and A|| = 195.4 Â 10–4 cm–1 (Fig. 3). These values are
due to the contemporaneous binding of three strong donors, and
are compatible with the coordination of two phenoxo and one al-
koxo oxygen atoms [58].
For the purposes of modeling the exchange interactions be-
tween copper pairs, the copper(II) atoms within the hexanuclear
cluster are reviewed in the following way: Cu1 and Cu2 are the
copper atoms in the top face of the cubane; Cu3 and Cu4 are the
copper atoms in the bottom face of the cubane; Cu5 and Cu6 are
the capping copper atoms for the top and bottom faces, respec-
tively (Fig. 5).
From the Fig. 5 and Table 4, it is clear that we need at least three
different J parameters to try to fit the magnetic results. In fact, in
the central [Cu4O4] cubane subcore there are six different J values
(the edges of the tetrahedron), but using all of them we will over-
parametrize the fit process. For this reason we have decided using
only two J values (J1 and J2) for the central cube and four identical J
values (J3) for the external Cu–O–Cu entities.
The fit of the susceptibility data has been carried out using the
CLUMAG program [59] which uses the irreducible tensor operator
(ITO) formalism, through the Hamiltonian: H = ÀJ1(S1S2 + S3S4) –
3.5. Variable temperature magnetic susceptibility measurements
The magnetic property of 1 is revealed by
vMT versus T plot
(where vM is the molar magnetic susceptibility for six copper(II)
1.6
1.2
0.8
T
m
χ
0.4
Fig. 2. X-band EPR spectra recorded on the polycrystalline powder of 1: (a) room
temperature and (b) liquid nitrogen temperature.
0.0
0
50
100 150 200 250 300
T / K
Fig. 4. Plot of
vMT vs. T for 1 (solid line represents the best fit of the data).
Table 4
Type of exchange interactions and coupling constants.
Interaction between
Bond type
Bond range
Coupling constants
Cu1–Cu2
Cu3–Cu4
eq–ap/eq–ap
eq–ap/eq–ap
long–long
long–long
J1
Cu1–Cu3
Cu1–Cu4
Cu2–Cu3
Cu2–Cu4
eq–eq/eq–ap
eq–eq/eq–ap
eq–eq/eq–ap
eq–eq/eq–ap
short–long
short–long
short–long
short–long
J2
Cu1–Cu5
Cu2–Cu5
Cu3–Cu6
Cu4–Cu6
eq–eq/eq–eq
eq–eq/eq–eq
eq–eq/eq–eq
eq–eq/eq–eq
short–short
short–short
short–short
short–short
J3
Fig. 3. X-band EPR spectra recorded at liquid nitrogen temperature on the
polycrystalline powder of 1 dissolved in: (a) DMF and (b) CH3CN.
eq = equatorial (short distance); ap = apical (long distance).

968
S. Shit et al. / Polyhedron 52 (2013) 963–969
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
J1
J2
J3
Cu 5
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