A new linear double phenoxide-bridged trinuclear Cu(II) Schiff base complex: Synthesis, crystallographic elucidation, magneto-structural correlation and DFT Study

Article abstract of DOI:10.1016/j.poly.2013.12.006

The tetradentate (ONNO) donor Schiff base ligand LH₂, derived from the condensation of salicylaldehyde and 1,3-diaminopropane, has been synthesized and reacted with CuBr₂ to yield a trinuclear complex with the molecular formula [Cu₃L₂Br₂]. Single crystal X-ray diffraction study reveals that the two terminal copper atoms adopt a square pyramidal geometry, whereas the central copper atom, situated at the inversion centre, is surrounded by four phenoxide oxygen atoms in a square planar fashion. Variable temperature magnetic susceptibility measurement study shows strong antiferromagnetic intra-trimer interactions between the copper centers with a J value of -302 cm^-1. EPR study of the complex depicts that, due to the exchange coupling between the copper ions, the EPR spectra were unresolved, having very broad resonances with g = 2.133. A theoretical DFT calculation was also carried out to supplement the experimental results.

Full text of DOI:10.1016/j.poly.2013.12.006

Polyhedron 69 (2014) 262–269
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
A new linear double phenoxide-bridged trinuclear Cu(II) Schiff base
complex: Synthesis, crystallographic elucidation, magneto-structural
correlation and DFT Study
a
a
b
c
Sandeepta Saha , Ashok Sasmal , Chirantan Roy Choudhury , Carlos J. Gómez-Garcia ,
d
a,
⇑
Eugenio Garribba , Samiran Mitra
a
Department of Chemistry, Jadavpur University, Raja S.C. Mullick Road, Kolkata 700032, India
Department of Chemistry, West Bengal State University, Barasat, Kolkata 700126, India
Instituto de Ciencia Molecular (ICMol), Universidad de of Valencia, Parque Científico, 46980 Paterna, Spain
Department of Chemistry and Pharmacy, University of Sassari, Via Vienna 2, I-07100 Sassari, Italy
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
The tetradentate (ONNO) donor Schiff base ligand LH , derived from the condensation of salicylaldehyde
2
Received 27 September 2013
Accepted 4 December 2013
Available online 12 December 2013
and 1,3-diaminopropane, has been synthesized and reacted with CuBr
2
to yield a trinuclear complex with
the molecular formula [Cu Br ]. Single crystal X-ray diffraction study reveals that the two terminal cop-
3
L
2
2
per atoms adopt a square pyramidal geometry, whereas the central copper atom, situated at the inversion
centre, is surrounded by four phenoxide oxygen atoms in a square planar fashion. Variable temperature
magnetic susceptibility measurement study shows strong antiferromagnetic intra-trimer interactions
Keywords:
Tetradentate Schiff base
Trinuclear copper complex
X-ray study
Antiferromagnetic interaction
EPR study
ꢀ1
between the copper centers with a J value of ꢀ302 cm . EPR study of the complex depicts that, due
to the exchange coupling between the copper ions, the EPR spectra were unresolved, having very broad
resonances with g = 2.133. A theoretical DFT calculation was also carried out to supplement the experi-
mental results.
DFT calculation
Ó 2013 Elsevier Ltd. All rights reserved.
1
. Introduction
Metal complexes containing salen type Schiff base ligands show
oxidase [10]. Trinuclear copper clusters comprise the active sites
of ceruloplasmin, laccase, ascorbate oxidase etc. [11–13]. Not only
that, the copper clusters also play different roles in many proteins,
like hemocyanine, rubredoxin, ceruloplasmin etc.
various types of properties, such as unusual coordination, high
thermodynamic stability, good fluorescent properties, catalytic
activities and interesting biological activities, etc. [1,2] and they
have proved themselves as useful chelators in the synthesis of
transition metal complexes. Di- and trinuclear complexes have
especially been the main point of interest in these fields to the
inorganic chemist in the past few decades. For example, Ferredox-
ins, which contains iron–sulfur proteins, are the main components
in electron storage and transfer [3–5] or as a co-factor (FeMoCo) or
P-cluster in nitrogenase. In phospholipase C (Bacillus circus), a tri-
nuclear zinc cluster comprises the active site, which catalyzes the
hydrolysis of phosphodiester bonds in phospholipids [6,7]. How-
ever, there are no proteins or metal complexes that possess such
versatile properties as those of copper clusters. The study of trinu-
clear copper(II) systems holds a special position in coordination
chemistry since the discovery of the trinuclear active site in laccase
The rich chemistry of these complexes has been applied to the
investigation of magnetic exchange interactions between metal
ions. The predominant magnetic interaction between metal cen-
ters in the commonly known polynuclear Cu(II) complexes is anti-
ferromagnetic, as observed in the antiferromagnetically coupled
metal centers present in the active sites of multicopper enzymes
[14], whereas ferromagnetic interactions in linear trinuclear Cu(II)
Schiff base complexes are comparatively rare [14–17]. Although
the field of magnetically important Cu(II) complexes has been
extensively studied, fewer Schiff base complexes containing a lin-
ear trinuclear array of copper centers have been reported [18,19].
One approach for the synthesis of multinuclear complexes of cop-
per is the introduction of tri- or tetradentate Schiff base ligands
[20–22]. Tetradentate Schiff base complexes of nickel [23–24], zinc
[25–27] and to a lesser extent copper [28] adopt a trimetallic motif
when prepared from the relevant metal salt and a symmetric li-
gand constructed from two rigid chelating salicylidene units sepa-
[
8], the ferromagnetically coupled Cu(II) triad in methane mono-
oxygenase [9] and the angled trinuclear Cu(II) unit in ascorbate
rated by
a flexible spacer (e.g. 1,3-propanediamine, 1,4-
butanediamine etc.). This motif creates two different metal binding
environments to form multinuclear complexes.
⇑
Corresponding author. Tel.: +91 9836883638.
E-mail address: samiranju92@gmail.com (S. Mitra).
0
277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.12.006

S. Saha et al. / Polyhedron 69 (2014) 262–269
263
2
In recent years, our group has reported a number of such trinu-
clear complexes using ONNO donor Schiff base ligands and in most
of the cases the central metal atom is copper [29–31]. The study of
the magnetic exchange interaction is controlled by the ligands, by
their different bridging modes, and it becomes more unusual or
complicated by the insertion of other bridging ligands. An addi-
tional bridging anionic ligand, like carboxylate, is usually needed
to provide the required rigidity and stability of the polynuclear
structure [32]. Interestingly, most of the trinuclear complexes re-
ported so far contain an additional anion bridge. Depending on sev-
eral factors, such as the identity of the bridge, the dihedral angles
between the planes of the magnetic orbitals, the distance between
the metal centers, the stereochemistry around the metal ion and
the topology of the bridging framework, the complexes show dif-
ferences in the nature and extent of the magnetic exchange inter-
action (antiferromagnetic and ferromagnetic) [33–36]. In the case
of magnetostructural correlations in multinuclear Cu(II) complexes
bridged by pairs of alkoxide or phenoxide groups, the major factor
controlling the spin coupling (J) between the metal centers is the
trix least-squares based on F using the program SHELXL-2013 [41b].
All non-hydrogen atoms were refined anisotropically. C-bound
hydrogen atoms were placed geometrically and refined using a rid-
ing model approximation. The molecular graphics and crystallo-
graphic illustrations for the complex were prepared using ORTEP
programs [42a,42b].
2
.4. Magnetic susceptibility and EPR study
The magnetic susceptibility measurements were carried out in
the temperature range 2–300 K with an applied magnetic field of
.1 T on polycrystalline samples of the compound (with masses
of 13.75 mg) with a Quantum Design MPMS-XL-5 SQUID suscep-
tometer. The isothermal magnetization as performed on the same
sample at 2 K with magnetic fields up to 5 T. The susceptibility data
were corrected for the sample holders, previously measured using
the same conditions, and for the diamagnetic contributions of the
0
salt, as deduced using Pascal’s constant tables (
v
dia = -438.24 x
emu mol for complex 1) [43]. The EPR spectra were re-
corded from 0 to 10000 Gauss in the temperature range 77–
98 K with an X-band (9.15 GHz) Varian E-9 spectrometer. The
ꢀ6
ꢀ1
1
0
Cu–O–Cu angle (U) [37,38]. There is only one reported complex
containing a salicylaldehyde Schiff base and it is ferromagnetically
coupled [39].
In continuation of our earlier studies [37] on phenoxide-bridged
multinuclear Cu(II) Schiff base complexes, this paper contains the
synthesis, spectral and structural characterization, magnetic sus-
ceptibility measurement and DFT study of a new trinuclear cop-
2
EPR parameters reported in the text were obtained by simulating
the spectra with the computer program Bruker WinEPR SimFonia
[
44].
2.5. Computational details
3 2 2
per(II) complex having the molecular formula [Cu L Br ], where
LH is the 2:1 condensation product of salicylaldehyde and 1,3-
2
The gas phase geometry of the copper(II) trinuclear complex
diaminopropane. Interestingly, our reported complex does not in-
volve extra anion bridge, but the bromide ion is connected to the
terminal copper in a monodentate fashion. An EPR study of the
complex was also carried out. Vafazadeha et. al. [40] synthesized
a trinuclear complex using same ligand, but with copper chloride
instead of copper bromide, and the geometry of the complex was
quite different to the complex we are reporting here.
was optimized without any symmetry restrictions with spin-unre-
stricted density functional theory (DFT) calculations using the hy-
brid DFT (B3LYP) method and the basis set 6-31g(d) with the
GAUSSIAN 03 software package [45]. The starting structure of com-
plex 1 was generated from its X-ray crystallographic data. No con-
straint was applied in the calculations and all atoms were free to
optimize.
2
. Experimental
2.6. Synthesis of the ligand and the complex
2.1. Materials
2.6.1. Synthesis of the ligand [LH2]
2
The ligand LH was synthesized by refluxing 2 mmol of salicyl-
Salicylaldehyde, 1,3-diaminopropane and copper bromide were
aldehyde (0.21 mL) and 1 mmol of 1,3-diaminopropane (0.08 mL)
for 1 h in methanol medium, and it was used without further
purification.
purchased from Sigma–Aldrich (USA) and were used without fur-
ther purification. All the solvents were of AR grade and were used
as received.
2.6.2. Synthesis of the complex [Cu
3
L
Br
2 2
] (1)
2.2. Physical measurements
CuBr (0.3315 g, 1.5 mmol) and LH
2
2
(0.268 g, 1 mmol) were ta-
ken in 25 mL methanol and the mixture was refluxed for 2 h. After
that, the resulting brown solution was filtered and kept for slow
evaporation. After 1 week, brown block shaped crystals appeared,
which were found to be suitable for X-ray diffraction study.
The infrared spectrum of the complex was recorded on a Perkin-
Elmer RX1 FT-IR spectrophotometer with KBr discs (4000–
ꢀ1
4
00 cm ). Elemental analyses (C, H, N) were carried out using a
Perkin-Elmer 2400 II elemental analyzer. Ground state absorptions
for the complex were recorded at 300 K with a Perkin-Elmer Lamb-
da-40 (UV–Vis) spectrophotometer using HPLC grade acetonitrile.
Yield: 56%. Anal. Calc. for [C34
N, 6.14. Found: C, 44.75; H, 3.47; N, 6.27%.
2 3 4 4
H32Br Cu N O ]: C, 44.78; H, 3.54;
3
. Results and discussion
.1. FT-IR spectrum
The infrared spectrum of complex 1 is consistent with the struc-
2.3. X-ray crystallography
3
X-ray diffraction data of the complex were recorded on a Bruker
APEX II CCD diffractometer, equipped with graphite monochroma-
tized Mo K radiation (k = 0.71073 Å) fine-focus sealed tubes. For
the complex, intensity data were collected at 296(2) K using
and scans. Data refinement and reduction were performed using
ꢀ1
a
tural data given in this paper. A broad band around 3436 cm is
assigned to (OH) of the triply bridging hydroxyl group. A strong
and sharp absorption band around 1617 cm in the spectrum of
complex 1 may be assigned to the imine stretching frequency,
i.e. coordination of the nitrogen atom of the imine group to the me-
u
m
ꢀ1
x
Bruker SAINT software. Multiscan absorption corrections were ap-
plied empirically to the intensity values (Tmin = 0.499 and
T
max = 0.643) using SADABS [41a]. The structure was solved by direct
tal ions [46,47]. The phenolic
quency region at about 1181–1186 cm
m(Ar–O) band observed in the fre-
ꢀ1
methods using the program SHELXS-2013, and refined with full-ma-
provides evidence for

2
64
S. Saha et al. / Polyhedron 69 (2014) 262–269
coordination to the metal ions through the deprotonated phenolic
oxygen atoms [48]. Sharp bands appearing at nearly 496 and
The coordination number 5 for copper(II) is very common,
where it may possess either a square pyramidal (SP) or trigonal
bipyramidal (TBP) geometry. For a penta-coordinated geometry,
the distortion of the structure from SP to TBP can be evaluated
ꢀ1
4
21 cm correspond to the Cu–N and Cu–O stretching frequen-
cies, respectively. Other characteristic peaks are located at
ꢀ
1
ꢀ1
1
621 cm for
m
(C@C) and at 1540 cm for
m(C@N), respectively
by the Addison distortion parameter,
s
= (h ꢀ /)/60, where h and
[
48].
/ are the two largest coronation angles [51]. In this case the
s
va-
lue for the terminal Cu1 centre is 0.23, which clearly suggests a
distorted SP geometry for the terminal symmetry related Cu1
centers. The equatorial plane of each of the two equivalent termi-
nal copper atoms is formed by the imine nitrogen atoms (N1 and
N2) and two phenolic oxygen atoms (O1 and O2). The apical po-
sition is occupied by one bromine atom (Br1). Cu1 is displaced
from the mean equatorial plane by 0.237 Å towards the axial bro-
mine ligand.
3
.2. Electronic spectrum
⁄
In complex 1, a band at 358 nm may be assigned to the n ?
p
transition of the imine group [49] and the band at higher energy
⁄
(
269 nm) is associated with the aromatic
p
?
p
intraligand
charge-transfer transition. A broad band in the range 620–
50 nm can be correlated to a d-d transition, which is typical for
a Cu(II) Schiff base complex [50].
6
The Cu1–imine nitrogen (N1 and N2) bond distances are
1
.968(3) and 1.947(2) Å, and the Cu1-phenolic oxygen (O1 and
O2) bond distances are 1.952(2) and 1.961(2) Å, respectively,
which are comparable to similar systems [52]. The axial Cu1–Br1
distance is 2.7149(5) Å, which is longer than the equatorial bond
lengths as the size of the bromine atom is very large. The deviation
from the square pyramidal geometry is also indicated by the bond
angles involving the cis-positions, varying from 77.93(8)° to
3
.3. Description of the crystal structure of 1
The ORTEP diagram of complex 1, with the atom numbering
scheme of the trimeric asymmetric unit, is presented in Fig. 1
and the packing diagram is shown in Fig. 2. Crystallographic data
and structural refinements are presented in Table 1, while selected
bond lengths and bond angles of complex 1 are summarized in Ta-
ble 2. The centrosymmetric complex shows a linear trinuclear ar-
ray of three copper(II) ions. There are two types of geometrically
different copper(II) centers, with an N
for Cu1 and an O donor set of atoms for Cu2. In the complex,
the two terminal copper centers are penta-coordinated, bearing
identical environments, owing to the crystallographic inversion
symmetry.
9
7.42(1)°, and the angles involving the trans-positions, which vary
from 154.66(9)° to 168.62(9)°.
The geometry around the Cu2 centre can be described as per-
fectly square planar. The square plane of the Cu2 centre is formed
by two bridging phenoxide oxygen atoms (O1 and O2) from one
Schiff base and another two symmetry related bridging phenoxide
2 2
O Br donor set of atoms
4
i
i
oxygen atoms (O1 and O2 ) coming from another Schiff base li-
gand. As the central Cu2 lies on an inversion centre, all the trans
angles are exactly 180°. The equatorial bond distances are
1
.9544(19) and 1.9350(19) Å for Cu2–O1 and Cu2–O2, respectively,
In the trimeric unit, the copper(II) atoms are joined by doubly
which are comparable to similar systems [52]. In this complex, the
Cu1ꢁ ꢁ ꢁCu2 distance is 2.8550(3) Å, which is slightly lower than for
similar systems in the literature [52].
bridging
2
l -phenolate oxygen atoms of the tetradentate Schiff base
ligand. Here, the Schiff base behaves as an N
bromide ion acts as a terminal ligand.
2 2
O donor set and the
Fig. 1. ORTEP diagram of complex 1 with the atom numbering scheme (thermal ellipsoids are plotted at the 40% probability level).

S. Saha et al. / Polyhedron 69 (2014) 262–269
265
Fig. 2. Packing diagram of complex 1.
Table 1
that, after dissolution, the trinuclear structure of 1 is destroyed,
as noticed for other polynuclear Cu(II) compounds [30,53–57].
The EPR signals were simulated with the software WinEPR, [44]
and are countersigned by a very slight rhombicity. They were
Crystallographic data and structural refinement of complex 1.
1
Empirical formula
Formula weight (g molꢀ1)
Temperature
Crystal system
Space group
a (Å)
34 2 3 4 4
C H32Br Cu N O
911.09
296(2)
monoclinic
P21/n (14)
8.9531(2)
10.7736(3)
17.3347(5)
90.00
101.4780(10)
90.00
1638.62(8)
2
1.847
4.413
simulated with the following parameters: g
z
= 2.243, A
z
= 189
ꢀ
4
ꢀ1
ꢀ4
ꢀ1
ꢂ 10 cm , g
y
= 2.055, A
(CH CN, Fig. 4a). g
= 22 ꢂ 10 cm
= 2.241,
= 2.053, A
y
= 25 ꢂ 10 cm , g
x
= 2.052, A
x
= 14
ꢀ4
ꢀ1
ꢀ4
ꢀ1
ꢂ 10 cm
3
z
= 2.243, A
= 2.052,
z
= 188 ꢂ 10 cm
,
ꢀ4
ꢀ1
ꢀ4
ꢀ1
g
y
= 2.056,
A
y
,
g
x
A
x
= 14 ꢂ 10 cm
ꢀ
4
ꢀ1
ꢀ1
(
A
DMSO, Fig. 4b);
g
z
A
z
= 184 ꢂ 10 cm
y
, g = 2.056,
b (Å)
c (Å)
ꢀ4
ꢀ1
ꢀ4
y
= 20 ꢂ 10 cm , g
x
x
= 14 ꢂ 10 cm (DMF). These
a
(°)
b (°)
(°)
are consistent with a dx
2ꢀy2
ground state and a square planar
geometry [58].
c
3
The values simulated are lower than those observed for the
V (Å )
0
Z
D
species formed by salen (N,N -ethylenebis(salicylidenimine))
calc (g cmꢀ3)
derivatives, for which g
z
values in the range 2.18–2.22 and A
z
ꢀ
1
l
(mm
)
ꢀ4
ꢀ1
in the range 199–212 ꢂ 10 cm are reported [59–62]. The g
z
/
F(000)
Crystal size (mm )
h Range (°)
Measured reflections
Independent reflections
906
3
z
A ratio, which measures the distortion degree of the equatorial
0.10 ꢂ 0.14 ꢂ 0.16
2.2–27.2
22263
3628
0.041
plane of a Cu(II) complex [63], between 103 and 112 indicates
a high planarity of such complexes. On the contrary, for salpn
0
(
A
N,N -propylenebis(salicylidenimine)) derivatives g
z
is 2.261 and
R
(int)
ꢀ4
ꢀ1
176 ꢂ 10 cm , [64] with a g
z z
/A ratio of 128, suggesting a
2
z
Goodness-of-fit (GOF) on F
Final R indices [I > 2
0.917
tetrahedral distortion of the equatorial plane. This means that
for Cu(II) complexes with three fused chelate rings, the passage
r
(I)]
R
1
= 0.0324, wR
wR = 0.1063
= 0.0520, wR
wR = 0.0984
ꢀ0.49 and 0.77
2
= 0.0857
= 0.0755
2
R indices (all data)
R
1
2
from (6,5,6) to (6,6,6) rings causes an increase of the distortion.
ꢀ
2
2
This in agreement with the observation that from the [CuLH ]
max (e Å 3
ꢀ
)
D
qmin and
D
q
,
species formed by the tripeptide bAla-Gly-bAla ((6,5,6)-chelate
rings) to that formed by bAla-bAla-bAla ((6,6,6)-chelate rings),
g
z
and g
z z z
/A increase, and A decreases significantly [65]. The val-
3.4. EPR spectra
ues of g , A and the ratio g /A (between 119 and 122) for 1 are
z
z
z
z
intermediate between those measured for the salen and salpn
derivatives. Therefore, the species formed by 1 is characterized
by a higher planarity than [Cu(salpn)]. This can be related to
the tendency of the ligand L in 1 to stabilize a penta-coordinated
geometry around the Cu centres. Therefore, we can explain the
The EPR spectra, recorded on a polycrystalline powder of 1, are
shown in Fig. 3. They show very broad unresolved resonances, due
to exchange coupling between the copper ions. As can be noticed,
the line width significantly decreases at 100 K (Fig. 3b), but no
anisotropy of g nor hyperfine coupling with the Cu nuclei can be
detected. The isotropic values of g (giso) are 2.120 at RT and
ꢀ
results postulating the formation of [CuLBr] or [CuL(solvent)],
where the solvent can be DMSO, DMF or CH CN, with the two
3
2
.100 at 100 K.
phenolate oxygens and the two imine nitrogens in the equatorial
ꢀ
Anisotropic spectra of 1, dissolved in organic solvents (DMSO,
plane, plus the binding of a Br or a solvent molecule in the axial
DMF or CH
3
CN), are typical of mononuclear Cu(II) centres and
position, which hinders the tetrahedral distortion of the equato-
rial plane.
show a well-resolved hyperfine structure (Fig. 4) This suggests

2
66
S. Saha et al. / Polyhedron 69 (2014) 262–269
Table 2
Selected bond lengths (Å) and angles (°) for complex 1.
Complex 1
Bond lengths
Experimental
Theoretical
Bond angles
Experimental
Theoretical
Br1–Cu1
Cu1–O1
Cu1–O2
Cu1–N1
Cu1–N2
Cu2–O1
Cu2–O2
2.715(5)
1.952(2)
1.961(2)
1.968(3)
1.947(2)
1.954(19)
1.935(19)
2.720
1.960
1.951
1.967
1.948
1.954
1.935
Br1–Cu1–O1
Br1–Cu1–O2
Br1–Cu1–N1
Br1–Cu1–N2
O1–Cu1–O2
O1–Cu1–N1
O1–Cu1–N2
O2–Cu1–N1
O2–Cu1–N2
N1–Cu1–N2
O1–Cu2–O2
O1–Cu2–O2i
86.92(6)
91.41(6)
111.45(7)
95.46(8)
77.93(8)
92.02(9)
168.62(9)
154.66(9)
90.88(9)
97.42(10)
78.47(8)
101.53(8)
86.93
91.40
111.19
95.42
77.97
90.87
168.73
154.73
92.05
97.39
78.48
101.55
0
0
0
0
0
0
0
0
.70
.65
.60
.55
.50
.45
.40
.35
0
0
0
0
0
0
0
.425
.420
.415
.410
.405
.400
.395
0
10
20
30
40
50
T (K)
0
50
100
150
200
250
300
T (K)
Fig. 3. X-band EPR spectra of the polycrystalline complex 1: (a) RT and (b) 100 K.
Diphenylpicrylhydrazyl (dpph) is the standard field marker (g = 2.0036).
Fig. 5. Thermal variation of the vmT product per Cu(II) trimer for compound 1 (inset
shows the low temperature region; dashed and solid lines are the best fit lines for
the symmetrical linear S = 1/2 trimer model without and with inter-trimer
interactions, respectively).
v
m
T value shows an almost linear decrease from room temperature
down to ca. 100 K. Between 100 and 10 K, T reaches a plateau of
T shows an additional de-
v
m
ꢀ1
0
.42 emu K mol (Fig. 5). Below 5 K, v
m
ꢀ
1
crease to reach a value of 0.40 emu K mol at 2 K. This behavior
indicates that compound 1 presents strong antiferromagnetic in-
tra-trimer interactions, resulting in an important reduction of the
magnetic moment already observed at room temperature. The pla-
teau in
m
v T observed between 100 and 5 K corresponds to the va-
lue expected for an isolated S = 1/2 ion with g ꢃ 2.12, which is the
expected ground state for a symmetric linear S = 1/2 trimer with
antiferromagnetic interactions (the three S = 1/2 spin states cannot
cancel and the ground state is, therefore, a S = 1/2 one). Since the
structure of 1 shows the presence of centro-symmetric Cu(II) tri-
mers with double oxido bridges, we have fit the magnetic proper-
ties of compound 1 with a symmetric linear S = 1/2 trimer model
[
55]. This model reproduces very satisfactorily the magnetic prop-
erties of compound 1 in the temperature range 5–300 K, with
ꢀ
1
g = 2.120 and J = ꢀ308 cm
.
Fig. 4. Anisotropic X-band EPR spectra of complex 1 dissolved in: (a) CH
DMSO. Diphenylpicrylhydrazyl (dpph) is the standard field marker (g = 2.0036).
3
CN and (b)
The spin hamiltonian considered was H = ꢀJ(S
1 2 2 3
S + S S ), the
dashed line in Fig. 5. As expected, the coupling is strong and anti-
ferromagnetic, but the model could not reproduce the additional
decrease observed below 5 K, since this decrease is probably due
to very weak inter-trimer antiferromagnetic interactions (inset in
Fig. 5). In order to reproduce this additional decrease, we have in-
3
.5. Magnetic properties
The global feature of the
m
v T versus T curve for 1 is shown in
0
Fig. 5. The
m
v T product per Cu(II) trimer for compound 1 shows,
cluded inter-trimer interactions (zj ) modelled with the molecular
ꢀ
1
at room temperature, a value of 0.66 emu K mol , well below
the expected value for three non-interacting Cu(II) ions
field approximation [66] in the model. With this addition, the mag-
netic properties can be very well reproduced over the whole tem-
ꢀ1
ꢀ1
0
ꢀ1
(
1.125 emu K mol
for g = 2). When the sample is cooled, the
perature range, with g = 2.133, J = ꢀ302 cm and zj = ꢀ0.2 cm

S. Saha et al. / Polyhedron 69 (2014) 262–269
267
(
solid line in Fig. 5). As expected, this second model is able to
reproduce the decrease observed in T at very low temperatures
v
m
with a very weak antiferromagnetic inter-trimer exchange.
A further confirmation of the antiferromagnetic coupling in
compound 1 and of the S = 1/2 ground state resulting from this
antiferromagnetic coupling is provided by the isothermal magneti-
zation at 2 K (Fig. 6). The magnetization shows a saturation value
close to 1.1
B
l , which is the expected value for an S = 1/2 spin state
with g ꢃ 2.2. Furthermore, it can be well reproduced with a Brillou-
in function for a spin S = 1/2 with a reduced g value due to the pres-
ence of inter-trimer weak antiferromagnetic interactions (solid line
in Fig. 6).
Fig. 7. Coordination environments and structural parameters of the Cu(II) trimer in
complex 1.
Finally, the strong and antiferromagnetic coupling observed in
compound 1 through the double phenoxido bridges can be ex-
plained from the magneto-structural correlations established for
double oxido and phenoxido bridges [67]. These correlations estab-
lish that the magnetic coupling strongly depends on the Cu–O–Cu
magnetic orbitals, thereby decreasing the antiferromagnetism,
although there is sufficient overlap of each dx2
2
orbital with the
ꢀy
phenoxide oxygen p orbital to generate negative J values of inter-
mediate magnitude (Fig. 7) [30].
bond angle (
a
) and on the dihedral angle in the Cu
2
O
2
entity. Fur-
In complex 1, the calculated
torted SP geometry for the terminal copper atoms in 1. This implies
that unlike the perfectly SP geometry where the d magnetic
s value is 0.23, suggesting a dis-
thermore, for double phenoxido bridged Cu(II) complexes, there
exists a linear correlation between the Cu–O–Cu bond angle and
x2ꢀy2
ꢀ
1
the coupling constant (J (cm ) = ꢀ31.95 ꢄ
relationship has only been established for almost planar Cu
entities with almost perfect square pyramidal geometries for the
a + 2462) [67]. This
orbital is majorly located in the basal plane in order to ensure a
more efficient overlap, resulting in a relatively larger J value, for
2
O
2
complex 1, a small distortion of the SP geometry toward the TBP
2
2
2
z
Cu(II) ions (so that the magnetic orbital is the x ꢀ y one) and
when the phenoxido bridges are connecting basal positions [67].
Despite the fact that Cu1 in compound 1 does not present a perfect
geometry allows the magnetic orbital to acquire some d character,
causing a diminution of the magnetic coupling. This explains well
the relatively lower experimentally found J value in 1.
square pyramidal geometry (
s
2 2
= 0.23) and the Cu O entities are
It has already been observed by Thompson et al. that the lack of
planarity of the Cu O unit, the presence of distortions in the Cu(II)
not planar (the dihedral angle in the Cu
2 2
O
unit is 29.1°, Fig. 7),
2
2
the linear correlation mentioned above works quite well in com-
environment and the presence of electronegative atoms directly
linked to the Cu(II) ion (as Br in 1) lead to a significant reduction
of the J value, as observed in 1. In fact, the dihedral angle of 1
(29.1°) is much larger than those of the complexes used to estab-
pound 1 and yields a strong antiferromagnetic exchange coupling
ꢀ
1
of ꢀ545 cm
for the Cu1–Cu2 exchange coupling, above the
ꢀ1
experimental value of ꢀ308 cm
.
It is well known that the magnetic behavior of divalent copper
complexes bridged equatorially by a pair of phenoxide [52] oxygen
atoms is highly dependent on the Cu–O–Cu bridge angle. Addition-
ally, the Cu–O (bridge) distance, the Cuꢁ ꢁ ꢁCu separation, the geom-
etries around the copper center and the bridging oxygen atom can
also influence the coupling, even though in smaller measure. In our
case too, the exchange coupling (J) can be explained in terms of the
geometrical distortions that affect the copper coordination
spheres, the geometry of the bridges and the planarity of the bridge
core. In the case of complex 1, the equatorial planes containing the
lish the above mentioned correlation (where the Cu
2 2
O units are
planar or almost planar) [66,67].
3.6. Theoretical calculations
The geometry of complex 1, as derived from X-ray crystallo-
graphic data, was optimized without any symmetry restrictions
by spin-unrestricted density functional theory (DFT) calculations
using the hybrid DFT (B3LYP) method with the basis set 6-31g(d)
and the GAUSSIAN 03 software package [45]. The calculated geometric
parameters of selected bond lengths and angles of complex 1 are
given in Table 2, which preserve their conformation, and these
are in very good agreement with the experimental data, suggesting
that the DFT/B3LYP method and the basis sets used in the calcula-
tions are reasonable.
2ꢀy2
dx orbitals are rotated, leaving a relatively perpendicular orien-
tation. For complex 1, the relatively perpendicular orientation of
the dx
2ꢀy2
orbitals reduces the extent of overlap between the
1
.0
.8
.6
.4
.2
.0
The
a-spin LUMO to LUMO+2 (Figs. S5–S7) of complex 1 are
mainly delocalized on the central and terminal Cu(II) centres and
the Schiff base. The HOMO to HOMOꢀ2 (Figs. S2–S4) molecular
orbitals are located mainly on the terminal Cu(II) centre and one
of two Schiff bases.
The calculated Mullikan charges on the copper atoms in com-
plex 1 (Cu1 = 0.67 and Cu2 = 1.22) are considerably lower than
the formal charge of +2, confirming a significant charge donation
from the ligands. The calculated Mullikan charges for the atoms
N1 and N2 are ꢀ0.52 and atoms O1 and O2 are also less than ꢀ1
(ꢀ0.79). So, both values are less than the formal charge (ꢀ1). All
these results indicate electron transfer occurs from the donor
atoms to the central metal.
0
0
0
0
0
0
.0
0.5
1.0
H/T (T.K )
1.5
2.0
2.5
-
1
4. Conclusion
In this paper, we report a trinuclear copper complex with an
ONNO donor Schiff base complex. The complex is characterized
Fig. 6. Isothermal magnetization at 2 K of complex 1 (solid line is the best fit to
Brillouin function for the S = ½ spin ground state) .

2
68
S. Saha et al. / Polyhedron 69 (2014) 262–269
[26] J. Reglinski, S. Morris, D.E. Stevenson, Polyhedron 21 (2002) 2175.
by spectroscopic and analytical techniques. The crystal structure of
the complex is described along with magneto structural correla-
tions. The antiferromagnetic interaction between the copper(II)
centers through the double phenoxide bridge is also rationalized.
The EPR spectra are unresolved, with broad resonances due to ex-
change coupling between the copper ions, and they are isotropic in
nature in the solid state, whereas in solution, the spectra show
anisotropy and a well resolved hyperfine structure. The relatively
moderate exchange coupling for complex 1 has been attributed
to the small distortion towards TBP, which diminishes the contri-
[
[
[
27] S. Akine, T. Taniguchi, T. Nabeshima, Inorg. Chem. 43 (2004) 6142.
28] O. Atakol, C. Arici, F. Ercan, D. Ulku, Acta Crystallogr. Sect. C C55 (1999) 511.
29] A. Datta, C.R. Choudhury, P. Talukder, S. Mitra, L. Dahlenburg, T. Matsushita, J.
Chem. Res. (S) (2003) 642.
[
[
[
[
30] S. Thakurta, J. Chakraborty, G. Rosair, J. Tercero, M.S. El Fallah, E. Garribba, S.
Mitra, Inorg. Chem. 47 (2008) 6227.
31] A. Ray, D. Sadhukhana, G. Rosair, C. Gómez-García, S. Mitra, Polyhedron 28
(2009) 3542.
32] E. Colacio, J.M. Dominguez-Vera, J.M. Moreno, J. Riuz, R. Kivekäs, A. Romerosa,
Inorg. Chim. Acta 212 (1993) 115.
33] O. Kahn, Molecular Magnetism, VCH Publishers, New York, 1993.
[34] L.K. Thompson, Z.Q. Xu, A.E. Goeta, J.A.K. Howard, H.J. Clase, D.O. Miller, Inorg.
Chem. 37 (1998) 3217.
bution of the dx
2ꢀy2
orbital, causing unfavorable overlaps of the
[
35] B. Graham, M.T.W. Hearn, P.C. Junk, C.M. Kepert, F.E. Mabbs, B. Moubaraki, K.S.
Murray, L. Spiccia, Inorg. Chem. 40 (2001) 1536. and references therein.
magnetic orbitals. A theoretical DFT calculation was also carried
out to supplement the experimental results. Overall, a reasonable
interpretation of the magnetic behavior shown by the complex
has been provided from the results of these studies.
[36] P. Manikandan, R. Muthukumaran, K.R.J. Thomas, B. Varghese, G.V.R.
Chandramonli, P.T. Manoharan, Inorg. Chem. 40 (2001) 2378.
[
37] (a) S.K. Dey, B. Bag, K.M.A. Malik, M.S. El Fallah, J. Ribas, S. Mitra, Inorg. Chem.
2 (2003) 4029;
b) S.K. Dey, B. Bag, Z. Zhou, A.S.C. Chan, S. Mitra, Inorg. Chim. Acta 357 (2004)
991;
c) M.K. Saha, D.K. Dey, B. Samanta, A.J. Edwards, W. Clegg, S. Mitra, Dalton
Trans. (2003) 488.
[38] (a) P.J. Orvan, W.E. Estes, R.R. Weller, W.E. Hatfield, Inorg. Chem. 19 (1980)
297;
b) R.L. Carlin, K. Kopinga, O. Kahn, M. Verdaguer, Inorg. Chem. 25 (1986)
786;
4
(
1
(
Acknowledgements
We highly acknowledge DST-FIST, India for funding the Single
Crystal Diffractometer Facility at the Department of Chemistry,
Jadavpur University. Financial assistance from the University Grant
Commission to S. S. is highly appreciated. We also thank the Span-
ish MINECO (CTQ2011-26507) and the Generalitat Valenciana
1
(
1
(c) D.B. Dell’Amico, R. Alessio, F. Calderazzo, F.D. Pina, U. Englert, G.
Pampaloni, V. Passarelli, J. Chem. Soc., Dalton Trans. (2000) 2067;
(
d) D.K. Towle, S.K. Hoffman, W.E. Hatfield, P. Singh, P. Choudhuri, Inorg.
Chem. 27 (1988) 94;
e) P.R. Levstein, R. Calvo, Inorg. Chem. 29 (1990) 1581.
39] H.D. Bian, J.Y. Xu, W. Gu, S.P. Yan, P. Cheng, D.Z. Liao, Z.H. Jiang, Polyhedron 22
2003) 2927.
40] R. Vafazadeha, B. Khaledi, A.C. Willis, M. Namazian, Polyhedron 30 (2011)
815.
41] (a) G.M. Sheldrick, SADABS, Programs for Area Detector Adsorption Correction,
Institute for Inorganic Chemistry, University of Göttingen: Göttingen,
Germany, 1996;
(
Prometeo program).
(
[
[
[
Appendix A. Supplementary data
(
1
CCDC 882000 contains the supplementary crystallographic data
for complex 1. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: +44 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2013.12.006.
(
b) G.M. Sheldrick, SHELX-2013-Programs for Crystal Structure Analysis: i.
Structure Determination (SHELXS) and ii. Refinement (SHELXL-2013), University of
Gottingen, Germany, 2013.
[42] (a) G.M. Sheldrick, SHELXTL, Program for the Solution and Refinement of Crystal
Structures, Version 5.1, Bruker AXS Inc., Madison, WI, 1999;
(
b) . ORTEP3 for Windows:L.J. Farrugia, J. Appl. Crystallogr. 30 (1997) 565.
[
[
43] G.A. Bain, J.F. Berry, J. Chem. Educ. 85 (2008) 532.
44] WINEPR SimFonia, version 1.25, Bruker Analytische Messtechnik GmbH,
Karlsruhe, 1996.
References
[
45] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
Gaussian 03, Revision D.01, Gaussian Inc., Wallingford, 2004.
46] S.H. Rahaman, R. Ghosh, T.H. Lu, B.K. Ghosh, Polyhedron 24 (2005) 1525.
47] (a) M. Dolaz, M. Tümer, M. Di gˆ rak, Transition Met. Chem. 29 (2004) 528;
[
[
1] G.A. Morris, H. Zhou, C.L. Stern, S.T. Nguyen, Inorg. Chem. 40 (2001) 3222.
2] P. Mukherjee, C. Biswas, M.G.B. Drew, A. Ghosh, Polyhedron 26 (2007) 3121.
and references therein.
3] R.H. Holm, P. Kennepohl, E.I. Solomon, Chem. Rev. 96 (1996) 2239.
4] E. Munck, E.L. Bominaar, Science 321 (2008) 1452.
5] H. Beinert, J. FASEB 4 (1990) 2483.
6] E. Hough, L.K. Hansen, B. Birknes, K. Jynge, S. Hansen, A. Hordvik, C. Little, E.
Dodson, Z. Derewenda, Nature 338 (1989) 357.
[
[
[
[
[
[
(
b) Z.L. You, H.L. Zhu, Z. Anorg. Allg. Chem. 630 (2004) 2754.
[
48] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination
Compounds, Theory and Applications in Inorganic Chemistry, 5th ed., John
Wiley and Sons Inc., New York, 1997.
[
[
[
49] A. Golcu, M. Tümer, H. Demirelli, R.A. Wheatley, Inorg. Chim. Acta 358 (2005)
[
[
7] S. Hansen, L.K. Hansen, E. Hough, J. Mol. Biol. 231 (1993) 870.
8] D.J. Spira-Solomon, M. Allendorf, E.I. Solomon, J. Am. Chem. Soc. 108 (1986)
1
785.
50] M. Dieng, I. Thiam, M. Gaye, A.S. Sall, A.H. Barry, Acta Chim. Slov. 53 (2006)
17.
5
316.
4
[
9] H.H.T. Nguyen, A.K. Shiemke, S.J. Jacobs, B.J. Hales, M.E. Linstrom, S.I. Chan, J.
Biol. Chem. 269 (1994) 14995.
51] (a) B.J. Hathaway, G. Wilkinson, R.D. Gillard, J.A. McCleverty (Eds.),
Comprehensive Coordination Chemistry, vol. 5, Pergamon, New York, 1987,
p. 607;
[
[
[
[
10] A. Messerschmidt, H. Leucke, R. Huber, J. Mol. Biol. 230 (1993) 997.
11] E.I. Solomon, U.M. Sundaram, T.E. Machonkin, Chem. Rev. 96 (1996) 2563.
12] D. Kosman, J. Biol. Inorg. Chem. 15 (2010) 15.
13] E.I. Solomon, J.W. Ginsbach, D.E. Heppner, M.T. Kieber-Emmons, C.H.
Kjaergaard, P.J. Smeets, L. Tian, J.S. Woertink, Faraday Discuss. 148 (2011) 11.
14] S. Mukhopadhyay, D. Mandal, P.B. Chatterjee, C. Desplanches, J.P. Sutter, R.J.
Butcher, M. Chaudhury, Inorg. Chem. 43 (2004) 8501.
(
b) A.W. Addison, T.N. Rao, J. Reedijk, J. Van Rijn, G.C. Verschoor, J. Chem. Soc.,
Dalton Trans. (1984) 1349.
[
52] (a) J. Chakraborty, B. Samanta, G. Pilet, S. Mitra, Inorg. Chem. Commun. 10
(
2007) 40;
[
[
[
[
[
[
[
(
b) A. Majumder, G. Rosair, A. Mallick, N. Chattopadhyay, S. Mitra, Polyhedron
25 (2006) 1753;
15] C. Biswas, S. Chattopadhyay, M.G.B. Drew, A. Ghosh, Polyhedron 26 (2007)
(
(
(
c) A.G. Blackman, Polyhedron 24 (2005) 1;
d) A. Mukherjee, M.K. Saha, M. Nethaji, A.R. Chakravarty, Polyhedron 23
2004) 2177.
4
411. and references therein.
16] M.S. Ray, S. Chattopadhyay, M.G.B. Drew, A. Figuerola, J. Ribas, C. Diaz, A.
Ghosh, Eur. J. Inorg. Chem. (2005) 4562. and references therein.
17] P. Mukherjee, M.G.B. Drew, M. Estrader, C. Diaz, A. Ghosh, Inorg. Chim. Acta
[
[
[
[
[
53] M.A. Ali, A.H. Mirza, R.J. Fereday, R.J. Butcher, J.M. Fuller, S.C. Drew, L.R. Gahan,
G.R. Hanson, B. Moubaraki, K.S. Murray, Inorg. Chim. Acta 358 (2005) 3937.
54] S. Thakurta, P. Roy, G. Rosair, C.J. Gómez-García, E. Garribba, S. Mitra,
Polyhedron 28 (2009) 695.
55] J.C. Jeffery, J.P. Maher, C.A. Otter, P. Thornton, M.D. Ward, J. Chem. Soc., Dalton
Trans. (1995) 819.
56] W.A. Alves, R.H. de Almeida Santos, A. Paduan-Filho, C.C. Becerra, A.C. Borin,
A.M. Da Costa Ferriera, Inorg. Chim. Acta 357 (2004) 2269.
57] (a) I.A. Koval, M. Sgobba, M. Huisman, M. Lüken, E. Saint-Aman, P. Gamez, B.
Krebs, J. Reedijk, Inorg. Chim. Acta 359 (2006) 4071;
3
61 (2008) 161.
18] S. Mukhopadhyay, D. Mandal, P.B. Chatterjee, C. Desplanches, J.P. Sutter, R.J.
Butcher, M. Chaudhury, Inorg. Chem. 43 (2004) 8501.
19] L. Gutierrez, G. Alzuet, J.A. Real, J. Cano, J. Borrás, A. Castiñeiras, Inorg. Chem.
3
9 (2000) 3608.
20] F. Tuna, L. Patron, Y. Journaux, M. Andruh, W. Plass, J.C. Trombe, J. Chem. Soc.,
Dalton Trans. (1999) 539.
21] X. Chen, S. Zhan, C. Hu, Q. Meng, Y. Liu, J. Chem. Soc., Dalton Trans. (1997) 245.
22] A.R. Paital, P.K. Nanda, S. Das, G. Aromí, D. Ray, Inorg. Chem. 45 (2006) 505.
23] J. Reglinski, S. Morris, D.E. Stevenson, Polyhedron 21 (2002) 2167.
24] S. Akine, T. Nabeshima, Inorg. Chem. 44 (2005) 1205.
[
[
[
[
[
(
b) C.P. Pradeep, S.K. Das, Polyhedron 28 (2009) 630.
[
[
58] E. Garribba, G. Micera, J. Chem. Ed. 83 (2006) 1229.
59] A.H. Maki, B.R. McGarvey, J. Chem. Phys. 29 (1958) 35.
25] L. Zhang, R.A. Jones, V.M. Lynch, Chem. Commun. (2002) 2986.

S. Saha et al. / Polyhedron 69 (2014) 262–269
269
[
60] E.H. Charles, L.M.L. Chia, J. Rothery, E.L. Watson, E.J.L. McInnes, R.D. Farley, A.J.
Bridgeman, F.E. Mabbs, C.C. Rowlands, M.A. Halcrow, J. Chem. Soc., Dalton
Trans. (1999) 2087.
61] M.M. Bhadbhade, D. Srinivas, Inorg. Chem. 32 (1993) 5458.
62] R. Klement, F. Stock, H. Elias, H. Paulus, P. Pelikán, M. Valko, M. Mazúr,
Polyhedron 18 (1999) 3617.
[63] H. Yokoi, A.W. Addison, Inorg. Chem. 16 (1977) 1341.
[64] K.M. Ananth, M. Kanthimathi, B.U. Nair, Transition Met. Chem. 26 (2001) 333.
[65] D. Sanna, C.G. Ágoston, G. Micera, I. Sóvágó, Polyhedron 20 (2001) 3079.
[66] C.J. O’Connor, Prog. Inorg. Chem. 29 (1982) 203.
[67] L.K. Thompson, S.K. Mandal, S.S. Tandon, J.N. Bridson, M.K. Park, Inorg. Chem.
35 (1996) 3117.
[
[