Synthesis, crystal structure and magnetic properties of quasi-linear tetranuclear copper(II) Schiff base complexes formed by covalent linkage of asymmetrically dibridged dicopper(II) units

Article abstract of DOI:10.1016/j.ica.2003.10.011

Alkoxo-phenoxo bridged tetranuclear copper(II) complexes [Cu ₄L₂(O₂CC₆H₄-p-OH) ₂] (1) and [Cu₄L₂(O₂CC ₆H₄-o-OH)₂] (2) containing pentadentate Schiff base ligand N,N′-(2-hydroxypropane-1,3-diyl)bis(salicylaldimine) (H ₃L) are prepared and structurally characterized. Crystal structures of the complexes show the covalent linkage between two {Cu₂L(O ₂CR)}(R = C₆H₄-p-OH, C₆H ₄-o-OH) units through the phenoxo atoms of the Schiff base ligand showing axial/equatorial bonding modes. The Cu(1)-O(2)-Cu(2) alkoxo bridge angle is 131° in 1 and 2. The pendant ortho- and para- OH groups of the three-atom bridging carboxylate ligands show no apparent bonding interactions with the metal or other group(s). The complexes show a d-d band near 635 nm in CH₂Cl₂. Variable temperature magnetic susceptibility measurements in the temperature range 300-18 K show antiferromagnetically coupled spin system. A theoretical fit of the magnetic data using exchange parameters J₁ and J₂ for the intradimer and interdimer units of the quasi-linear tetrameric core gave values as: J₁ = -132,J₂ = -72 cm^-1 for 1 and J₁ = -167,J ₂ = -67 cm^-1 for 2.

Full text of DOI:10.1016/j.ica.2003.10.011

Inorganica Chimica Acta 357 (2004) 1077–1082
www.elsevier.com/locate/ica
Synthesis, crystal structure and magnetic properties of quasi-linear
tetranuclear copper(II) Schiff base complexes formed by
covalent linkage of asymmetrically dibridged dicopper(II) units q
a
Arindam Mukherjee , Manas K. Saha , Indranil Rudra ,
a
b
b
Suryanarayanasastry Ramasesha , Munirathinam Nethaji , Akhil R. Chakravarty
a
a,
*
a
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Sir C.V. Raman Avenue, Bangalore 560012, India
b
Solid State and Structural Chemistry Unit, Indian Institute of Science, Sir C.V. Raman Avenue, Bangalore 560012, India
Received 14 June 2003; accepted 4 October 2003
Abstract
Alkoxo-phenoxo bridged tetranuclear copper(II) complexes [Cu4L2(O2CC6H4-p-OH)2] (1) and [Cu4L2(O2CC6H4-o-OH)2] (2)
containing pentadentate Schiff base ligand N; N0-(2-hydroxypropane-1,3-diyl)bis(salicylaldimine) (H3L) are prepared and struc-
turally characterized. Crystal structures of the complexes show the covalent linkage between two {Cu2L(O2CR)}(R ¼ C6H4-p-OH,
C6H4-o-OH) units through the phenoxo atoms of the Schiff base ligand showing axial/equatorial bonding modes. The Cu(1)–O(2)–
Cu(2) alkoxo bridge angle is 131° in 1 and 2. The pendant ortho- and para- OH groups of the three-atom bridging carboxylate
ligands show no apparent bonding interactions with the metal or other group(s). The complexes show a d–d band near 635 nm in
CH2Cl2. Variable temperature magnetic susceptibility measurements in the temperature range 300–18 K show antiferromagnetically
coupled spin system. A theoretical fit of the magnetic data using exchange parameters J1 and J2 for the intradimer and interdimer
units of the quasi-linear tetrameric core gave values as: J1 ¼ ꢀ132; J2 ¼ ꢀ72 cmꢀ1 for 1 and J1 ¼ ꢀ167; J2 ¼ ꢀ67 cmꢀ1 for 2.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Copper complexes; Crystal structures; Pentadentate Schiff base; Magnetic properties
1. Introduction
in modeling the dicopper active sites of hemocyanin and
to probe their unusually low magnitude of the exchange
parameter (ꢀ2J) which is proposed to be due to the
Ôcounter-complementaryÕ nature of overlap of the mono-
atomic bridging hydroxo/alkoxo and the three-atom
bridging carboxylato ligands [7–10]. This is in contrast
with the complementary effect of the overlap known for
the other two classes of dicopper(II) complexes having
{Cu2(l-OH/OR)22þ} and [Cu2(l–O2CR)4] cores [11–14].
Multi-dentate Schiff base ligands have generally
been used to stabilize the asymmetrically dibridged
dicopper(II) core. The present work stems from our
interest to study the magneto-structural properties of
dinuclear complexes having a pentadentate Schiff base
N; N0-(2-hydroxypropane-1,3-diyl)bis(salicylaldimine) (H3L)
using ortho- and para-substituted benzoic acid for
generation of possible supramolecular architectures
utilizing the pendant phenolic OH groups. Earlier
Dinuclear copper(II) complexes have been extensively
used to derive magneto-structural correlations to un-
derstand the nature of spin–spin coupling phenomena in
different structural arrangements and as precursors in the
synthesis of high nuclearity copper clusters showing
novel supramolecular structural features [1–6]. Among
three major dinuclear copper(II) structural types, the
asymmetrically dibridged dicopper(II) complexes with a
{Cu2(l-OH/OR)(l-O2CR)2þ} core have drawn consid-
erable current interests for their use in catalytic reactions,
q Supplementary data associated with this article can be found, in the
online version, at doi:10.1016/j.ica.2003.10.011.
* Corresponding author. Tel.: +91-802932533; fax: +91-803600683.
E-mail address: arc@ipc.iisc.ernet.in (A.R. Chakravarty).
0020-1693/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2003.10.011

1078
A. Mukherjee et al. / Inorganica Chimica Acta 357 (2004) 1077–1082
Nishida and Kida have shown that this Schiff base
ligand in presence of benzoic acid forms a discrete
dimeric species, structurally characterized as [Cu2L(O2
CPh)] [7]. Interestingly, using substituted benzoic acids,
we are successful in isolating new tetrameric copper(II)
complexes of formulations [Cu4L2(O2CC6H4-p-OH)2]
(1) and [Cu4L2(O2CC6H4-o-OH)2] (2), resulting from
the covalent linkage of two dimeric {Cu2L(O2CR)}
units. While several types of tetranuclear copper(II)
complexes are known showing a variety of arrange-
ments of four copper(II) centers like linear [15–19],
cubane [20–22], rectangular [23–25], and cyclic [26–29],
the linear complexes are of particular importance for
NLO applications [15]. Herein, we report the synthesis,
crystal structure and magnetic properties of complexes
1 and 2. Significant result of this study is the obser-
vation of a quasi-linear tetrameric copper(II) core
having pendant phenolic OH groups that could be
utilized for suitable binding to a metal center or in the
formation of supramolecular architecture.
758s, 702m, 593w, 440w. UV–vis in MeOH, kmax, nm (e,
ꢀ1 cmꢀ1): 631 (280), 365 (10500), 272 (38500).
M
2.3. Measurements
The infrared and electronic spectral data were re-
corded on a Bruker Equinox-55 FT-IR and Hitachi U-
3400 spectrophotometers, respectively. Elemental anal-
ysis data were obtained from Heraeus CHN–O rapid
instrument. Variable temperature magnetic susceptibility
data for the powdered samples of complexes 1 and 2 were
measured in the temperature range 18–300 K using a
Model 300 Lewis Coil Force Magnetometer (George
Associates Inc., Berkeley, CA), equipped with a Cahn
balance and a closed cycle cryostat (Air Products). The
experimental susceptibility data were corrected for dia-
magnetic contributions and for temperature-indepen-
dent paramagnetism (Na). Hg[Co(NCS)4] was used as a
standard.
2.4. X-ray crystallography
2. Experimental
Single crystals of complex 1 were obtained by slow
evaporation of methanol–isopropanol solution (6:1 v/v)
at an ambient temperature. Single crystals of 2 were
obtained from the mother liquor on slow evaporation.
Crystal mounting was done on glass fiber using epoxy
cement. All geometric and intensity data were collected
using an automated Enraf-Nonius CAD4 diffractometer
2.1. Materials
All chemicals used were of reagent grade and pur-
chased from commercial sources. The Schiff base N; N0-
(2-hydroxypropane-1,3-diyl)bis(salicylaldimine) (H3L)
and the precursor complex [Cu2L(O2CMe)] were pre-
pared by literature procedures [7]. The solvents used in
the spectral measurements were distilled prior to use.
ꢀ
equipped with Mo Ka radiation (k ¼ 0:71073 A). In-
tensity data, collected using x–2h scan mode, were cor-
rected for Lorentz-polarization effects and for
absorption [30]. The structures were solved by heavy
atom method and refined on F 2 by full matrix least
squares using SHELX programs [31]. There was one
lattice isopropanol molecule in the asymmetric unit of
the crystal structure of 1. The non-hydrogen atoms other
than two carbon atoms of the solvent molecule in 1 and
the positionally disordered hydroxy oxygen atom of the
carboxylate in 2 were refined anisotropically. The O(6)
and O(6A) atoms were refined with a site occupancy
factor of 0.6 and 0.4, respectively. Few hydrogen atoms
were located from the difference Fourier map and the rest
were generated and assigned fixed thermal parameters,
and refined using riding model. The hydrogen atoms
were used for structure factor calculation only. Selected
crystallographic data are given in Table 1. Perspective
views of the molecules were obtained by ORTEP [32].
2.2. Synthesis of the complexes
Complexes [Cu4L2(O2CC6H4-p-OH)2] (1) and
[Cu4L2(O2CC6H4-o-OH)2] (2) were prepared by a gen-
eral preparative method in which the carboxylic acid (0.2
g, 1.45 mmol) was initially reacted with sodium hy-
droxide (0.05 g, 1.25 mmol) under stirring for 30 min in
10 ml MeOH. The sodium salt of the carboxylate was
then reacted with [Cu2L(O2CMe)] (0.7 g, 1.4 mmol)
under stirring for a period of 2 h. A green solid thus
formed was separated, washed with ethanol and finally
dried over P4O10 in vacuo. Yield: 0.6 g (65%). Anal.
Found: C, 51.3; H, 3.9; N, 4.7. Calc. for C48H40
N4O12Cu4 (1): C, 51.5; H, 3.6; N, 5.0%. FT-IR (KBr
phase), cmꢀ1: 3388br, 3021w, 2911w, 1643s, 1600s,
1550s, 1448s, 1394s, 1300m, 1198m, 1153m, 1053w,
857w, 759m, 699m, 637w, 571w, 461w, 426w [s, strong;
m, medium; w, weak; br, broad]. UV–vis in MeOH, kmax
,
3. Results and discussion
nm (e, Mꢀ1 cmꢀ1): 640 (390), 362 (8500), 268 (27500), 243
(43000). Anal. Found: C, 51.21; H, 3.78; N, 4.87. Calc.
for C48H40N4O12Cu4 (2): C, 51.52; H, 3.60; N, 5.01%.
FT-IR (KBr phase), cmꢀ1: 3442br, 3021w, 2908w, 1635s,
1583s, 1535s, 1449s, 1329s, 1312s, 1196m, 1149m, 1032w,
3.1. Synthesis and general aspects
The tetranuclear complexes are prepared in high yield
from a reaction of [Cu2L(O2CMe)] with the sodium salt

A. Mukherjee et al. / Inorganica Chimica Acta 357 (2004) 1077–1082
1079
Table 1
Crystallographic data for 1.2iPrOH and 2
Table 2
Selected bond distances (A) and angles (°) of coordination polyhedra
in complex 1.2iPrOH and 2
ꢀ
1.2iPrOH
2
1.2iPrOH
2
Empirical formula
Formula weight
Temperature (K)
C54H56Cu4N4O14 C48H40Cu4N4O12
1239.19
293(2)
0.71073
1119.0
293(2)
Cu(1)ꢂ ꢂ ꢂ Cu(2)
Cu(1)ꢂ ꢂ ꢂ Cu(1)#1
Cu(1)–O(1)
3.485(3)
3.170(3)
1.933(5)
1.926(5)
1.932(5)
1.930(5)
2.402(5)
1.896(5)
1.916(5)
1.919(4)
1.932(6)
131.5(2)
93.37(18)
173.7(2)
89.53(19)
86.63(18)
98.13(19)
89.25(19)
91.7(2)
3.481(2)
3.256(2)
1.917(4)
1.919(4)
1.953(4)
1.934(5)
2.482(5)
1.905(4)
1.894(4)
1.937(4)
1.922(5)
ꢀ
k (Mo Ka) (A)
Crystal system
0.71073
monoclinic
P21=nð14Þ
triclinic
ꢁ
Cu(1)–O(2)
Cu(1)–O(5)
Space group (no.)
Unit cell dimensions
P1ð2Þ
Cu(1)–N(1)
Cu(1)–O(1)#1
ꢀ
a (A)
9.943(3)
11.721(4)
11.776(10)
90.21(4)
100.48(4)
104.01(3)
1307.7(12)
1
11.680(4)
11.1939(15)
17.798(5)
90
ꢀ
b (A)
Cu(2)–O(2)
Cu(2)–O(3)
ꢀ
c (A)
a (°)
b (°)
c (°)
Cu(2)–O(4)
Cu(2)–N(2)
107.04(2)
90
Cu(1)–O(2)–Cu(2)
Cu(1)–O(1)–Cu(1)#1
O(1)–Cu(1)–O(2)
O(1)–Cu(1)–O(5)
O(1)–Cu(1)–O(1)#1
O(1)#1–Cu(1)–O(2)
O(1)#1–Cu(1)–O(5)
O(1)–Cu(1)–N(1)
O(1)#1–Cu(1)–N(1)
O(2)–Cu(1)–O(5)
O(2)–Cu(1)–N(1)
O(5)–Cu(1)–N(1)
O(2)–Cu(2)–O(3)
O(2)–Cu(2)–O(4)
O(2)–Cu(2)–N(2)
O(3)–Cu(2)–O(4)
O(3)–Cu(2)–N(2)
O(4)–Cu(2)–N(2)
131.1(2)
3
ꢀ
Volume (A )
2224.7(10)
2
1.670
94.72(16)
175.53(17)
88.03(15)
85.27(15)
97.90(15)
83.77(14)
92.54(18)
104.15(16)
95.44(16)
83.64(18)
172.08(18)
177.50(19)
95.45(16)
84.16(18)
86.25(18)
94.39(19)
172.6(2)
Z
qcalc (Mg/mꢀ3
)
1.574
1.675
636
l (mmꢀ1
F (0 0 0)
)
1.957
1136
Crystal size, mm3
0.56 ꢃ 0.34 ꢃ 0.15 0.56 ꢃ 0.5 ꢃ 0.3
h Range (°)
1.76–25.02
1.87–25.00
(data collection)
Index ranges
97.3(2)
94.55(19)
83.8(2)
0 6 h 6 11,
0 6 h 6 13,
0k 6 13
ꢀ21 6 l 6 20
4114 [0.0921]
3908 [2998]
ꢀ13 6 k 6 13
ꢀ13 6 l 6 13
4869 [0.1000]
4580 [3114]
173.4(2)
176.55(19)
96.1(2)
Reflections collected [Rint
Independent reflections
[I > 2rðIÞ]
]
84.1(2)
87.3(2)
No. of parameters refined
Maximum, minimum
transmission
384
0.5973, 0.3946
354
0.5542, 0.4175
92.5(2)
175.4(2)
Goodness-of-fit on F 2
R1½I > 2rðIÞꢄ½wR2ꢄ
1.020
1.077
Symmetry transformations used to generate equivalent atoms: #1:
1 ꢀ x, 1 ꢀ y, ꢀz for 1; 1 ꢀ x, 1 ꢀ y, 1 ꢀ z, for 2.
0.0731 [0.1785]
0.1083 [0.2039]
1.38 and )0.86
0.0561 [0.1506]
0.0788 [0.1703]
0.93 and )1.02
R1(all data) [wR2]
Largest difference peak
ꢀ3
ꢀ
and hole (e A
)
Weight factor:
w ¼ 1=½r2ðFo2Þ +(AP)2 +BP]
a ¼ 0:1346
b ¼ 0:2937
a ¼ 0:1023
b ¼ 3:9281
of the ortho- or para-hydroxybenzoic acid. The precur-
sor complex has a dicopper(II) unit linked by the alk-
oxide and acetate oxygen atoms [7]. Each copper is
coordinated to one nitrogen and two oxygen atoms of
the Schiff base and one oxygen of the acetate to form a
square planar geometry. Substitution of acetate by the
o- or p-hydroxybenzoate leads to the formation of the
tetrameric complexes that are characterized by X-ray
crystallography. The infrared spectra of the complexes
show characteristic phenolic OH stretch at ꢁ3400 cmꢀ1
.
The complexes display visible spectral band near 635
nm.
Fig. 1. An ORTEP view of complex 1 with 50% probability thermal
ellipsoids showing the atom numbering scheme.
3.2. Crystal structures
The complexes are formed from the dinuclear precursor
complex [Cu2L(O2CMe)] by substitution of the acetate
by ortho- and para-hydroxy benzoic acids and the re-
sulting {Cu2L(O2CR)} (R ¼ C6H4-p-OH, C6H4ꢀo-OH)
unit self assembles to the tetranuclear complex by
The molecular structures of the complexes have been
determined by single crystal X-ray diffraction studies.
Relevant bond distances and angles are given in Table 2,
and the ORTEP diagrams are shown in Figs. 1 and 2.

1080
A. Mukherjee et al. / Inorganica Chimica Acta 357 (2004) 1077–1082
Fig. 2. An ORTEP view with 50% probability thermal ellipsoids for
complex 2 displaying the atom numbering scheme. The O(6) and O(6a)
atoms are positionally disordered with a site occupancy factor of 0.6
and 0.4, respectively.
covalent linkage through the phenoxo oxygen atom of
the Schiff base by axial ligation. Both the complexes are
centrosymmetric showing a dimeric unit in the crystal-
lographic asymmetric unit. Within the dimeric unit, the
dicopper(II) core is bridged by the mono-atomic alkoxo
oxygen and the three-atomic carboxylato oxygen atoms
retaining the asymmetrically dibridged structure of the
precursor complex. The Cu(1)ꢂ ꢂ ꢂCu(2) distance of 3.48
ꢀ
A in 1 and 2 is similar to the Cuꢂ ꢂ ꢂCu distance in
[Cu2L(O2CPh)] [7]. The other structural features also
resemble with the benzoate species except that the Cu(1)
center in the tetrameric complexes has a square pyra-
midal (4 + 1) CuNO4 coordination geometry. The basal
planes at Cu(1) and Cu(2) centers are essentially planar
(dihedral angle: 11.5° for 1; 9.0° for 2). The alkoxo ox-
ygen atom O(2) has a planar geometry. The Cu(1)–
O(2)–Cu(2) angle of ꢁ131° is marginally smaller than
the 133° reported for the benzoato dicopper species.
In the Cu(1)Cu(1)0O(1)O(1)0 unit, the Cu(1)–O(1)
Fig. 3. Crystal packing diagram of 1 along a-axis showing the for-
mation of hydrogen bonds involving O(6) atom of the complex and the
O(7) atom of the lattice isopropanol solvent molecule (a). A packing
diagram of the complexes showing the formation of a staircase struc-
ture involving weak axial contact between the copper and oxygen at-
oms. Only the skeletal structure is shown for clarity (b).
ꢀ
axial bond length of ꢁ2.45 A is considerably long and
such a distance is likely to reduce the spin–spin coupling
between Cu(1) and Cu(1)0 centers. The Cu(1)ꢂ ꢂ ꢂCu(1)0
ꢀ
distance is 3.170 (3) A. The phenoxo-bridge angle
show interesting hydrogen bonding interactions in the
crystal packing diagrams (Fig. 3). The hydrogen-bond-
ing interactions in 1 involve the isopropanol molecule
and the phenolate oxygen of the Schiff base bound to the
terminal copper atom. A significant structural observa-
tion is the non-involvement of the ortho- and para-hy-
droxy groups of the bridging carboxylates in covalent
bonding with other atom(s) or group.
Cu(1)–O(1)–Cu(1)0 of 93.37 (18)° is significantly lower
than the Cu–OR–Cu angles reported for dialkoxo
bridged dicopper(II) complexes with a Cu2O2 unit. Kida
and coworkers [33] have studied the effects of structural
factors on the magnetism of di-l-alkoxodicopper(II)
complexes. They have shown that the Cu–O–Cu angle
has the major effect on the exchange parameter (J) va-
lue. The other factors like the dihedral angle between
two coordination planes, planarity of the Cu2O2 unit,
and tilt of the O–C bond at the bridging oxygen atom
play minor but significant role in the exchange process.
The Cu2O2 unit in 1 and 2 is planar with the phenoxo
oxygen atom O(1) having a planar geometry. The Cu2O2
plane makes an angle of 86.2° with the O(1), O(2), O(5),
and N(1) basal plane. The Cu(1) atom is displaced 0.08
3.3. Magnetic properties
The magnetic susceptibility data for the complexes in
the temperature range 300–18 K reveal overall antifer-
romagnetic behavior. Complex 1 gives a magnetic mo-
ment (per copper) that varies from 1.72 lB at 300 K to
0.34 lB at 18 K. The leff /Cu values of 2 are 1.75 lB at 300
K and 0.21 lB at 18 K. The data are indicative of a
diamagnetic ground state although at 300 K the complex
is essentially paramagnetic in nature. The magnetic sus-
ꢀ
A from this basal plane. Weak axial contact between
Cu(2) and O(3) atoms gives rise to a staircase type
network structure in the complexes. The complexes

A. Mukherjee et al. / Inorganica Chimica Acta 357 (2004) 1077–1082
1081
The magnetic susceptibility has been calculated as
v ¼ M=H, where ÔH Õ is the magnetic field and M is the
magnetization due to the field applied. The magnetiza-
tion is evaluated using the following expression:
2.0
1.8
1.6
1.4
1.2
1.0
0.4
0.3
0.2
0.1
0.0
-0.1
M ¼ ½RiMi expðꢀEiðHÞ=kBT Þ= expðꢀEiðHÞ=kBTÞꢄ;
ð2Þ
where EiðHÞ ¼ Eð0Þ þ gMiH. Eið0Þ is the eigenvalue of the
i
spin in the eigenstate ÔiÕ and ÔgÕ is the gyromagnetic ratio.
Since the value of ÔgÕ depends on the local chemical
environment, the ÔgÕ value is varied within 1.98 to 2.2,
instead of taking the free electron Ôg Õ value in the fitting.
The best theoretical fit gave average g -value as 2.1 for 1
and 2.05 for 2. The presence of a trace quantity of
paramagnetic impurity (q) in the analytically pure
sample has been taken care of using a Curie-like con-
tribution to the susceptibility in the form v ¼ C=T,
where ÔC Õ is a constant. The q values are 0.03% for 1 and
0.6% for 2. The theoretical model gave excellent fit to
the experimental data with R-value of 2.3 ꢃ 10ꢀ4 for 1
and 5.1 ꢃ 10ꢀ4 for 2. The magnitudes of the exchange
parameters are: J1 ¼ ꢀ132 cmꢀ1; J2 ¼ ꢀ72 cmꢀ1 for
complex 1 and J1 ¼ ꢀ167 cmꢀ1; J1 ¼ ꢀ67 cmꢀ1 for
complex 2. The magnitude of the J1 parameter is similar
to the values observed for the dimeric precursor com-
plex. The extent of exchange interaction in the phenoxo
bridged Cu2O2 unit is less due to poor overlap of the
metal orbitals with the axially/equatorially bound oxy-
gen atom. A lower magnitude of the Cu–O–Cu angle
also weakens the antiferromagnetic (AF) interaction
between the copper centers. The singlet ground state
results from the alignment of spins in the quasi-linear
arrangement of metal atoms as: Cu(2)"–Cu(1)#–
Cu(1)0"–Cu(2)0#. The greater magnitude of J1 in com-
plex 2 than in 1 could be due to the reduced extent of
‘‘counter-complimentary’’ effect in 2 [7,9]. It is likely
that the o-hydroxy group in 2 can form hydrogen bond
with the carboxylate oxygen atom. The presence of such
a bond is proposed from the longer Cu–O (carboxylato)
distances in 2 than in 1.
0
50
100
150
200
250
300
350
Temp (K)
Fig. 4. A plot of magnetic susceptibility data in the form of vM(O) and
vMTðrÞ vs. T for complex 1 in the temperature range 18–300 K. The
theoretical fit to experimental data is shown by solid line.
Fig. 5. Magnetic susceptibility data plotted in the form of vM(O) and
vMTðDÞ vs. T for complex 2 in the temperature range 18–300 K. The
theoretical fit to the experimental data is shown by solid line.
ceptibilities are shown as a function of temperature in
Figs. 4 and 5. The experimental data are fitted for a
theoretical model based on Heisenberg spin-1/2 Hamil-
tonian with nearest neighbor antiferromagnetic interac-
tion [34] using two different coupling constants J1 and J2
for the terminal {Cu(2), Cu(1)};{Cu(1)0, Cu(2)0} and
central {Cu(1), Cu(1)0} dicopper(II) units, in the form as,
4. Conclusion
Two new quasi-linear tetranuclear copper(II) Schiff
base complexes are prepared by covalent linkage of the
dimeric precursor involving phenoxo bridges in the
presence of o- or p-hydroxybenzoic acid which shows
three-atom bridging mode. Both the complexes display
staircase like array in the crystal structure. The variable
temperature magnetic susceptibility data show antifer-
romagnetic behavior of the complexes. A theoretical
fitting of the magnetic data displays higher magnitude of
the antiferromagnetic interaction for the terminal di-
copper(II) unit in comparison with the central one.
Complexes 1 and 2 present a new structural motif in-
volving the pentadentate Schiff base ligand. A significant
H ¼ ꢀJ1ðS1S2 þ S3S4Þ ꢀ J2S2S3:
ð1Þ
The spin labels S1; S2; S3; S4 correspond to the copper
atoms Cu(2), Cu(1), Cu(1)0, Cu(2)0 as shown in the
crystal structures. The energy levels are calculated by
exact diagonalization of the Hamiltonian matrix set up
in the constant total MS basis [35]. Using the spin–spin
correlation function, the expectation value of St2otal has
been evaluated along with the identification of the total
spin of each eigen state. The ground and the first excited
state are found to be a singlet and a triplet state,
respectively.

1082
A. Mukherjee et al. / Inorganica Chimica Acta 357 (2004) 1077–1082
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The complexes may be suitable for generation of su-
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Variable temperature magnetic susceptibility data for
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been deposited with the Cambridge Crystallographic
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211826 for 1.2iPrOH and 211827 for 2. Copies of the
data can be obtained free of charge on application to
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+44-1223-336-003; e-mail: deposit@ccdc.cam.ac.uk).
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Acknowledgements
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Financial support received from the Department of
Science and Technology, Government of India, is
gratefully acknowledged (SP/S1/F-01/2000). We thank
the Bioinformatics Centre of Indian Institute of Science,
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