The key role of hydrogen bonding in the nuclearity of three copper(II) complexes with hydrazone-derived ligands and nitrogen donor heterocycles

Article abstract of DOI:10.1016/j.ica.2011.01.008

Three new Cu(II) complexes of formula [Cu(L¹)(pyz)(CH ₃OH)]ClO₄ (1), [Cu(L¹)(4,4′-bpy)(ClO ₄)]·0.5H₂O (2) and [{Cu(L²)(ClO ₄)}₂(μ-4,4′-bpy)] (3) have been s

Full text of DOI:10.1016/j.ica.2011.01.008

Inorganica Chimica Acta 370 (2011) 18–26
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Inorganica Chimica Acta
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The key role of hydrogen bonding in the nuclearity of three copper(II)
complexes with hydrazone-derived ligands and nitrogen donor heterocycles
Shyamapada Shit a, Subrata K. Dey b, Corrado Rizzoli c, Ennio Zangrando d, Guillaume Pilet e,
Carlos J. Gómez-García f, Samiran Mitra a,
⇑
a Department of Chemistry, Jadavpur University, Kolkata 700 032, West Bengal, India
b Department of Chemistry, Bankura Sammilani College, Kenduadihi, Bankura 722 102, West Bengal, India
c Università degli Studi di Parma, Dip. di Chimica GIAF, Viale G.P. Usberti 17/A, I-43100 Parma, Italy
d Università degli Studi di Trieste, Dip. di Scienze Chimiche e Farmaceutiche, Via Licio Giorgieri, 1, 34127 Trieste, Italy
e Groupe de Cristallographie et Ingénierie Moléculaire, Laboratoire des Multimatériaux et Interfaces, UMR 5615, Université Claude Bernard Lyon 1, Bât. Jules Roulin,
43 Bd du 11 Novembre, 1918-69622 Villeurbanne Cedex, France
f Instituto de Ciencia Molecular, ICMol, Universidad de Valencia, Parque Científico, 46980 Paterna, Spain
a r t i c l e i n f o
a b s t r a c t
Three new Cu(II) complexes of formula [Cu(L1)(pyz)(CH3OH)]ClO4 (1), [Cu(L1)(4,40-bpy)(ClO4)]ꢀ0.5H2O (2)
Article history:
Received 10 May 2010
Received in revised form 28 November 2010
Accepted 8 January 2011
Available online 19 January 2011
and [{Cu(L2)(ClO4)}2( -4,40-bpy)] (3) have been synthesised by using pyrazine (pyz) and 4,40-bipyridine
l
(4,40-bpy) and tridentate O,N,O-donor hydrazone ligands, L1H and L2H, obtained by the condensation
of 1,1,1-trifluoro-2,4-pentanedione with salicyloylhydrazide and benzhydrazide, respectively. The
ligands and their complexes have been characterized by elemental analyses, FT-IR, and UV–Vis spectros-
copies. Single crystal X-ray structure analysis evidences the metal ion in a slightly deformed square
pyramidal geometry in all the complexes. However complexes 1 and 2 are mononuclear with pyz and
4,40-bpy, respectively, showing an unusual monodentate behavior, while complex 3 is dinuclear with
4,40-bpy adopting the typical bridging coordination mode. Self assembly of the complex units by hydro-
gen bonding interactions produces one-dimensional arrangement in each crystal packing. The magnetic
characterization of complex 3 indicates a weak antiferromagnetic exchange interaction between the
Cu(II) ions (J = ꢁ0.96 cmꢁ1) mediated through the long 4,40-bpy bridge. Electrochemical behavior of the
complexes is also discussed.
Keywords:
Cu(II) hydrazone complexes
Crystal structures
Monodentate pyz and 4,40-bpy
Self assembly
Hydrogen bonding
Antiferromagnetic coupling
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
such as nature and concentration of the metal, pH of the medium
and hydrazone ligand properties [5]. Aryl hydrazones are well
The self-assembly of molecular building blocks containing or-
ganic ligand and transition metal ions provides a reliable and effi-
cient approach for the design and construction of metal-organic
frameworks (MOFs). The interest in this field is due to the potential
use of the compounds as metal-based molecules, host guest prop-
erties, magnetic materials, etc. [1]. Among the various organic
ligands, Schiff bases have become an obvious choice not only for
their synthetic flexibility, structural variety and varied denticity
[2], but also for their selectivity as well as sensitivity towards tran-
sition metal ions. In this regard, hydrazones have got special
importance due to their chemical as well as biological applications
[3]. Moreover, due to keto-enol tautomerism, they can provide
neutral [4], mono- [5], di- [6–8], or tetraanionic [9] species leading
to unusual coordination numbers [6,10]. Mono- or di-nuclear com-
plexes are synthesized depending upon the reaction conditions
known as a class of versatile ligands capable of forming different
molecular architectures and coordination polymers. Their transi-
tion metal complexes show electrical and magnetic properties
[11] and are used in synthetic and analytical chemistry as novel
heterogeneous catalysts in oxido-reduction processes, various
chemical and photochemical reactions [12,13], as well as in
numerous industrial applications. Arylhydrazone metal complexes
are also important for their possible biological applications [14,15–
21]. In this regard, Cu(II) hydrazones can be mentioned for their
structural richness, biological activities as well as for understand-
ing the effects of structural and chemical factors that govern the
exchange coupling between paramagnetic centers [22–31].
Self-assembly of molecular building blocks can also be achieved
in a controlled manner through the use of bridging heterocyclic
bidentate amine ligands such as 4,40-bipyridine (4,40-bpy), pyra-
zine (pyz) and their derivatives. Many examples are known where
4,40-bpy and pyz act as a linear rigid spacer between metal centers
to form coordination polymers of different dimensionalities [32].
⇑
Corresponding author. Tel.: +91 33 2414 6666x2505; fax: +91 33 2414 6414.
E-mail address: smitra_2002@yahoo.com (S. Mitra).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.01.008

S. Shit et al. / Inorganica Chimica Acta 370 (2011) 18–26
19
Moreover, hydrogen bonding has been applied in self-assembly of
molecular building blocks containing appropriate groups with the
aim of generating discrete or extensive supramolecular entities.
Sometimes self assembly through hydrogen bonding interaction
acts as a structure directing force and formation of higher dimen-
sionality structures are hindered regardless of the presence of such
bridging ligands [33–35].
sation of 1,1,1-trifluoro-2,4-pentanedione (0.770 g, 5 mmol) with
salicyloylhydrazide (0.760 g, 5 mmol) or benzhydrazide (0.680 g,
5 mmol) for L1H and L2H, respectively, in 100 mL of methanol for
2 h. The volume of the resultant yellow liquid was reduced to
about 40 mL in a rotary evaporator. The crystalline ligands were
obtained by slow evaporation of the mother liquor. L1H, yield:
1.224 g (85%). Anal. Calc. for C12H11F3N2O3: C, 50.01; H, 3.85; N,
9.72. Found: C, 50.01; H, 3.83; N, 9.70%. L2H, yield: 1.20 g (88%).
Anal. Calc. for C12H11F3N2O2: C, 52.94; H, 4.07; N, 10.29. Found:
C, 52.92; H, 4.05; N, 10.28%.
In this paper, we describe the synthesis, spectral and structural
characterization of three new Cu(II) complexes of formulation
[Cu(L1)(pyz)(CH3OH)]ClO4 (1), [Cu(L1)(4,40-bpy)(ClO4)]ꢀ0.5H2O (2),
and [{Cu(L2)(ClO4)}2( -4,40-bpy)] (3), derived from two different
l
O,N,O-donor tridentate hydrazone ligands L1H and L2H (Scheme 1),
and N-donor heterocycles such as pyrazine (pyz), and 4,40-bipyri-
dine (4,40-bpy). We have also explored the conjugate effect of
4,40-bpy and pyz and the hydrogen bonding interaction on the
self-assembly process of the building blocks. The magnetic proper-
ties of the three compounds are also reported. In complex 3 the
thermal variation of the magnetic susceptibility shows the pres-
ence of a weak antiferromagnetic exchange interaction that has
been satisfactorily reproduced with a simple S = 1/2 dimer model
2.2.2. Syntheses of the complexes
2.2.2.1. [Cu(L1)(pyz)(CH3OH)]ꢀClO4 (1). A methanolic solution
(20 mL) of the Schiff base L1H (0.288 g, 1 mmol) was added to a
methanolic solution (10 mL) of Cu(ClO4)2ꢀ6H2O (0.371 g, 1 mmol)
and stirred for 10 min. Then a 10 mL methanolic solution of pyra-
zine (0.80 g, 1 mmol) was added dropwise to the solution. The
resulting solution was filtered and the filtrate kept undisturbed
at room temperature for 4 days. Light green crystals suitable for
X-ray diffraction quality were obtained after 4 days. Yield: 70%
with respect to metal substrate. Anal. Calc. for C17H18CuF3N4O8Cl:
C, 36.31; H, 3.23; N, 9.96; Cu, 11.30. Found: C, 36.30; H, 3.21; N,
9.95; Cu, 11.29%.
with J = ꢁ0.96(1) cmꢁ1
.
2. Experimental
2.2.2.2. [Cu(L1)(4,40-bpy)(ClO4)]ꢀ0.5H2O (2). To a methanolic solution
(30 mL) of Cu(ClO4)2ꢀ6H2O (0.371 g, 1 mmol) and L1H (0.288 g,
1 mmol), a 20 mL methanolic solution of 4,40-bipyridine (0.156 g,
1 mmol) was added dropwise with constant starring. The solution
was filtered and deep green single crystals of X-ray diffraction qual-
ity were obtained from the solution kept undisturbed at room tem-
perature for 7 days. Yield: 74% with respect to metal substrate. Anal.
Calc. for C22H19CuF3N4O7.5Cl: C, 43.50; H, 3.15; N, 9.92; Cu, 10.33.
Found: C, 43.47; H, 3.14; N, 9.91; Cu, 10.31%.
2.1. Materials
1,1,1-Trifluoro-2,4-pentanedione, salicyloylhydrazide, benzhy-
drazide, pyrazine, 4,40-bipyridine, and Cu(ClO4)2ꢀ6H2O were pur-
chased from Aldrich Chemical Co. and used without further
purification. All other chemicals and solvents were of analytical re-
agent grade and used as received.
2.2. Syntheses of the ligands and complexes
2.2.2.3. [{Cu(L2)(ClO4)}2( -4,40-bpy)] (3). A methanolic solution
l
Caution! Perchlorate salts of transition metal ions in presence of
organic ligands are potentially explosive, though no problems have
been encountered during the syntheses of the complexes, they
should be prepared in small quantities and handled with much
care.
(20 mL) of the Schiff base L2H (0.272 g, 1 mmol) was added to a
methanolic solution (10 mL) of Cu(ClO4)2ꢀ6H2O (0.371 g, 1 mmol),
and the resulting mixture was stirred for 30 min. Then a solution
of 4,40-bipyridine (0.062 g, 0.4 mmol) in 10 mL methanol was added
dropwise. A white precipitate that appeared immediately was fil-
tered and the filtrate was kept undisturbed for 7 days at room tem-
perature from which deep green single crystals suitable for X-ray
diffraction quality were obtained. Yield: 73% with respect to metal
substrate. Anal. Calc. for C34H28Cl2Cu2F6N6O12: C, 39.86; H, 2.75; N,
8.20; Cu, 12.40. Found: C, 39.85; H, 2.73; N, 8.18; Cu, 12.38%.
2.2.1. Syntheses of the Schiff base ligands (L1H and L2H)
2.2.1.1. [CF3C (OH)@CH–C(CH3)@N–NH–CO–C6H4(OH)] (L1H) and
[CF3C (OH)@CH–C(CH3)@N–NH–CO–C6H5] (L2H). The Schiff bases
were prepared following a similar procedure by the reflux conden-
2.3. Physical measurements
X
H3C
F3C
O
X
H
N
Elemental analyses (carbon, hydrogen and nitrogen) were car-
ried out with a Perkin–Elmer 2400 II elemental analyser. Copper
content in the complexes have been estimated quantitatively by
standard iodometric procedure. The FT-IR spectra (4000–
200 cmꢁ1) were recorded on a Perkin–Elmer Spectrum RX I FT-IR
system with solid KBr pellets. The electronic spectra (800–
200 nm) were recorded on a Perkin–Elmer Lambda 40 UV–Vis spec-
trometer using HPLC grade acetonitrile as solvent. The concentra-
tions of the solutions were 1 ꢂ 10ꢁ5 M. Electrochemical studies
were performed in acetonitrile on a VersaStat II cyclic voltammeter
instrument and tetrabutylammonium perchlorate as the support-
ing electrolyte at a scan rate of 50 mV sꢁ1 using platinum wire as
working electrode and saturated calomel electrode (SCE) as refer-
ence. Variable temperature magnetic susceptibility measurements
were carried out on polycrystalline samples with a Quantum
Design MPMS-XL5 SQUID susceptometer, under an applied magnetic
field of 0.1 T working in the temperature range of 2–300 K. The iso-
thermal magnetization was performed on compound 3 at 2 K with
O
O
H2N
H3C
N
+
H
N
O
O
H
H
F3C
X
H
N
H3C
For L1H; X= -OH
For L2H; X= -H
N
O
OH
F3C
Scheme 1. Synthetic scheme of Schiff base ligands.

20
S. Shit et al. / Inorganica Chimica Acta 370 (2011) 18–26
Table 1
Crystallographic data collection and structure refinement for 1, 2 and 3.
Parameters for
1
2
3
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C
17H18CuF3N4O8Cl
C22H19CuF3N4O7.5Cl
615.40
C34H28Cl2Cu2F6N6O12
1024.61
orthorhombic
Fddd (No. 70)
562.35
triclinic
P1 (No. 2)
8.328(2)
triclinic
ꢀ
ꢀ
P1 (No. 2)
9.893(3)
20.2537(10)
b (Å)
11.056(3)
11.588(4)
25.8187(12)
c (Å)
12.662(3)
11.866(4)
30.1135(9)
a
(°)
76.006(5)
105.05(3)
90.0
b (°)
89.407(6)
94.51(3)
90.0
c
(°)
76.211(5)
1097.3(5)
99.56(2)
1284.8(7)
90.0
15747.1(12)
V (Å3)
Z
2
2
16
T (K)
295
1.702
1.195
293
1.591
1.027
293
1.729
1.315
Dcalc (g cmꢁ3
)
l
(mmꢁ1
)
F(0 0 0)
570
624
8256
h Range for data collection (°)
1.7–25.3
2.9–25.3
1.4–27.9
Total data
11 366
15 834
15 850
Rint
0.049
0.035
0.047
Unique data
3963
4046
4706
Observed data [I > 2
r
(I)]
2309
2022
2329
Number parameters
Final R indices [I > 2
R indices (all data)a
307
359
280
r
(I)]a
R1 = 0.0475, wR2 = 0.0658
R1 = 0.0972, wR2 = 0.0697
1.105
0.35, ꢁ0.33
R1 = 0.0566, wR2 = 0.1556
R1 = 0.1030, wR2 = 0.1705
0.869
0.83, ꢁ0.29
R1 = 0.0517, wR2 = 0.0568
R1 = 0.0994, wR2 = 0.1018
1.122
0.96, ꢁ0.52
Goodness-of-fit (GOF) on F2
Residuals (e Åꢁ3
)
a
½
R =
R
(|Fo–Fc|)/
R
|Fo|, wR = {
R
[w(|Fo–Fc|)2]/
R
[w|Fo|2]}
.
magnetic fields up to 8 T in a Quantum design PPMS9 equipment.
Susceptibility data were corrected for the sample holder, previ-
ously measured under the same conditions, used for the diamag-
netic contributions of complex 3, as deduced by using Pascal’s
Cu(ClO4)2ꢀ6H2O with pyz or 4,40-bpy in 1:1:1 ratio resulted in
mononuclear complexes 1 and 2, respectively, while the dinuclear
complex 3 was obtained on reaction of L2H and Cu(ClO4)2ꢀ6H2O
with 4,40-bpy in 1:1:0.4 ratio. Taking into account the structural
results, we have performed a similar reaction of L2H and Cu
(ClO4)2ꢀ6H2O with pyz varying their stoichiometric ratios, but
any attempt to get single crystals was unsuccessful.
constant tables (
v
dia = ꢁ550.2 emu molꢁ1).
2.4. X-ray crystallography
An ORTEP view of complexes 1, 2 and 3 with atom numbering
scheme is shown in Figs. 1–3, respectively, and relevant bond
lengths and angles are summarized in Table 2. X-ray crystallo-
graphic analyses reveal that compounds 1 and 2 consist of mono-
nuclear complexes of formula [Cu(L1)(pyz)(CH3OH)]ꢀClO4 and
[Cu(L1)(4,40-bpy)(ClO4)]ꢀ0.5H2O, respectively, where pyz and 4,40-
bpy ligands coordinate the metal in the less usual monodentate
fashion [45–50] in spite of their bridging behavior through the
equivalent nitrogen donors. In fact, a survey of the CCDC database
shows that of ca. 150 structures presenting Cu(II) ions and pyra-
zine, this ligand acts as monodentate only in eight cases, being a
bridge in the remaining structures. Furthermore, of these eight
structures only in four cases the copper complex is a monomer,
as for compound 1. Similarly the CCDC database was searched
for structures comprising the 4,40-bpy ligand and Cu(II) ions. Of
ca. 450 structures retrieved, the 4,40-bpy acts as a monodentate li-
gand only in 30 cases, being a bridge in the remaining structures.
Of these 30 structures only in eight cases the copper complex is
a monomer, as for compound 2.
An air stable single crystal of 1 (of dimensions 0.18 ꢂ 0.06 ꢂ
0.05 mm3),
2
(0.15 ꢂ 0.12 ꢂ 0.08 mm3) and
3
(0.25 ꢂ 0.26 ꢂ
0.31 mm3) were selected for diffraction data collections, which
were performed at room temperature on a Bruker SMART 1000CCD,
a Nonius DIP1030H system, and Nonius KappaCCD area diffrac-
tometer, respectively [36,37]. All the systems were equipped
with graphite monochromated Mo K
a radiation (k = 0.71073 Å).
Cell refinements and data reductions were carried out by using
Bruker SAINT [38] for 1, DENZO and SCALEPACK [39] for 2 and 3.
All the structures were solved by direct methods [40–42] com-
bined to Fourier difference syntheses and refined with full-matrix
least-squares based on F2 with all observed reflections with all
non-hydrogen atoms anisotropically treated. The contribution of
hydrogen atoms at calculated positions were included in final
cycles of refinements. A residual in
DF map of 2 was interpreted
as water oxygen (0.5 factor occupancy, H atoms not included).
The molecular graphics and crystallographic illustrations were pre-
pared using the ORTEP [43], and CAMERON program contained in the
WinGX system [44]. Relevant crystallographic data and structure
refinement parameters for the three complexes are summarized
in Table 1.
On the other hand, 3 is a dinuclear species [{Cu(L2)(ClO4)}2
(l
-4,40-bpy)], where the 4,40-bipyridine ligand, located on a crys-
tallographic two-fold axis, connects the symmetry related [Cu(L2)
(ClO4)] units and spaces the metals by 10.968 Å.
Since most of the structural features are close comparable in 1,
2, and 3, we describe first the coordination sphere, and then we
will illustrate some differences in each structure and in the crystal
packing. In all the complexes the metal ion has a slightly deformed
square pyramid coordination sphere with a N2O3 chromophore
satisfied by the hydrazone ligand and the heterocyclic organic N
donor located in the basal plane, while an oxygen atom from
3. Results and discussion
3.1. Description of molecular structures
For the syntheses of metal complexes two different hydrazone
ligands (L1H and L2H) have been employed. Reaction of L1H and

S. Shit et al. / Inorganica Chimica Acta 370 (2011) 18–26
21
It is worth to note the Cu–N(pyz) of 2.020(4) Å in 1 is sensibly
longer than the corresponding values for the 4,40-bpy in 2 and 3,
of 1.984(5) and 1.965(4) Å, respectively. Since both L1H and L2H
have almost similar structures with identical donor atoms, the dif-
ference in the bond lengths may be due to the higher donor
strength of 4,40-bpy than that of pyz, which may also be inferred
from their relative pK1 values of 3.17 and 0.6, respectively [52].
The axial Cu–O bond distance is, as expected, considerably longer
than the equatorial ones and show significantly differences in
length, increasing on going from 1 to 3, (2.320(3), 2.449(5),
2.506(5) Å).
The distortions in the geometry of the coordination sphere are
evident from the deviation of the cisoid [81.60(17)–97.68(12)°]
and transoid measured angles [169.37(14)–174.55(18)°] from the
Fig. 1. ORTEP view of the complex cation of 1 with atom numbering scheme (thermal
displacement ellipsoids are drawn at the 40% probability level).
ideal values. The Addison parameter
s
[53], defined as (b ꢁ
a)/60,
where b and are the two largest coordination angles, provides
a
an index of degree of trigonality, being 0 and 1 for a perfect square
pyramidal (SP) geometry and trigonal bipyramidal (TBP), respec-
tively. The calculated value of
s for 1, 2, and 3 is 0.05, 0.01 and
0.07, respectively. The metal Cu(II) ion is slightly displaced by
0.13, 0.08 and 0.05 Å from the squared basal plane towards the api-
cal oxygen atom, a feature also evidenced by the coordination an-
gles involving the apical donor, the majority being larger than 90°
(Table 2).
In all the complexes the Schiff base atoms are coplanar within
0.09 (in 1), 0.26 (2), and 0.60 Å (3), indicating an electron delo-
calization over this ligand. The dihedral angle formed by N2O2 ba-
sal coordination plane with the pyz ring in 1 and with the pyridine
ring in 2 is of ca. 8°, while the coordinating pyridine in 3 is consid-
erably more tilted (30.9°). The 4,40-bpy presents a linear conforma-
tion in 2 with py rings almost coplanar (4.9°). In 3 the 4,40-bpy
rings are tilted by 10.6° and slightly bowed being the Cuꢀ ꢀ ꢀXꢀ ꢀ ꢀCui
angle (X = centroid of 4,40-bpy) of 171°.
The phenolic group O(3) in (L1)ꢁ (complexes 1 and 2), which
acts as H-bond acceptor for the protonated nitrogen N(2), appears
to affect the nuclearity of the complexes and is responsible for the
polymeric chains observed in the crystal packing (see below).
Finally the mean basal coordination planes (O(1)/O(2)/N(1)/
N(3)) of the symmetry related copper ions in 3 form a dihedral an-
gle of ca. 53.6° (Fig. 3).
Fig. 2. ORTEP view of
ellipsoids are drawn at 40% probability level).
2 with atom numbering scheme (thermal displacement
Table 2
Selected bond lengths (Å) and angles (°) for 1, 2, and 3.
1
2
3
Cu–O(1)
Cu–O(2)
Cu–N(1)
Cu–N(3)
Cu–O(4)
1.911(3)
1.977(3)
1.940(3)
2.020(4)
2.320(3)
1.897(4)
1.969(4)
1.929(4)
1.984(5)
2.449(5)
1.901(4)
1.973(4)
1.930(4)
1.965(4)
2.506(5)
3.2. Crystal packing
In complexes 1 and 2, beside the intra-molecular interaction
N(2)–Hꢀ ꢀ ꢀO(3), the phenolic group O(3) in turn behaves as H donor
to form an intermolecular hydrogen bond O(3)–Hꢀ ꢀ ꢀN(4) with the
uncoordinated pyz nitrogen of an adjacent unit in 1, and similarly
with pyridine nitrogen N4 in 2, giving rise to a 1D chain (Figs. 4 and
5). The distances of 2.689(4) and 2.566(6) Å, respectively, indicate
a strong Oꢀ ꢀ ꢀN interaction. Moreover, the perchlorate anion in 1 is
appended to the cationic unit via O(4)–Hꢀ ꢀ ꢀO(8) hydrogen bonding
interaction involving the coordinated methanol molecule (Fig. 4).
Fig. 6 shows a perspective view of the crystal packing down the
direction of propagation of the 1D chain in 1. The relevant hydro-
gen bonding parameters for all the complexes are listed in Table 3.
Due to the lack of the phenolic group in (L2)ꢁ, complex 3 exhib-
its inter-molecular hydrogen bonding interaction only, forming a
wavy 1D polymeric chain structure (Fig. 7). Here the units are dou-
bly interlocked via N(2)–Hꢀ ꢀ ꢀO(5) interactions involving the hydra-
zinic hydrogen and the oxygen atom O(5) of the coordinated
perchlorate of the neighboring unit and vice versa. The Nꢀ ꢀ ꢀO dis-
tances of 2.849(6) Å indicate a less tight interaction than those
measured in 1 and 2. However the crystal packing is reinforced
O(1)–Cu–O(2)
O(1)–Cu–N(1)
O(1)–Cu–N(3)
O(1)–Cu–O(4)
O(2)–Cu–N(1)
O(2)–Cu–N(3)
O(2)–Cu–O(4)
N(1)–Cu–N(3)
O(4)–Cu–N(1)
O(4)–Cu–N(3)
172.44(12)
91.93(13)
91.43(13)
92.66(12)
82.11(12)
93.64(12)
92.75(11)
169.37(14)
97.68(12)
92.24(12)
172.24(15)
92.30(18)
92.28(18)
88.64(17)
81.60(17)
93.35(17)
96.65(16)
172.90(2)
95.23(19)
90.33(19)
170.18(16)
92.83(17)
92.28(16)
94.10(18)
81.38(17)
93.82(16)
93.56(17)
174.55(18)
87.40(2)
90.40(2)
methanol (in 1) or perchlorate anion (2 and 3) occupies the apical
position of the pyramid. The Schiff base ligand coordinates the me-
tal ion in a tridentate fashion forming a five- and a six-membered
chelate rings, through the alkoxo oxygen O(1), hydrazinic nitrogen
N(1), and the keto oxygen O(2). Inspection of Table 2 indicates that
the Cu–O(1), Cu–O(2), and Cu–N(1) bond distances involving the
Schiff base are close comparable, being in a range from 1.897(4)
to 1.911(3), 1.969(4)–1.977(3), and 1.929(4)–1.940(3) Å, in 1, 2
and 3, respectively. These Cu–O and Cu–N bond distances agree
with those found in other square pyramidal complexes reported
in the literature with similar O,N,O-donor ligands [51].
by
p–p interactions occurring between pyridine rings (centroid-
to-centroid distance of 3.54 Å) of adjacent chains forming a 2D lay-
ered structure parallel to planes (0 1 0).

22
S. Shit et al. / Inorganica Chimica Acta 370 (2011) 18–26
Fig. 3. ORTEP view of 3 located on a crystallographic two-fold axis with atom numbering scheme of the independent unit (thermal displacement ellipsoids are drawn at 40%
probability level, i = x, 1/4 ꢁ y, 5/4 ꢁ z).
Fig. 4. 1D chain in 1 built up by hydrogen bonds.
In the close comparable mononuclear complexes 1 and 2 the
hydrazone ligand L1H differs from L2H for the phenolic-OH side
arm that appears as structure directing. In fact, as described above,
in these species the phenolic group acts as acceptor for intra-ligand
hydrogen bonding interaction with the amide hydrogen and at the
same time as donor in a strong intermolecular hydrogen bond with
the uncoordinated nitrogen atom of pyz and 4,40-bpy in 1 and 2,
respectively. Possibly, this strong intermolecular interaction ham-
pers the uncoordinated nitrogen atom to form a dinuclear complex,
thus propagating the structure in one dimension through hydrogen
bonding instead. On the other hand the absence of the phenolic-OH
side arm in the ligand L2H in 3 induces the 4,40-bpy to act as a
bridging species giving rise to a dinuclear complex. However, the
formation of one dimensional chain in the solid state of 3 is real-
ized through pairs of weaker intermolecular H-bonds involving
the amide N2–H fragment of L2H and an oxygen atom of the coor-
dinated perchlorate anion of a nearby complex.
3.3. Fourier transform infrared spectra
In the FT-IR spectra of ligands, a strong band assignable to mC@N
stretching frequency and a strong C@O absorption band are ob-
served around 1640 cmꢁ1 and in the region of 1650–1657 cmꢁ1
,
respectively. Broad N–H absorption bands in the range 3370–
3390 and 3205–3220 cmꢁ1 are also observed for both ligands. A
weak broad band in the region of 3145–3310 cmꢁ1 also indicates
the presence of an enolic –OH group in both the ligands, while
an additional band at 3040 cmꢁ1 is observed for L1H, due to the
phenolic –OH group of benzene ring [54].
Upon complexation the mC@N stretching vibration is bathochro-
mically shifted by 25–35 cmꢁ1, and the coordination of azomethine
nitrogen is well supported by the peaks at 1615, 1612 and
1605 cmꢁ1 observed for 1, 2 and 3, respectively [55,56]. In all the
complexes the azomethine coordination is further substantiated
by the appearance of a new band in the range 420–427 cmꢁ1

S. Shit et al. / Inorganica Chimica Acta 370 (2011) 18–26
23
Fig. 5. 1D chain in 2 built up by hydrogen bonds.
Fig. 6. Crystal packing of 1 viewed down the direction of propagation of polymeric chains.
Table 3
Hydrogen bonding parameters (Å, °) for 1, 2, and 3.
Complex
D–Hꢀ ꢀ ꢀA
d(D–H)
d(Hꢀ ꢀ ꢀA)
d(Dꢀ ꢀ ꢀA)
\(D–Hꢀ ꢀ ꢀA)
1
N(2)–H(1N)ꢀ ꢀ ꢀ(O3)
O(3)–H(10)ꢀ ꢀ ꢀN(4)a
O(4)–Hꢀ ꢀ ꢀO(8)
0.86
0.82
0.82
1.91
1.90
2.01
2.582(4)
2.689(4)
2.830(5)
133
162
175
2
3
N(2)–Hꢀ ꢀ ꢀO(3)
0.96(6)
0.91(6)
1.84(6)
1.66(6)
2.559(6)
2.566(6)
130(5)
172(7)
O(3)–Hꢀ ꢀ ꢀN(4)b
N(2)–Hꢀ ꢀ ꢀO(5)c
1.11
1.82
2.849(6)
152
Symmetry codes: a1 + x, ꢁ1 + y, z; b1 + x, y, ꢁ1 + z; c5/4 ꢁ x, 1/4 ꢁ y, z.

24
S. Shit et al. / Inorganica Chimica Acta 370 (2011) 18–26
Fig. 7. 1D chain formed by pairs of hydrogen bonds in 3 with indication of p–p interactions between facing pyridine rings of adjacent chains (in black).
assignable to
m
Cu–N vibration [57]. Moreover a broad band at 3368–
SP or distorted SP geometries exhibit a band in the range 550–
660 nm, whereas the corresponding TBP complexes usually show
a maximum at k > 800 nm with a higher energy shoulder [65,66].
Therefore, these bands inferred the presence of distorted SP geom-
etry of the metal centers in all these complexes which results from
2E ? 2B1 transition.
3385 cmꢁ1 indicates the existence of amide functionality but the
broad band in the region of 3145–3310 cmꢁ1 assigned to the enolic
–OH group is not observed here indicating the deprotonation dur-
ing complexation. This is further confirmed by the appearance of a
new band at 395, 397 and 405 cmꢁ1 for 1, 2, and 3, respectively,
attributed to mCu–O stretching. This indicates that the free Schiff
base ligands undergo keto-enol tautomerisation in the uncon-
densed b-ketonic end during complexation. In the complexes, the
band corresponding to mC@O stretching vibration appeared within
1625–1630 cmꢁ1 region. Shifting of this band to lower energy re-
gion compared to that of the free ligands also indicates the coordi-
nation of ketonic oxygen to the metal center. Monodentate
coordination mode of pyz in 1 is indicated by the band around
453 cmꢁ1 while the bands at 1605, 1411, 1220 and 805 cmꢁ1 for
2 and bands at 1540, 1522, 1425, 1225 and 825 cmꢁ1 for 3 indicate
the monodentate and bridging coordination mode of 4,40-bpy,
respectively [58,59]. In addition, a well resolved peak correspond-
3.5. Electrochemistry
Electrochemical properties of the complexes were revealed by
the cyclic voltammograms, recorded in the potential region +1.5 to
ꢁ1.5 V. On cathodic scan, cyclic voltammograms of 1 and 2 show a
single reductive response at the potential ꢁ0.42 and ꢁ0.37 V (Epc),
respectively while 3 shows two reductive responses at ꢁ0.44 (E1
)
pc
and ꢁ0.87 V (E2pc). On anodic scan, the oxidative responses for 1
and 2 appeared at ꢁ0.32 and ꢁ0.26 V (Epa), respectively and for 3
theseappearedatꢁ0.32(E1pa)andꢁ0.68 V (E2pa). Thus peak-to-peak
separation (
D
Ep = Epc ꢁ Epa) for 1 and 2 are 100 and 110 mV while for
ing to the m
asym(CF3) vibration appears at 1200 cmꢁ1 while a broad
3 the peak-to-peak separations are 120 (
has been observed for all the complexes that upon increasing the
scan rate Ep increases, which is always greater than 60 mV and
D D
Ep1) and 190 mV ( Ep2). It
trifurcated band in the range 1140–1100 cmꢁ1 is also observed in
all the spectra assignable to lattice, coordinated perchlorate anions
D
and msym(CF3) vibration [60,61].
the cathodic peak currents (Ipc) are always greater than the anodic
peak currents (Ipa). These observations indicate the irreversible nat-
ureof theredox processes[67]. Controlledpotentialelectrolysisfor 1
and 2 carried out at 100 mV more negative to the first reduction
wave consumed approximately one electron per molecule
(n ꢃ 0.93 for 1 and n ꢃ 0.95 for 2) indicates that the redox process in-
volves CuII + e ? CuI. Similar controlled potential electrolysis for 3
performed at 100 mV more negative to the first reduction wave
consumed approximately one electron (n ꢃ 0.91) and at 100 mV
more negative to the second reduction wave consumed approxi-
mately two electrons (n ꢃ 1.90) per molecule which indicates
the involvement of two electrons corresponds to the process;
CuIICuII ? CuIICuI ? CuICuI. The redox behaviors of the complexes
are very similar to some of the previously reported mono- and
binuclear complexes in the literature [58,68,69].
3.4. Electronic spectra
Electronic spectra of L1H and L2H show a weak band around
370 nm (log
255 nm (log
e
e
= 3.30) and another considerably strong band around
= 3.90), respectively, assignable to n ? * (forbidden)
p
and
In the spectra of the complexes, the bands of the azomethine
chromophore (n ? *) transition are shifted to lower frequencies
p ? p* transitions [62,63].
p
indicating that the imine nitrogen atom is involved in coordination
to the metal ion. Electronic spectra of 1, 2, and 3 show strong
absorption bands at 258 (log
294 nm (log = 4.92), respectively, which are clearly charge trans-
fer in origin [64]. The UV absorption bands at 375 (log = 4.52),
380 (log = 4.50), and 373 nm (log = 4.60), observed for 1, 2 and
e = 4.94), 292 (loge = 4.93), and
e
e
e
e
3, respectively, can be correlated to the ligand to Cu(II) charge
3.6. Magnetic properties
transfer [LMCT] transition [62]. Complexes 1, 2 and 3 also exhibit
a broad band at 667 (log
e
= 2.72), 673 (log
e
= 2.75), and 665 nm
The magnetic properties of 1, 2 and 3 were evaluated by means
(log = 2.81), respectively. In general typical Cu(II) complexes of
e
of bulk magnetization measurements. The value of the product of

S. Shit et al. / Inorganica Chimica Acta 370 (2011) 18–26
25
1.0
0.8
0.6
0.4
0.2
0.0
complex 3 that can be well reproduced with a simple S = 1/2 dimer
model with J = ꢁ1.38(1) K = ꢁ0.96(1) cmꢁ1, in agreement with the
expected behavior for a dinuclear copper(II) complex with metals
separated by more than 10 Å through the long 4,40-bipyridine
spacer.
2.5
2.0
1.5
1.0
0.5
0.0
Acknowledgments
S.S. gratefully acknowledges Dr. J. Chakroborty, DRDO, Kanpur,
India for his valuable suggestions. S.K.D. and S.M. acknowledge
University Grants Commission, New Delhi, India for financial
assistance. We also acknowledge the European Union (MAGMANet
network of excellence), the Spanish Ministerio de Educación y
Ciencia (Projects MAT2007-61584 and CSD 2007-00010 Consolider-
Ingenio in Molecular Nanoscience) and the Generalitat Valenciana
(Project PROMETEO/2009/095) for funding this research work.
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
-1
H/T (T.K
)
0
50
100
150
200
250
300
T (K)
Fig. 8. Thermal variation of the vmT product per Cu(II) dimer in compound 3. Inset
shows the isothermal magnetization at 2 K. Solid lines represent the best fit to the
models (see text).
Appendix A. Supplementary material
CCDC 671674, 671675, and 671676 contain the supplementary
crystallographic data for 1, 2, and 3. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data
associated with this article can be found, in the online version, at
doi:10.1016/j.ica.2011.01.008.
the molar magnetic susceptibility times the temperature,
vmT at
300 K (0.428 emu K molꢁ1) for the powdered samples of 1 and 2
are close to the spin-only value expected for a S = ½ d9 Cu(II) com-
pound with g = 2.15.
The thermal variation of vmT product for compound 3 shows at
room temperature a value of ca. 0.80 emu K molꢁ1 (Fig. 8), which is
close to that expected for two noninteracting Cu(II), S = 1/2 ions
(0.75 emu K molꢁ1 for g = 2.0). When cooling down the sample,
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