Magneto-structural study on a tetracopper(II) Schiff base complex stabilizing a decanuclear water aggregate

Article abstract of DOI:10.1016/j.poly.2005.12.025

A new tetranuclear copper(II) complex [Cu₄L₂(3-ppz)₂] (1) has been prepared from the reaction of [Cu₂LBr] and 3-phenylpyrazole (3-ppz), where H₃L is a pentadentate Schiff base N,N′-(2-hydroxypropane-1

Full text of DOI:10.1016/j.poly.2005.12.025

Polyhedron 25 (2006) 2135–2141
www.elsevier.com/locate/poly
Magneto-structural study on a tetracopper(II) Schiff base
complex stabilizing a decanuclear water aggregate
a
a
b
a
Sunil G. Naik , Arindam Mukherjee , Rajamani Raghunathan , Munirathinam Nethaji ,
b,
a,
*
Suryanarayanasastry Ramasesha *, Akhil R. Chakravarty
a
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Sir C.V. Raman Avenue, Bangalore 560012, Karnataka, India
b
Solid State and Structural Chemistry Unit, Indian Institute of Science, Sir C.V. Raman Avenue, Bangalore 560012, Karnataka, India
Received 13 November 2005; accepted 23 December 2005
Available online 20 February 2006
Abstract
A new tetranuclear copper(II) complex [Cu4L2(3-ppz)2] (1) has been prepared from the reaction of [Cu2LBr] and 3-phenylpyrazole
(3-ppz), where H3L is a pentadentate Schiff base N,N0-(2-hydroxypropane-1,3-diyl)bis(salicylaldimine). The complex as 1 Æ 6H2O has
been structurally characterized by X-ray crystallography. The structure shows the presence of a tetranuclear copper(II) complex with
˚
a core showing Cuꢀ ꢀ ꢀCu distances in the range of 3.261(1)–3.492(1) A. The tetrameric complex is formed from the self-assembly of
two dimeric {Cu2L}+ units in the presence of two anionic 3-ppz ligands as linkers. The alkoxo oxygen atoms bind three copper(II)
centers giving two equatorial and one axially elongated Cu–O bond. The variable temperature magnetic susceptibility data in the range
300–17 K show antiferromagnetic spin–spin coupling giving magnetic moments of 3.3 lB at 300 K and 0.56 lB at 17 K for the Cu4 unit.
A theoretical fitting to the magnetic data gives J values of ꢁ86.3, ꢁ47.32 and 11.13 cmꢁ1 with a diamagnetic ground state. The crystal
ꢀ
structure shows the presence of two complexes and 12 water molecules in the unit cell belonging to the triclinic space group P1. Among
them, 10 water molecules form an aggregate in which six water molecules are in a chair conformation of the cyclohexane-type with two
water dimers hydrogen bonded to the cyclic structure. The discrete decameric water assembly is anchored onto the metalloorganic host
involving four phenoxo oxygen atoms of two Cu4 units and four water molecules of the cyclic chair hexameric unit through hydrogen
bonding interactions.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Keywords: Tetracopper(II) complex; Crystal structure; Magnetic properties; Decameric water aggregate; Supramolecular assembly; Schiff base
1. Introduction
ground state [6,7]. While the reaction of N,N0-(2-hydroxy-
propane-1,3-diyl)bis(acetylacetoneimine) with
(H3L0)
Discrete molecular tetranuclear transition metal com-
plexes are of interest for their magnetic properties, as mod-
els for the multimetal active sites of metalloproteins and as
precursors for high nuclearity cluster synthesis [1–6]. We
have recently reported the synthesis and magneto-struc-
tural properties of copper(II) cubanes having multidentate
Schiff base ligands and the complexes show a spin triplet
copper(II) perchlorate Æ hexahydrate forms the cubane
complex [Cu4(HL0)4] with a Cu:Schiff base ratio of
1:1, the dimeric precursor [Cu2LBr], where H3L is
N,N0-(2-hydroxypropane-1,3-diyl)bis(salicylaldimine), self
assembles to a cubane cluster [Cu4L2(OR)2] (R = H, Me)
with a Cu:Schiff base ratio of 2:1 in the presence of a base.
The ferromagnetically coupled cubane complexes show
magnetic ground state [6,7].
The present work stems from our interest to probe this
chemistry further. Using [Cu2LBr] as a precursor, we have
carried out the reaction with 3-phenylpyrazole (3-ppz) as a
possible linker of the dimeric units to generate a discrete
*
Corresponding authors. Fax: +91 80 236 00683 (S. Ramasesha), Tel.:
+91 80 229 32533; fax: +91 80 236 01552 (A.R. Chakravarty).
E-mail addresses: ramasesh@sscu.iisc.ernet.in (S. Ramasesha), arc@
ipc.iisc.ernet.in (A.R. Chakravarty).
0277-5387/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2005.12.025

2136
S.G. Naik et al. / Polyhedron 25 (2006) 2135–2141
high nuclearity structure. The product we have obtained
from the reaction is a new tetranuclear copper(II) complex
[Cu4L2(3-ppz)2] (1) which crystallizes in the hexahydrate
form (1 Æ 6H2O). The crystal structure shows the presence
of a discrete decanuclear water aggregate anchored onto
two tetranuclear host units through hydrogen bonding
interactions. Herein, we present the synthesis, crystal struc-
ture, magnetic properties of the complex along with the
supramolecular structure of the water assembly. Stabiliza-
tion of water aggregates of a larger size in metalloorganic
hosts is of current interest to unravel their properties in
restricted confinements as observed in crystal lattices and
biological systems [8–14]. A detailed theoretical study of
the variable-temperature magnetic susceptibility data of 1
shows the antiferromagnetic nature of the spin coupling
in the complex.
over P4O10 [Yield: 0.36 g (ꢂ78%)]. Anal. Calc. for
C52H44Cu4N8O6 (1): C, 55.21; H, 3.92; N, 9.91. Found: C,
55.09; H, 3.95; N, 9.75%. IR (KBr disc): 3415br, 1628s,
1601s, 1534m, 1464m, 1449s, 1396m, 1318m, 1195m,
1149w, 1132m, 1072w, 1042w, 981w, 860w, 759s,
694m cmꢁ1 [s, strong; m, medium; w, weak; br, broad].
Electronic spectral data in MeOH [kmax, nm (e, Mꢁ1 cmꢁ1)]:
620 (340), 367 (19100). Magnetic moment per molecule
(leff): 3.33 lB at 300 K; 0.56 lB at 17 K.
2.4. X-ray crystallography
Single crystals of 1 Æ 6H2O were obtained on slow evap-
oration of an aqueous methanol solution of the complex. A
crystal of approximate size 0.25 · 0.2 · 0.14 mm3 was
mounted on a glass fiber with epoxy cement. The X-ray dif-
fraction data were measured in frames with increasing x
(width of 0.3ꢁ per frame) at a scan speed of 8 s per frame
using a Bruker SMART APEX CCD diffractometer,
equipped with a fine focus sealed tube X-ray source. The
SMART software was used for data acquisition and the
SAINT software for data extraction [18]. Empirical absorp-
tion corrections were made on the intensity data [19]. The
structure was solved by the heavy atom method and refined
by full-matrix least-squares using the SHELX system of pro-
grams [20]. All non-hydrogen atoms of the complex except
the positionally disordered solvent water molecule were
refined anisotropically. All the hydrogen atoms belonging
to the complex were generated, assigned isotropic thermal
parameters and refined using the riding model. One water
oxygen atom was found to be positionally disordered to
2. Experimental
2.1. Materials
All reagents and chemicals were procured from commer-
cial sources and used without further purification. The
Schiff base N,N0-(2-hydroxypropane-1,3-diyl)bis(salicyl-
aldimine) (H3L) and 3-phenylpyrazole were prepared fol-
lowing literature methods [15,16]. The precursor complex
[Cu2LBr] was prepared from the reaction of H3L and
CuBr2 Æ H2O using a method that is similar to the one used
for an analogous complex with a related Schiff base [15].
2.2. Measurements
The elemental analyses were done using a Heraeus
CHN-O Rapid instrument. The electronic and infrared
spectral data were obtained from Perkin–Elmer Lambda
35 UV–Vis and Bruker Equinox 55 spectrophotometers,
respectively. Variable temperature magnetic susceptibility
data in the temperature range 300–17 K were obtained
for the polycrystalline sample using George Associates
Inc. Lewis-coil-force magnetometer system (Berkeley,
CA), equipped with a closed cycle cryostat (Air Products)
and a Cahn balance. Hg[Co(NCS)4] was used as a stan-
dard. Experimental susceptibility data were corrected for
Table 1
Selected crystallographic data for [Cu4L2(3-ppz)2] Æ 6H2O (1 Æ 6H2O)
1 Æ 6H2O
Empirical formula
Formula mass
Temperature (K)
C52H56Cu4N8O12
1239.21
293(2)
0.71073
triclinic
˚
Wavelength (A)
Crystal system
Space group
ꢀ
P1
˚
a (A)
12.609(3)
12.719(3)
18.171(4)
81.429(4)
77.644(4)
66.691(4)
2607.5(11)
2
˚
b (A)
˚
diamagnetic contributions [vdia = ꢁ521.7 · 10ꢁ6 cm3 Mꢁ1
]
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
[17]. The magnetic moments at various temperatures were
calculated in lB units [lB = 9.274 · 10ꢁ24 J Tꢁ1]. TGA
analysis was done on a Mettler Toledo Star thermal
analyzer.
3
˚
Unit cell volume (A )
Z
Dcalc (g cmꢁ3
)
1.578
1.680
10223
l(Mo) (mmꢁ1
)
2.3. Synthesis
Reflections collected
Independent reflections
Restraints/parameters
Rint
6038
0/684
0.1033
0.0562 [0.1153]
0.1167 [0.1417]
[Cu2LBr] (0.4 g, 0.8 mmol) in MeCN (10 ml) was reacted
with a methanol (10 ml) solution of 3-phenylpyrazole
(0.12 g, 0.8 mmol) under refluxing conditions for 1 h. The
solution was cooled to an ambient temperature and a green
solid thus obtained was isolated, washed with a mixture of
methanol and diethyl ether (1:1 v/v), and dried in vacuo
R (observed data) [R (all data)]
Rw (observed data) [Rw (all data)]
Weight factor
w ¼ 1=½r2ðF 2oÞ þ ðAPÞ þ BPꢃ
2
A = 0.0633, B = 1.1454

S.G. Naik et al. / Polyhedron 25 (2006) 2135–2141
2137
two sites that were refined as O(11) and O(11A) with site
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
occupancy values of 0.6 and 0.4, respectively. The structure
refinement gave a goodness-of-fit (GoF) value of 0.896.
Selected crystallographic data are given in Table 1. The
perspective view of the complex was obtained by ORTEP
[21].
3. Results and discussion
3.1. Synthetic and general aspects
Dicopper(II) complexes of trianionic pentadentate Schiff
base ligands have been studied for magneto-structural cor-
relations that show a counter-complimentary spin coupling
effect in the {Cu2(l-alkoxo)(l-carboxylato)} core [15,22–
27]. The {Cu2L} unit has also been used in designing tri-
copper(II) complexes as models for multicopper oxidases,
in the synthesis of high nuclearity clusters and for designing
supramolecular host structures stabilizing different types of
water clusters based on the carboxylate ligand used [6,7,28–
31]. In the present work, we have attempted to synthesize a
complex having a {Cu2L} unit using 3-phenylpyrazole as a
linker to form either a binuclear complex with a {Cu2-
(l-alkoxo)(l-pyrazolato)} core or a higher nuclearity com-
plex. The reaction of [Cu2LBr] with 3-ppz is found to form
a discrete tetracopper(II) species [Cu4L2(3-ppz)2] (1) in
good yield (Scheme 1). The complex has been characterized
from analytical and spectral methods, and by X-ray crys-
tallography. It exhibits a d–d band at 620 nm in MeOH
in its electronic spectrum. The complex shows antiferro-
magnetic behavior due to spin–spin coupling through
alkoxo and pyrazolato bridges giving magnetic moment
values of 3.33 and 0.56 lB at 300 and 17 K, respectively
(Fig. 1). The magnetic behavior is similar to those of anti-
ferromagnetically coupled dicopper(II) complexes having a
{Cu2(l-alkoxo)(l-carboxylato)} core [15,22–27]. Thermo-
gravimetric studies of 1 Æ 6H2O show loss of water (ꢂ8%
weight loss) in the temperature range 30–140 ꢁC followed
by further weight loss due to 3-ppz and Schiff base decom-
position. The gradual weight loss is observed till 250 ꢁC,
possibly due to the loss of 3-phenylpyrazole along with
the water aggregate, thus, making it difficult for an accu-
rate estimate of the water loss.
0
50
100
150
200
250
300
Temperature /K
Fig. 1. vMT vs. T plot for the complex [Cu4L2(3-ppz)2] (1) with the
theoretical fit to the experimental data shown by the solid line.
3.2. Crystal structure
The complex with six solvent water molecules crystal-
lizes in the centrosymmetric triclinic space group P1 with
a Z value of 2. Selected bond distances and angles are given
in Table 2. The ORTEP diagram of the complex is shown
ꢀ
Table 2
˚
Selected bond distances (A) and angles (ꢁ) for [Cu4L2(3-ppz)2] Æ 6H2O
(1 Æ 6H2O)
Cu(1)ꢀ ꢀ ꢀCu(2)
Cu(1)ꢀ ꢀ ꢀCu(3)
Cu(1)ꢀ ꢀ ꢀCu(4)
Cu(2)ꢀ ꢀ ꢀCu(3)
Cu(3)ꢀ ꢀ ꢀCu(4)
Cu(1)–O(1)
Cu(1)–N(1)
Cu(1)–N(5)
Cu(1)–O(2)
Cu(1)–O(5)
Cu(2)–O(3)
Cu(2)–N(2)
Cu(2)–N(8)
Cu(2)–O(2)
Cu(3)–O(4)
Cu(3)–N(3)
Cu(3)–N(7)
Cu(3)–O(5)
Cu(3)–O(2)
Cu(4)–O(6)
Cu(4)–N(4)
Cu(4)–N(6)
Cu(4)–O(5)
N(5)–N(6)
N(7)–N(8)
Cu(1)–O(2)–Cu(2)
Cu(1)–O(2)–Cu(3)
O(2)–Cu(3)–N(3)
O(2)–Cu(3)–N(7)
O(4)–Cu(3)–N(3)
O(4)–Cu(3)–N(7)
O(5)–Cu(3)–N(3)
O(5)–Cu(3)–N(7)
N(3)–Cu(3)–N(7)
3.472(1)
3.261(1)
3.457(1)
3.492(1)
3.457(1)
1.937(4)
1.941(5)
1.959(4)
2.019(4)
2.360(4)
1.909(4)
1.931(5)
1.945(5)
1.986(3)
1.927(4)
1.953(4)
1.975(4)
2.000(4)
2.375(4)
1.890(4)
1.935(5)
1.941(5)
1.978(4)
1.361(6)
1.361(6)
120.14(18)
95.47(14)
106.41(16)
87.61(15)
91.64(18)
91.37(18)
83.06(17)
93.97(17)
165.19(18)
Cu(1)–O(5)–Cu(3)
Cu(1)–O(5)–Cu(4)
Cu(3)–O(5)–Cu(4)
Cu(2)–O(2)–Cu(3)
Cu(1)– N(5)–N(6)
Cu(2)–N(8)–N(7)
Cu(3)– N(7)–N(8)
Cu(4)– N(6)–N(5)
O(1)–Cu(1)–O(2)
O(1)–Cu(1)–O(5)
O(2)–Cu(1)–O(5)
O(1)–Cu(1)–N(1)
O(1)–Cu(1)–N(5)
O(2)–Cu(1)–N(1)
O(2)–Cu(1)–N(5)
O(5)–Cu(1)–N(1)
O(5)–Cu(1)–N(5)
N(1)–Cu(1)–N(5)
O(2)–Cu(2)–O(3)
O(2)–Cu(2)–N(2)
O(2)–Cu(2)–N(8)
O(3)–Cu(2)–N(2)
O(3)–Cu(2)–N(8)
N(2)–Cu(2)–N(8)
O(2)–Cu(3)–O(4)
O(2)–Cu(3)–O(5)
O(4)–Cu(3)–O(5)
O(5)–Cu(4)–O(6)
O(5)–Cu(4)–N(4)
O(5)–Cu(4)–N(6)
O(6)–Cu(4)–N(4)
O(6)–Cu(4)–N(6)
N(4)–Cu(4)–N(6)
96.49(14)
105.34(15)
120.70(18)
106.03(15)
120.9(3)
126.7(3)
119.1(3)
124.1(3)
175.15(16)
98.14(16)
83.89(14)
92.11(19)
90.00(19)
83.08(17)
94.49(17)
107.20(16)
87.63(16)
164.55(19)
175.88(17)
84.17(17)
93.07(17)
92.33(18)
90.54(17)
175.7(2)
96.82(15)
83.92(15)
174.63(16)
173.91(18)
84.14(17)
94.05(17)
91.99(19)
89.70(19)
177.8(2)
N
N
N
N
O
O
Cu
Cu
Cu
Cu
O
O
N
O
O
Br
N
[Cu2LBr]
N
MeOH:MeCN (1:1)
Reflux 1h
N
N
+
O
O
N
Cu
Cu
O
NH
N
3-phenylpyrazole (ppz)
[Cu4L2(ppz)2]
Scheme 1. The synthetic methodology for the tetranuclear copper(II)
complex [Cu4L2(3-ppz)2] (1).

2138
S.G. Naik et al. / Polyhedron 25 (2006) 2135–2141
in Fig. 2. The structure consists of a tetranuclear copper(II)
complex in which two dimeric {Cu2L} units are covalently
linked by two mono-anionic 3-phenylpyrazole ligands. The
trianionic Schiff base shows a pentadentate mode of coor-
dination, providing the alkoxo-bridge. The coordination
geometries at the copper centers are shown in Fig. 3. The
Cu(1) and Cu(3) atoms have five-coordinate square-pyra-
midal geometry in which the nitrogen, alkoxo and phenoxo
oxygen atoms are in the basal plane. The alkoxo oxygen
atom belonging to the Schiff base displays the l3-mode of
bonding with two equatorial bonds and one significantly
long axial Cu–O bond. The Cu(2) and Cu(4) atoms have
four-coordinate planar geometry. The four copper basal
planes in the structure are not coplanar. The dihedral angle
between the Cu(1) and Cu(2) planes is ꢂ70ꢁ and the same
for the Cu(3) and Cu(4) planes is ꢂ68ꢁ. The alkoxo oxygen
atoms are not planar due to the l3-bonding mode. The
˚
Cuꢀ ꢀ ꢀCu distances are in the range 3.261(1)–3.492(1) A.
The Cu(1)ꢀ ꢀ ꢀCu(3) distance in the unit having axial–equa-
˚
Fig. 2. An ORTEP view of the tetranuclear copper(II) complex showing
the atom labelling scheme for the metal and hetero atoms, with thermal
ellipsoids at the 50% probability level. The axial Cu(1)–O(5) and Cu(3)–
O(2) bonds are not shown for clarity.
torial bridging alkoxo oxygen atoms is 3.261(1) A. The
Cu(1)–O(2)–Cu(2) and Cu(3)–O(5)–Cu(4) angles of ꢂ120ꢁ
are expected to give moderately strong antiferromagnetic
1
spin-coupling involving two fdx2ꢁy2 g orbitals of the metal
centers. In contrast, the other Cu–O–Cu angles of ꢂ95ꢁ
should promote ferromagnetism. However, magnetic inter-
actions involving these super-exchange pathways are
expected to be negligible as the dx2ꢁy2 and dz2 orbitals of
the metal centers are orthogonal to each other and the axial
Cu–O bond is significantly longer than the equatorial ones.
The tetranuclear complex is involved in hydrogen bond-
ing interactions with the lattice water molecules to form a
supramolecular octanuclear copper(II) species in which
two tetranuclear units are non-covalently linked by a deca-
nuclear water cluster (Table 3 and Fig. 4). Besides, each
complex is hydrogen bonded to an isolated lattice water
molecule. The decanuclear water cluster is formed by
hydrogen bonding interactions of two dimeric water aggre-
gates with a cyclic hexameric water cluster that is anchored
on two tetranuclear complexes involving four water mole-
cules and four phenoxo oxygen atoms. Interestingly, the
Fig. 3. Aperspectiveviewof thetetranuclearcorein [Cu4L2(3-ppz)2] Æ 6H2O
showing the coordination geometry at the copper(II) centers.
Table 3
Selected hydrogen bonding distances (A), angles (ꢁ) and torsional angles (ꢁ) in [Cu4L2(3-ppz)2] Æ 6H2O (1 Æ 6H2O)
a
˚
O(1)ꢀ ꢀ ꢀO(7)
2.875(11)
2.880(7)
O(8)ꢀ ꢀ ꢀO(7)ꢀ ꢀ ꢀO(9)#1
149.5(6)
74.3(6)
143.3(6)
ꢁ23.1(7)
92.8(7)
O(4)ꢀ ꢀ ꢀO(12)
O(8)ꢀ ꢀ ꢀO(9)ꢀ ꢀ ꢀO(10)
O(6)ꢀ ꢀ ꢀO(8)
2.772(12)
2.964(17)
2.382(19)
2.744(19)
2.778(17)
O(9)ꢀ ꢀ ꢀO(10)ꢀ ꢀ ꢀO(11)
O(7)ꢀ ꢀ ꢀO(8)
O(7)ꢀ ꢀ ꢀO(8)ꢀ ꢀ ꢀO(9)ꢀ ꢀ ꢀO(7)#1
O(1)ꢀ ꢀ ꢀO(7)ꢀ ꢀ ꢀO(9)#1ꢀ ꢀ ꢀO(8)#1
O(1)ꢀ ꢀ ꢀO(7)ꢀ ꢀ ꢀO(8)ꢀ ꢀ ꢀO(9)
O(6)ꢀ ꢀ ꢀO(8)ꢀ ꢀ ꢀO(7)ꢀ ꢀ ꢀO(1)
O(6)ꢀ ꢀ ꢀO(8)ꢀ ꢀ ꢀO(9)ꢀ ꢀ ꢀO(7)#1
O(6)ꢀ ꢀ ꢀO(8)ꢀ ꢀ ꢀO(7)ꢀ ꢀ ꢀO(9)#1
O(8)ꢀ ꢀ ꢀO(7)ꢀ ꢀ ꢀO(9)#1ꢀ ꢀ ꢀO(8)#1
O(9)ꢀ ꢀ ꢀO(8)ꢀ ꢀ ꢀO(7)ꢀ ꢀ ꢀO(9)#1
O(8)ꢀ ꢀ ꢀO(9)ꢀ ꢀ ꢀO(10)ꢀ ꢀ ꢀO(11)
O(8)ꢀ ꢀ ꢀO(9)
O(9)ꢀ ꢀ ꢀO(10)
ꢁ95.5(6)
O(10)ꢀ ꢀ ꢀO(11)
39.6(6)
O(1)ꢀ ꢀ ꢀO(7)ꢀ ꢀ ꢀO(8)
O(1)ꢀ ꢀ ꢀO(7)ꢀ ꢀ ꢀO(9)#1
O(6)ꢀ ꢀ ꢀO(8)ꢀ ꢀ ꢀO(7)
O(6)ꢀ ꢀ ꢀO(8)ꢀ ꢀ ꢀO(9)
O(7)ꢀ ꢀ ꢀO(8)ꢀ ꢀ ꢀO(9)
O(8)ꢀ ꢀ ꢀO(9)ꢀ ꢀ ꢀO(7)#1
109.7(4)
ꢁ129.5(8)
ꢁ179.5(9)
ꢁ51(1)
94.6(5)
103.4(5)
134.3(7)
87.6(6)
45(1)
ꢁ97(1)
113.1(7)
#1: ꢁx + 1, ꢁy + 1, ꢁz.
a
For the positionally disordered water oxygen atom, hydrogen bonding parameters are given only for O(11) with a higher site occupancy.

S.G. Naik et al. / Polyhedron 25 (2006) 2135–2141
2139
stants between the four magnetic Cu(II) ions in the com-
plex. The symmetry of the molecule further reduces them
to three unique exchange constants (Fig. 5). The Cu–O–
Cu bond angles involving the pathway with exchange con-
stant J1 is 120.18ꢁ and that involving J2 is 106.03ꢁ. This sug-
gests that the interactions J1 and J2 are both expected to be
antiferromagnetic [34]. The Cu(1)–O(2)–Cu(3) and Cu(1)–
O(5)–Cu(3) bond angles for the pathway involving J3 are
95.47ꢁ and 96.49ꢁ suggesting that this exchange interaction
could be ferromagnetic [35]. The 3-phenylpyrazole ligands
that act as linkers of the two binuclear units are unlikely
to play any significant role in the magnetic exchange pro-
cesses involving the copper(II) centers. The bond angles
and bond lengths suggest that |J1| > |J2| > |J3|. Based on
the structural data, the magnetic interactions in the system
are modelled using the Hamiltonian
Fig. 4. A view of the discrete decanuclear water cluster anchored onto two
tetracopper(II) units.
hexameric water aggregate resembles cyclohexane in its
chair conformation. This rare and high-energy form of
the cyclic hexameric structure is previously known in liquid
helium droplets, organic supramolecular hosts and metal-
organic framework structures [11,13,31]. The av. Oꢀ ꢀ ꢀO
ꢀ ꢀ ꢀO angle of ꢂ116.7ꢁ in the cyclic structure compares well
to the value of 116.5ꢁ observed for water hexamers in
organic hosts, but differ from the angle of 109.3ꢁ in hexag-
onal ice [13,32]. This could be due to the steric constraints
imposed by the synthetic host. The Oꢀ ꢀ ꢀO distances do,
however, compare well with the corresponding distance
^ ^
^ ^
^ ^
^ ^
^ ^
^
H ¼ ꢁJ1ðS1S2 þ S3S4Þ ꢁ J2ðS1S4 þ S2S3Þ ꢁ J3ðS1S3Þ
X
X
z
0
z
z
^
^
þ glBH
Si ꢁ zJ hStot
i
Si
ð1Þ
i
i
where the positive and negative sign of J corresponds to
ferromagnetic and antiferromagnetic interactions, respec-
tively, and z is the number of nearest neighboring mole-
^
cules. All the spin operators ðSsÞ correspond to spin-1/2
˚
of 2.759 A reported for hexagonal ice (Ih) at ꢁ90 ꢁC [33].
objects. The last term in the Hamiltonian takes into ac-
count intermolecular interactions in the system within the
mean field approximation with |J0| ꢄ {|J1|,|J2|,|J3|} due to
the large separation of the tetranuclear complexes from
each other in the presence of the diamagnetic water cluster.
Using the above model we can compute the square of the
magnetization, M2(T), as a function of temperature
The important hydrogen bonding parameters are given in
Table 3.
3.3. Magnetic properties
The temperature dependence of the magnetic susceptibil-
ity of the tetranuclear copper(II) complex is shown as a vMT
versus T plot in Fig. 1. The vMT value decreases gradually
in the beginning from 300 K and then rapidly, as the tem-
perature is decreased below 150 K. The magnetic behavior
shows that an antiferromagnetic interaction is predominant
in the system. The structure of the {Cu4} core shows that
the Cu(1) and Cu(3) atoms are five-coordinate with a
square-pyramidal geometry in which the nitrogen and phe-
noxo oxygen atoms are in the basal plane. The alkoxo oxy-
gen atom belonging to the Schiff base displays the l3-mode
of bonding with two short equatorial and one axially elon-
gated Cu–O bond. The Cu(2) and Cu(4) atoms have a four-
coordinated planar structure. The four basal planes in the
structure are not coplanar giving dihedral angles of ꢂ70ꢁ.
The alkoxo oxygen atoms are not planar due to the l3-
bonding mode. There are no bridging ligand atoms between
Cu(2) and Cu(4) atoms and hence there is no exchange
pathway between these two sites. Besides, the long Cuꢀ ꢀ ꢀCu
P P
S
M2S expðꢁEðS; MSÞ=kBTÞ
Ms¼ꢁS
S
Ms¼ꢁS
M2ðT Þ ¼
ð2Þ
P P
S
expðꢁEðS; MSÞ=kBTÞ
S
^
where the eigenvalues E(S,Ms) of H are given by
E(S,Ms) = Eo(S,Ms) + Ms(gbH ꢁ zJ0ÆSzæ), Eo(S,Ms) being
the eigenvalues of the exchange Hamiltonian in the absence
of an external magnetic field and intermolecular interac-
tions. The temperature dependent magnetic susceptibility,
vM(T) is then given by
˚
distances in the range 3.261(1)–3.492(1) A precludes any
direct bonding between the metal centers. Hence, the
exchange constant for the magnetic interaction between
Cu(2) and Cu(4) sites is zero. The Cu(1)ꢀ ꢀ ꢀCu(3) distance
˚
in the {Cu2(l-OR)2} unit is 3.261(1) A. The presence of
two axial–equatorial bridging alkoxo oxygen atoms in this
unit is likely to make the exchange interaction (J3) different
from the other units. There exists only five exchange con-
Fig. 5. (a) Energy levels of the {Cu4} cluster from solving the Hamiltonian
in Eq. (1) for the best-fit exchange constants. (b) The spin–spin coupling
scheme in the complex.

2140
S.G. Naik et al. / Polyhedron 25 (2006) 2135–2141
vMðTÞ ¼ g2M2ðTÞ=½T ꢁ zJ0M2ðTÞꢃ
ð3Þ
port (grant SP/S1/F-01/2000). We thank the Convener, Bio-
informatics Centre of our Institute, for the database search.
where g is the gyromagnetic ratio. To account for trace
concentrations of paramagnetic impurities present in the
analytically pure sample, an additional Curie-like contribu-
tion to the magnetic susceptibility has been assumed and
the total magnetic susceptibility is given by
Appendix A. Supplementary data
Crystallographic data for 1 Æ 6H2O in the CIF format
have been deposited with the Cambridge Crystallographic
Data Center, CCDC No. 280462. Copies of this informa-
tion may be obtained free of charge from The Director,
CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax:
+44 1223 366 033; e-mail: deposit@ccdc.ac.uk or http://
www.ccdc.cam.ac.uk). Supplementary data associated with
this article can be found, in the online version, at
doi:10.1016/j.poly.2005.12.025.
vtotðTÞ ¼ vMðT Þ þ C=T
where C is the Curie constant. The error in the fit (R) is cal-
ð4Þ
culated from the expression,
X
2
2
R ¼
½ðvobsðTiÞ ꢁ vcalðTiÞÞ =vobsðTiÞ ꢃ
ð5Þ
i
where vobs(Ti) and vcal(Ti) are the observed and calculated
magnetic susceptibilities at temperature Ti, respectively.
The magnetic data for the complex are fitted using the
model Hamiltonian. The best fit gives J1, J2 and J3 values
as ꢁ86.3 cmꢁ1, ꢁ47.32 cmꢁ1 and 11.13 cmꢁ1, respectively.
The intermolecular interaction (J0) is weak and antiferro-
magnetic in nature, giving a value of ꢁ0.765 cmꢁ1. The g
value used for the theoretical fitting of the magnetic data
are 2.02. The Curie constant obtained from the fit corre-
sponds to an impurity concentration of 0.03 free-spins
per tetranuclear unit. The error (R) of the fit is
1.7 · 10ꢁ4. The energy levels of the copper cluster are
shown in Fig. 5. The ground state is a low-spin singlet with
a triplet state lying above because of the dominant antifer-
romagnetic interactions present in the system. The singlet
triplet gap is 74.2 cmꢁ1 and the quintet state is
207.6 cmꢁ1 above the singlet ground state.
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