Two new pseudohalide bridged di- and poly-nuclear copper(II) complexes: Synthesis, crystal structures and magnetic studies

Article abstract of DOI:10.1016/j.poly.2006.12.015

Two new pseudohalide bridged di- and poly-nuclear copper(II) complexes [Cu₂(L)₂(μ_1,1-N₃)₂] (1), [Cu(L)(μ_1,3-NCS)]_n (2), [LH = C₆H₅C(O)NHN{double bond, long}C(CH₃)C₅H₄N] have been synthesised and characterised by elemental analysis, IR and UV-Vis spectral studies. Structures of both the complexes have been established by X-ray crystallography. A tridentate hydrazone ligand (LH) obtained by the condensation of benzhydrazide and 2-acetylpyridine is used for the preparation of complexes 1 and 2. Variable temperature magnetic susceptibility measurement studies indicate there is a weak ferromagnetic interaction between the two copper(II) ions in 1 (J = +0.75 cm^-1), while weak antiferromagnetic interaction is observed with J values -0.23 cm^-1 for 2.

Full text of DOI:10.1016/j.poly.2006.12.015

Polyhedron 26 (2007) 1740–1744
www.elsevier.com/locate/poly
Two new pseudohalide bridged di- and poly-nuclear
copper(II) complexes: Synthesis, crystal structures and magnetic studies
a
a,
b
c
d
*
´
Soma Sen , Samiran Mitra , David L. Hughes , Georgina Rosair , Cedric Desplanches
a
Department of Chemistry, Jadavpur University, Kolkata 700 032, India
School of Chemical Sciences and Pharmacy, University of East Anglia, Norwich NR4 7TJ, UK
b
c
Department of Chemistry, Heriot-Watt University, Edinburgh EH14 4AS, UK
´
d
`
´
Institut de Chimie de la Matiere Condensee de Bordeaux – CNRS, Universite Bordeaux 1, Avenue du Dr. Schweitzer, F-33608 Pessac, France
Received 21 November 2006; accepted 8 December 2006
Available online 16 December 2006
Abstract
Two new pseudohalide bridged di- and poly-nuclear copper(II) complexes [Cu2(L)2(l1,1-N3)2] (1), [Cu(L)(l1,3-NCS)]n (2),
[LH = C6H5C(O)NHN@C(CH3)C5H4N] have been synthesised and characterised by elemental analysis, IR and UV–Vis spectral studies.
Structures of both the complexes have been established by X-ray crystallography. A tridentate hydrazone ligand (LH) obtained by the
condensation of benzhydrazide and 2-acetylpyridine is used for the preparation of complexes 1 and 2. Variable temperature magnetic
susceptibility measurement studies indicate there is a weak ferromagnetic interaction between the two copper(II) ions in 1
(J = +0.75 cmꢀ1), while weak antiferromagnetic interaction is observed with J values ꢀ0.23 cmꢀ1 for 2.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Keywords: Copper(II) complexes; Tridentate hydrazone ligand; Crystal structures; Magnetic studies
1. Introduction
of magnetic properties of its compounds. Less literature
is available for magneto-structural studies on copper(II)
complexes containing bridging thiocyanate groups as this
ligand is less efficient as a transmitter of magnetic interac-
tions than azide.
Multinuclear metal complexes have demonstrated many
important properties in systems such as catalysis [1], clath-
ration [2] and molecular sieving [3]. In addition, they are
very useful for developing new functional molecule-based
materials [4]. The design and synthesis of polynuclear coor-
dination complexes, aimed at understanding the structural
and chemical factors that govern the exchange coupling
between paramagnetic centres, are of continuing interest
in biology, chemistry and physics [5]. Pseudohalide bridged
polynuclear and dinuclear complexes continue to be the
subject of much interest, and intensive investigations have
taken place as a result of their diverse structures and poten-
tial applications as magnetic materials. Among the pseud-
ohalides, the azido group has received much attention
due to its versatility as a ligand and to the wide variety
To examine the versatility of the pseudohalides, several
pseudohalide bridged di- and poly-nuclear transition
metal complexes have already been reported by our group
using various Schiff bases, amines etc. as co-ligands [6–8].
In continuation of this work, here we have synthesised
one dinuclear and a polynuclear copper(II) complexes
[Cu2(L)2(l1,1-N3)2] (1), [Cu(L)(l1,3-NCS)]n (2), using
azide, thiocyanate as bridging ligands with the ions of a
hydrazone (LH = C6H5C(O)NHN@C(CH3)C5H4N) as
coligand. The hydrazone ligand is the condensation prod-
uct of benzhydrazide and 2-acetylpyridine. The present
paper describes the syntheses, X-ray crystal structures,
variable temperature magnetic (VTM) susceptibility stud-
ies and spectroscopic results of two new Cu(II) complexes
1 and 2.
*
Corresponding author. Tel.: +91 33 2668 2017; fax: +91 33 2414 6266.
E-mail address: smitra_2002@yahoo.com (S. Mitra).
0277-5387/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2006.12.015

S. Sen et al. / Polyhedron 26 (2007) 1740–1744
1741
2. Experimental
2.3. X-ray crystallography
2.1. Materials and measurements
The determination of the unit cell and the data collec-
tion for dark green coloured crystals of 1 and 2 was per-
formed on a Bruker-Nonius X8 Apex2 Diffractometer.
All data were collected with MoKa radiation
Benzhydrazide, 2-acetylpyridine, copper(II) perchlorate
hexahydrate (Fluka), sodium thiocyanate and sodium
azide (Aldrich) were used as received. The solvents used
were of reagent grade.
˚
(k = 0.71073 A). The structures of all the complexes were
determined by direct method procedures in SHELXS [10a],
and refined by full-matrix least-squares methods in SHELXL
[10b]. Carbon-bonded hydrogen atoms were included in
idealised positions and set to ride on the parent atoms.
The crystallographic data and the refinement results are
listed in Table 1.
Elemental analyses were carried out using a Perkin–
Elmer 2400 II elemental analyser. The infrared spectra
were recorded on a Perkin–Elmer RX 1 FT-IR spectropho-
tometer with a KBr disc. The electronic spectra were
recorded, in acetonitrile, on a Perkin–Elmer Lambda 40
(UV–Vis) spectrophotometer. Magnetic susceptibility mea-
surements for the complexes 1 and 2 were carried out with
a Quantum Design MPMS-5S SQUID magnetometer
under an applied magnetic field of 5000 Oe. The tempera-
ture dependence of the molar magnetic susceptibility, vM,
for the complexes 1 and 2 was measured on polycrystalline
samples in the temperature range 2–300 K. Diamagnetic
corrections were estimated from Pascal tables and magnetic
data were corrected for diamagnetic contributions of the
sample holder.
3. Results and discussion
3.1. Description of the crystal structures
3.1.1. [Cu(L)2(l1,1-N3)2] (1)
An ORTEP diagram of 1 with atom numbering scheme
is given in Fig. 1. Bond dimensions about the copper atom
in 1, together with those 2, are in Table 2; dimensions in the
main chains of the L ligand and in the bridging ligands in
both the complexes 1 and 2 are collated in Table 3. The
structural study reveals that 1 is a semi-bridging centro-
symmetric dimer. The azide ion bridges in an asymmetric
(basal-apical) fashion so the bridging Cu–N bond lengths
2.2. Synthesis of ligand and metal complexes
2.2.1. [C6H5C(O)NHN@C(CH3)C5H4N] (LH)
˚
are significantly different (Kd = 0.566 A). The copper cen-
The ligand LH was prepared by the condensation of
benzhydrazide (1.36 g, 10 mmol) with 2-acetylpyridine
(1.12 ml, 10 mmol) in methanol (50 ml) according to the
procedure reported elsewhere [9]. The resulting light yellow
liquid was used without further purification.
tres are five-coordinate, bonded to three coordinating
Table 1
Crystallographic and refinement data for complexes 1 and 2
Complex
1
2
Caution! Perchlorate salts are potentially explosive and
should be used in small quantity and with much care.
Chemical formula
Formula weight
Crystal system
Space group
Z
C
687.68
triclinic
28H24N12O2Cu2 C15H12N4OSCu
359.89
monoclinic
Cc
ꢀ
P1
2.2.2. [Cu2(L)2(l1,1-N3)2] (1)
1
4
To a methanolic solution (20 ml) of copper perchlorate
hexahydrate (0.370 g, 1 mmol), the solution of the hydra-
zone ligand (LH) (1 mmol) was added, followed by the
addition, with constant stirring, of a solution of sodium
azide (0.065 g, 1 mmol) in minimum volume of water–
methanol mixture. The final solution was kept at refrigera-
tor temperature yielding dark green, hexagonal crystals
suitable for X-ray diffraction after 6 days. Crystals were
isolated by filtration and were air-dried. Yield – 61%. Anal.
Calc. for C14H12N6OCu: C, 48.86; H, 3.52; N, 24.42.
Found: C, 48.61; H, 3.32; N, 24.09%.
˚
a (A)
7.877(13)
10.465(7)
10.616(7)
115.71(2)
97.73(3)
103.04(3)
740.9(14)
100(2)
11.424(3)
11.281(3)
11.173(3)
90
93.466(4)
90
1437.3(6)
100(2)
1.663
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A )
Temperature (K)
Density (Mg/m3)
Absorption coefficient (mmꢀ1
F(000)
1.541
1.484
350
)
1.671
732
Crystal size (mm)
0.22 · 0.12 · 0.08 0.38 · 0.36 · 0.04
h Range for data collection (ꢁ)
Reflections collected
2.28–25.36
6690
4.05–37.24
4456
2.2.3. [Cu(L)(l1,3-NCS)]n (2)
Independent reflections (Rint
Number of ‘observed’ reflections
Goodness-of-fit, S
)
1961(0.117)
726
0.788
R1 = 0.196,
wR2 = 0.187
R1 = 0.0719,
wR2 = 0.158
3387(0.0327)
2551
1.053
R1 = 0.0597,
wR2 = 0.103
R1 = 0.0424,
wR2 = 0.0968
0.83 and ꢀ1.54
The complex 2 was synthesised in a similar manner as
described for 1, except that sodium thiocyanate was used
instead of sodium azide. Dark green, hexagonal crystals
suitable for X-ray diffraction were obtained after 7 days.
Crystals were collected by filtration and were air-dried.
Yield – 53%. Anal. Calc. for C15H12N4OSCu: C, 50.01;
H, 3.36; N, 15.56. Found: C, 49.00; H, 3.32; N, 14.94%.
R indices (all data)
Final R indices [I > 2r(I)]
Largest difference in peak and hole 0.88 and ꢀ0.98
ꢀ3
˚
(e A
)

1742
S. Sen et al. / Polyhedron 26 (2007) 1740–1744
Table 3
˚
Selected bond dimensions (A and ꢁ) in the main chains of the L ligand and
in the bridging ligands in complexes 1 and 2
Complex 1
Complex 2
C(29)–O(4)
N(4)–C(29)
N(1)–N(4)
N(1)–C(22)
C(22)–C(6)
N(2)–C(6)
1.326(11)
1.353(12)
1.424(11)
1.319(13)
1.502(14)
1.421(11)
O(1)–C(2)
C(2)–N(9)
N(9)–N(10)
N(10)–C(11)
C(11)–C(12)
C(12)–N(17)
1.295(5)
1.335(5)
1.379(5)
1.303(5)
1.466(6)
1.358(5)
C(29)–O(4)–Cu(1)
O(4)–C(29)–N(4)
C(29)–N(4)–N(1)
C(22)–N(1)–N(4)
N(1)–C(22)–C(6)
N(2)–C(6)–C(22)
C(6)–N(2)–Cu(1)
108.9(6)
C(2)–O(1)–Cu(1)
O(1)–C(2)–N(9)
110.9(2)
126.8(10)
105.3(8)
122.0(8)
112.5(9)
113.3(10)
112.4(7)
124.5(4)
107.1(3)
123.0(4)
113.3(4)
114.2(3)
113.0(3)
C(2)–N(9)–N(10)
C(11)–N(10)–N(9)
N(10)–C(11)–C(12)
N(17)–C(12)–C(11)
C(12)–N(17)–Cu(1)
Complex 1, azide ligand
Complex 2, thiocyanate ligand
N(21)–N(22)
N(23)–N(22)
1.209(11)
1.208(12)
S(2)–C(1)
C(1)–N(10)
1.648(4)
1.150(6)
N(22)–N(21)–Cu(1)
N(22)–N(21)–Cu(10)
Cu(1)–N(21)–Cu(10)
N(23)–N(22)–N(21)
123.4(8)
109.2(8)
89.9(4)
C(1)–S(2)–Cu(1)
N(10)–C(1)–S(2)
C(100)–N(1)–Cu(1)
97.01(13)
179.7(4)
160.0(4)
176.3(11)
E.s.ds are in parentheses.
Fig. 1. View of the dimeric unit of 1, with atom labels. Displacement
ellipsoids are shown at 50% probability level. Hydrogen atoms are omitted
for clarity.
b and a are the two largest angles around the central atom;
s = 0 and 1 for the perfect square pyramidal and trigonal
bipyramidal geometries, respectively] [11]. The square base
is formed by pyridyl and imine nitrogen atoms, a benzoyl
oxygen atom and the closer of bridging azido nitrogen
atoms. The nitrogen atom from the semi-bridging azide
group with the longer Cu–N distance is at the apex. The
distortion in the geometry is evident from the variation
of the basal angles (Table 2) from equivalent cis and trans
values.
atoms [N2O] from the ligand L and nitrogen atoms from
two bridging azide anions. Both azides bridge in an end-
on fashion and this leads to a relatively small Cu–Cu
˚
(3.205 A) separation.
The di-l1,1-azido bridging nitrogen atom is in a planar
Cu2N2 ring about a crystallographic inversion centre. The
azide ions are nearly linear with N–N–N angle of
176.3(11)ꢁ. The geometry of the Cu(II) centres is close to
square pyramidal with s = 0.268, [s = jb ꢀ aj/60ꢁ, where
3.1.2. [Cu(L)(l1,3-NCS)]n (2)
An ORTEP representation of 2 with the atom number-
ing scheme is shown in Fig. 2 and relevant bond dimen-
sions are included in Tables 2 and 3. The structure is
made up of infinite chains, in which Cu(II) centres are con-
nected by single end-to-end thiocyanate bridges. The cop-
per(II) pentacoordination sphere comprises the N2O
donor atoms from the tridentate deprotonated hydrazone
ligand L, and a nitrogen and a sulfur atom from two mol-
ecules of bridging thiocyanate ligands. The geometry
around the copper centre can be described as square pyra-
midal with s = 0.0135. The basal plane is formed by the
N2O donor atoms of L and the nitrogen atom of a thiocy-
anate ligand. The sulfur atom from the other thiocyanate
ligand is at the apex. The apical Cu–S bond distance is sig-
Table 2
˚
Molecular dimensions (bond lengths in A and angles in ꢁ) about the
copper atoms in complexes 1 and 2
Complex 1
Complex 2
Bond lengths
Cu1–O4
N1–Cu1
Cu1–N2
Cu1–N21
Cu1–N21A
1.988(7)
1.947(8)
2.028(9)
1.968(9)
2.534(10)
Cu1–O1
Cu1–N10
Cu1–N17
Cu1–N1
Cu1–S2
1.965(3)
1.928(3)
2.012(3)
1.965(4)
2.7110(11)
Bond angles
N1–Cu1–O4
O4–Cu1–N2
80.5(3)
161.1(3)
102.1(3)
92.8(3)
81.1(4)
177.2(4)
90.9(4)
96.2(4)
91.5(4)
90.1(4)
N10–Cu1–O1
O1–Cu1–N17
O1–Cu1–N1
O1–Cu1–S2
N10–Cu1–N17
N10–Cu1–N1
N10–Cu1–S2
N1–Cu1–N17
N17–Cu1–S2
N1–Cu1–S2
79.41(13)
158.96(13)
96.89(14)
102.22(8)
80.32(14)
159.77(14)
101.29(10)
100.17(15)
87.26(9)
N21–Cu1–O4
O4–Cu1–N21A
N1–Cu1–N2
N1–Cu1–N21
N1–Cu1–N21A
N21–Cu1–N2
N2–Cu1–N21A
N21–Cu1–N21A
˚
nificantly longer [2.7110(1) A] than the basal bond dis-
˚
tances [1.924(4)–2.020(5) A], even when taking into
account the larger covalent radius of sulfur . The bond
dimensions around the Cu(II) ion are comparable to those
in similar systems [7]. The thiocyanate ligand is almost lin-
ear at C1. The intrachain copper–copper separation is
98.93(11)

S. Sen et al. / Polyhedron 26 (2007) 1740–1744
1743
to the C@N stretching frequency of the coordinated ligand
L [13]. In the spectra of complexes 1 and 2, strong, well-
resolved sharp bands at 2086 and 2028 cmꢀ1 are assigned
to mN@N of the azide (N3) group and mC@N of the NCS
group respectively. The ligand coordination to the metal
centre is substantiated by two bands appearing for each
complex, at 422, 312 (1) and 424, 315 (2) cmꢀ1 attributable
to mCu–N and mCu–O respectively.
3.3. Electronic spectra
The electronic spectral data for the complexes in aceto-
nitrile are in good agreement with their geometries. The
UV absorption bands exhibit a charge transfer transition
(CT) in the range 350–420 nm for Cu(II) complexes and
may be assigned to the ligand-to-metal charge transfer
transition for each of the compounds 1 and 2. The spectra
of complexes 1 and 2 each display a broad band at 623 and
635 nm respectively, consistent with the expected five-coor-
dinate geometry.
Fig. 2. View of two units in a chain of 2, with atom labels. Displacement
ellipsoids are shown at 50% probability level.
˚
5.806 A which is comparable to previously reported singly
3.4. Magnetic studies
thiocyanate bridged copper complexes [12].
The crystal packing of the complex indicates that adja-
cent chains have pairwise p–p interactions, by overlap of
phenyl with pyridine rings, with four adjacent chains
(Fig. 3), thus connecting the chains in a three-dimensional
network.
The temperature dependence of vMT for 1 is depicted in
Fig. 4. The curve of vMT = f(T) is almost constant above
60 K with a value near 0.825 cm3 K molꢀ1, in good agree-
ment with the expected value for two Cu(II) ions. Below
60 K, vMT slightly increases as T decreases to reach
0.875 cm3 K molꢀ1 at 4 K. According to the dimeric struc-
ture of the complex, the vMT curve has been fitted using a
classical Bleaney and Bowers law, using the phenomeno-
logical Hamiltonian H = ꢀJSAÆSB. The resulting law has
the following form:
3.2. Infrared spectra
The infrared spectra of the complexes 1 and 2 display
absorption bands at ca. 1563 cmꢀ1 which can be assigned
2Ng2b2
k½3 þ expðꢀJ=kTÞꢁ
vMT ¼
0.88
0.87
0.86
0.85
T
0.84
0.83
0.82
0
50
100
150
200
250
300
T (K)
Fig. 3. Crystal packing showing the parallel chains of 2, and the alignment
of overlapping phenyl and pyridine rings.
Fig. 4. Experimental (h) and calculated (—) temperature dependence of
vMT for 1.

1744
S. Sen et al. / Polyhedron 26 (2007) 1740–1744
of the same hydrazone ligand (L) with single end-to-end
0.435
0.430
0.425
0.420
0.415
0.410
0.405
thiocyanate bridges have been isolated. Electronic spectra
of the complexes support their geometries as established
from X-ray analysis. Magnetic studies indicate a weak fer-
romagnetic interaction operates between the two metal
centres in the doubly azido bridged complex 1, whereas
complex 2 exhibits weak antiferromagnetic interactions.
T
5. Supplementary material
CCDC 615282 and 615283 contain the supplementary
crystallographic data for 1 and 2. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@
ccdc.cam.ac.uk.
0
50
100
150
200
250
T (K)
Fig. 5. Experimental (h) and calculated (—) temperature dependence of
MT for 2.
v
Acknowledgement
The best fit was obtained with a Lande factor, g of 2.1, iso-
tropic interaction parameter J = +0.75 cmꢀ1. The two cop-
per atoms of the dinuclear unit are linked by two azido
bridging ligands in an end-on fashion. The copper atoms
have a square pyramidal geometry and the bridging nitro-
gen atom of each azido ligand is in a basal position of one
copper and the apical position of the second copper.
The temperature dependence of vMT for 2 is shown in
Fig. 5. Due to the chain structure of the complexes, the
model of equally spaced copper(II) ions has been used.
We acknowledge the financial assistance to S. Sen from
the CSIR (New Delhi, India).
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nꢀ1
The spin Hamiltonian in zero-field is H ¼ ꢀJ
i¼1
S
ꢂ S
where the summation runs over the n sites of
Ai
Aiþ1
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Ng2b2
k
with x ¼ ꢀJ=kT:
0:25 þ 0:074975x þ 0:075235x2
vMT ¼
1:0 þ 0:9931x þ 0:172135x2 þ 0:757825x3
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2.14 and isotropic interaction parameter J = ꢀ0.23 cmꢀ1
.
The sulfur atom of the thiocyanate bridge occupies the api-
cal position of the copper cation.
The weak coupling for both the complexes may be
explained as follows: the magnetic orbital describing the
unpaired electron on a copper(II) ion in square pyramid
is of the dx2ꢀy2 type (the x and y axes being defined by
the short basal bonds). The overlap between the magnetic
orbitals of two neighbouring coppers is thus very small,
and the isotropic interaction parameter (roughly propor-
tional to the square of this overlap) is expected to be small.
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4. Conclusion
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A double end-on azido bridged dinuclear copper(II)
hydrazone complex, and a 1D chain copper(II) polymer