Synthesis and characterization of heterotrinuclear bis(μ2- chlorido)dicopper (II) mono zinc(II) complexes derived from succinoyldihydrazones

Article abstract of DOI:10.1016/j.saa.2013.08.063

Three new zinc (II)-copper (II) heterometallic trinuclear complexes of the composition [ZnCu₂(Lⁿ)(μ₂-Cl) ₂(H₂O)₆]×2H₂O (H ₄Lⁿ = H₄L¹, H₄L ², H₄L³) have been synthesized from substituted succinoyldihydrazones (H₄Lⁿ) in methanol medium. The composition of the complexes has been established on the basis of data obtained from analytical, mass spectral studies and molecular weight determinations in DMSO. The structure of the ligand H₄L² has been established by X-ray crystallography. The structure of the complexes has been discussed in the light of molar conductance, magnetic moment, electronic, EPR, IR and FT-IR spectral studies. The molar conductance values for the complexes fall in the region 1.2-1.7 ohm^-1 cm² mol^-1 in DMSO solution indicating that all of these are non-electrolyte. The magnetic moment values suggest weak M-M interaction in the structural unit of the complexes. The dihydrazone ligand is present in enol form in all of the complexes. Copper centre has tetragonally distorted octahedral stereochemistry. The EPR parameters of the complexes indicate that the copper centre has doublet state as the ground state. The electron transfer reactions of the complexes have been investigated by cyclic voltammetry.

Full text of DOI:10.1016/j.saa.2013.08.063

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 94–101
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis and characterization of heterotrinuclear bis
(
l2-chlorido)dicopper (II) mono zinc(II) complexes derived
from succinoyldihydrazones
a
b
a,
⇑
Rosmita Borthakur , Arvind Kumar , R.A. Lal
a
Department of Chemistry, North Eastern Hill University, Shillong 793022, Meghalaya, India
Department of Chemistry, Faculty of Science and Agriculture, The University of West-Indies, St Augustine, Trinidad and Tobago
b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
2 2 2 2 6
Three new heterotrinuclear copper (II)–zinc (II) complexes having composition [Cu Zn(L)(l -Cl) (H O) ]-
ꢀ
Three new zinc (II)–copper (II)
heterotrinuclear complexes have
been synthesized.
The structure of the ligand H
been established by X-ray
crystallography.
ꢁ
2H
2 4
O (H L = N, O donor schiff base ligands) were prepared and characterised by UV–vis, IR, magnetic
moment and EPR spectroscopy. The electron transfer reactions for the complexes have been studied by
2
ꢀ
L2 has
4
4
cyclic voltammetry. Single crystal structure of H L .
ꢀ
ꢀ
ꢀ
The dihydrazone ligand is present in
enol form in all of the complexes.
Copper centre has tetragonally
distorted octahedral stereochemistry.
EPR parameters indicate that copper
centre has doublet state as the
ground state.
a r t i c l e i n f o
a b s t r a c t
n
Article history:
Received 3 April 2013
Received in revised form 9 August 2013
Accepted 15 August 2013
Available online 30 August 2013
2
Three new zinc (II)–copper (II) heterometallic trinuclear complexes of the composition [ZnCu (L )
n
1
2
3
(
l
2
-Cl)
razones (H
of data obtained from analytical, mass spectral studies and molecular weight determinations in DMSO.
2
(H
2
O)
6
]ꢁ2H
2
O (H
4 4 4 4
L = H L , H L , H L ) have been synthesized from substituted succinoyldihyd-
n
4
L ) in methanol medium. The composition of the complexes has been established on the basis
2
4
The structure of the ligand H L has been established by X-ray crystallography. The structure of the
complexes has been discussed in the light of molar conductance, magnetic moment, electronic, EPR, IR
and FT-IR spectral studies. The molar conductance values for the complexes fall in the region 1.2–
Keywords:
Heteropolymetallic complexes
Succinoyldihydrazones
Zinc (II)
Copper (II) complexes
Spectroscopic studies and cyclic
voltammetric studies
ꢂ1
2
ꢂ1
1.7 ohm cm mol in DMSO solution indicating that all of these are non-electrolyte. The magnetic
moment values suggest weak M–M interaction in the structural unit of the complexes. The dihydrazone
ligand is present in enol form in all of the complexes. Copper centre has tetragonally distorted octahedral
stereochemistry. The EPR parameters of the complexes indicate that the copper centre has doublet state
as the ground state. The electron transfer reactions of the complexes have been investigated by cyclic
voltammetry.
Ó 2013 Elsevier B.V. All rights reserved.
⇑
Corresponding author. Tel.: +91 0364 2722616.
E-mail address: ralal@rediffmail.com (R.A. Lal).
1
386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.08.063

R. Borthakur et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 94–101
95
Introduction
interest to investigate the electron transfer reactions of the com-
plexes by cyclic voltammetry. The results of such an investigations
are presented in this paper.
Copper is a biologically important metal and occurs in enzymes
and metalloproteins like ascorbate oxidase copper monooxygen-
ase, copper dioxygenase, superoxide dismutase, cytochrome oxi-
dase and blue oxidase [1]. In living organisms, it is often present
in mono-, di-, or trinuclear assemblies. When blue copper site in
multinuclear copper oxidases is substituted by a redox innocent
R
1
R
2
X
Dihydrazone ligand
H
H
H
H
H
Disalicylaldehydesuccinoyldihydrazone
1
(
H
4
L )
5,6-benzo
H
H
Bis(2-hydroxy-1-naphthaldehyde)
2
succinoyldihydrazone (H
4
L )
2
mercuric ion, O -bond cleavage by the fully reduced trinuclear site
Br Bis(5-bromosalicyladehyde)
is significantly impeded [2]. The multimetallic copper complexes
are also important because of their relevance in the development
of novel functional materials showing molecular ferromagnetism
3
4
succinoyldihydrazone (H L )
[
3] and specific catalytic properties [4]. Homo- and hetero-trime-
tallic copper complexes offer the opportunities to test magnetic
exchange models on more complicated systems. Copper (II) com-
plexes find application as catalyst for the oxidation of alcohols into
aldehydes and ketones [5].
Experimental
Materials
Hydrazones are special kind of polyfunctional Schiff base
ligands derived from condensation of organic acid hydrazines
and o-hydroxy aromatic aldehydes and ketones [6–10] which have
potential for yielding homo- and hetero-metallic complexes. The
dihydrazones derived from condensation of succinoyldihydrazines
and o-hydroxy-aromatic aldehydes and ketones are unique in the
sense that their succinoyl fraction offers greater flexibility in three
dimentional space because of their capability for free rotation
about C–C single bond as compared to those in which the two
hydrazone groupings are joined together either directly (oxaloyl)
or through phenyl or pyridyl groups. Such dihydrazones, although,
might exist in only one configuration in free state, in metal com-
plexes can exist in staggered, anti-cis or syn-cis configuration
Zinc acetate dihydrate, copper chloride dihydrate, diethyl
succinate, hydrazine hydrate, substituted salicyldehyde and 2-hy-
droxy-1-naphthaldehyde were E-Merck, Qualigens, Hi-Media or
equivalent grade reagents.
Instrumentation and measurement
Copper and zinc were determined by standard literature proce-
dure [12]. Chloride was determined as AgCl [12]. C, H, N were
determined by micro-analytical method using Perkin-Elmer, 2400
CHNS/O Analyser II. All conductance measurements were made
at 1 kHz using Wayne Kerr B905 Automatic Precision Bridge. A
dip-type conductivity cell having a platinised platinum electrode.
The cell constant was determined using a standard KCl solution.
Room temperature magnetic susceptibility measurements were
made on a Sherwood Magnetic Susceptibility Balance MSB-Auto.
Diamagnetic corrections were carried out using Pascals Constant
[13]. Electronic spectra of the complexes were recorded in DMSO
[
10]. The resulting dihydrazones contain salicylaldimine, 2-hydro-
xy-naphthaldimine and 5-bromo-salicylaldimine in their molecu-
lar skeleton. Further, as we go from salicylaldimine dihydrazone
to naphthaldimine dihydrazone to 5-bromo-salicylaldimine dihy-
drazone, the bulkyness and electronegativity both increase in the
same order. It is quite interesting to see how the properties of
the complexes change as we go from one dihydrazone to next one.
A survey of literature reveals that although some isolated stud-
ies are available on metal complexes of succinoyldihydrazones [10]
and related dihydrazones [6–10] yet the synthesis and character-
ization of heterotrinuclear complexes of dihydrazones is quite
meagre [11]. Moreover, a systematic study on trinuclear metal
complexes of succinoyldihydrazones is absent to the best of our
knowledge inspite of their highly flexible nature. In view of the
above importance of copper complexes, absence of work on het-
erotrinuclear complexes of succinoyldihydrazones and highly flex-
ible polyfunctional nature of succinoyldihydrazones, it was of
interest to synthesize the heterotrinuclear copper complexes of
the title dihydrazones (Fig. 1) and to characterize them by various
physico-chemical and spectroscopic studies. Further, it was of
ꢂ3
solution at ꢃ10 M concentration on a Perkin–Elmer Lambda-25
spectrophotometer. Electron paramagnetic resonance spectra of
the complexes were recorded at X-band frequency on a Varian E-
112 E-Line Century Service EPR spectrometer using TCNE
(g = 2.0027) as an internal field marker. Variable temperature
experiments were carried out with a Varian Variable Temperature
accessory. Infrared spectra were recorded on a BX-III/FT-IR Perkin–
ꢂ1
Elmer Spectrophotometer in the range 4000–400 cm in KBr discs
ꢂ1
and 600–50 cm in CsI discs. The molecular weights of the com-
plexes were determined in DMSO by freezing point depression
method. Mass losses were determined by heating the complexes
at 110 °C and 180 °C in an electronic oven. APCI mass spectra of
the complexes were recorded on a water Zg 4000 Micromass Spec-
trometer in DMSO solution. Cyclic voltammetric measurements of
the compounds in DMSO were done using a CH instruments
Electrochemical analyzer under dinitrogen. The electrolytic cell
comprises of three electrodes, the working electrode was a Pt disk
while the reference and auxiliary electrodes were Ag/AgCl sepa-
X
ꢂ1
rated from the sample solution by a salt bridge; 0.1 mol L , TBAP
R 2
R 1
was used as the supporting electrolyte.
C
N
O
O
H
N
O
O
C
C
Molecular weight determination
H
H
N
H
N
C
The molecular weight of the complexes were determined in
DMSO (K = 4.07) [14] as a solvent, by lowering of the freezing
f
R 1
point of DMSO (f.p. 18.5 °C) using Beckman’s freezing point depres-
sion instruments. The instrument consisted of a Beckman ther-
mometer and a stirrer with a non-conducting handle of wood
fitted into a tube through a rubber stopper fixed at its upper end.
This tube was supported through a rubber stopper in a bigger glass
R 2
X
Fig. 1. Structure of ligands.

9
6
R. Borthakur et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 94–101
tube thoroughly purged with dry dinitrogen which acts as a dry
dinitrogen jacket and ensures slow and uniform cooling of the in-
ner tube. The outer tube is immersed in a 1,2-dimethyl-3-nitroben-
zene (F. P. 15 °C) freezing bath contained in a glass jar. The jar was
covered with a lid of plastic which carries three holes, one for
inserting the inner tube, other for large metal stirrer and third
for inserting the ordinary thermometer. The molecular weight
apparatus was enclosed in a dry box that was constantly purged
with dry nitrogen to minimize the error due to highly hygroscopic
nature of DMSO. The different samples of the solutions were pre-
pared in DMSO taking different amounts of complexes. All of the
samples were placed in an entry port and purged overnight with
dry dinitrogen before bringing them into the dry box. 20 g of DMSO
was transferred into the thoroughly cleaned and dried freezing
point tube purged with dry dinitrogen. About 0.6 g of the complex
was accurately weighed and inserted into the DMSO solution. The
tube was heated to dissolve the compound and then brought to
room temperature. Finally, the upper surface of the liquid in the
tube was purged with dry dinitrogen and then adjusted into the
outer tube. The freezing point temperatures of the solvent and
the solutions were directly measured with this apparatus through
a magnifying glass fitted into the dry box.
C, 36.74; H, 4.01; N, 6.80; Cl, 8.05. Mol. Wt. (theo.: (857.49),
ꢂ1
2
ꢂ1
exp.: (1150 ± 55). Molar Conductance (DMSO,
X
cm mol ): 1.5.
3
[
ZnCu
2
(L )(
l
2
-Cl)
2
(H
2
O)
6
]ꢁ2H
2
O (3). Yield: 2.45 g (85%), M.p.:270 °C
Cu ZnCl (%): Cu, 13.88; Zn, 7.14; C,
Anal: Calc for C18
H
28
N
4
O
12 Br
2
2
2
2
2
3.59; H, 3.06; N, 6.12; Cl, 7.75. Found: Cu, 13.53; Zn,7.32; C,
3.93; H, 3.03; N, 5.91; Cl, 7.98. Mol. Wt. (theo.: (915.49), exp.:
ꢂ1
2
ꢂ1
(
1250 ± 60). Molar Conductance (DMSO,
X
cm mol ): 1.2.
Results and discussion
2
4
Structural description of ligand (H L )
Crystallographic data and refinement details for the structural
2
4
analyses of the ligand (H L ) is summarized in (Table 1). Selected
bond lengths and bond angles are presented in (Table 2). A single
unit cell with atom numbering scheme and atom connectivity of
2
the ligand (H
4
L ) is shown in (Fig. 2). The X-ray single-crystal
2
4
structure determination of ligand (H L ) suggested that it was
crystallized in monoclinic crystal system with space group P 21/
c. The two procumbent naphthyl rings constitute a staggered con-
figuration about the central –CH
tended conformation (Fig. 2). The N(1)–C(11) bond length is
.273. This bond length is shorter than the mean reported bond
2 2
–CH – bond and adopt an ex-
2
4
X-ray crystallographic analysis of H L
1
length for a single C–N bond (1.437 Å) and is very close to the
mean bond length reported for a C@N double bond (1.289 Å). These
data clearly reveal complete localization of the electron lone pair of
N(1) atoms [18]. The two parts of the molecule have a centre of
symmetry. The carbonyl groups have anti–configuration and there
is an intramolecular hydrogen bonding between O(1)–H(1)ꢁ ꢁ ꢁN(1)
with bond length 2.55 Å to form six membered ring involving
atoms O(1)–H(1)–N(1)–C(11)–C(1)–C(2). In addition, there are
two intermolecular hydrogen bonds involving secondary N–H pro-
ton (N2) and oxygen of cocrystallized DMSO molecule (O3) [19]
The X-ray diffraction data were collected by mounting a single
crystal of the sample on glass fibers. Oxford diffraction XCALIBUR-
EOS diffractometer was used for the determination of cell parame-
ters and intensity data collection at 273 K. Monochromating Mo K
a
radiation (k = 0.71073 Å) was used for the measurements. The
crystal structures were solved by direct methods using SHELXL-
9
7 Program [15] and were refined by full matrix least-squares
SHELXL-97 [16]. Non-hydrogen atoms were refined with aniso-
tropic thermal parameters. Hydrogen atoms were placed into cal-
culated positions and refined using a riding model.
(
Fig. S1). The mode of packing of the compound in stereo projection
along b-direction is illustrated in (Fig. S2).
Synthesis of compounds
n
The precursor monometallic zinc complexes [Zn(H
2
L )(H
O)
2 2
]
Table 1
n
1
3
2
(
H
4
L = H
4
L –H
4
L ) were prepared by literature method [17].
Crystal data and structure refinement for [H
4
L ].
Emperical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system, space group
Unit cell dimensions
a (Å)
30 34 4 6 2
C H N O S
610.75
293
0.71073
Monoclinic, P 21/c
n
n
1
Synthesis of [ZnCu
2
(L )(
l
2
-Cl)
2
(H
2
O)
6
]ꢁ2H
2
O[where H
4
L = H
4
L (1),
2
3
H
4
L (2), H
4
L (3)]
1
[
2
Zn(H L )(H
2
O)
2
] (2.0 g; 4.43 mmol) was suspended in metha-
nol (30 mL) and stirred vigorously to get a homogeneous suspen-
sion. This suspension was added into copper chloride dihydrate
solution maintaining [Zn(H
at 1:3 and was refluxed for 3 h which precipitated a green coloured
compound. The compound was suction filtered in hot condition
and washed several times with methanol (30 mL each time) fol-
lowed by ether and finally dried over anhydrous CaCl
The complexes 2 and 3 were also prepared by following the
above procedure using [Zn(H
20.119(2)
4.6311(3)
17.5223(18)
90
114.680(13)
90
1483.5(3)
2
1.367
1
b (Å)
c (Å)
2
L )(H
2
O)
2
]:CuCl
2
ꢁ2H
2
O 1 molar ratio
a
(Å)
b (Å)
(Å)
Volume (Å )
c
3
2
.
3
Z, calculated density (Mg/m )
ꢂ3
2
Dcalc (g/cm
)
2
L )(H
2
O)
2
] (2.0 g; 3.28 mmol) and
ꢂ1
Absorption coefficient (mm
F(000)
)
0.230
644.0
3
[
[
Zn(H
Zn(H
2
L )(H
2
O)
O)
2
]
].
(2 g;3.63 mmol) respectively, instead of
1
2
L )(H
2
2
Crystal size (mm)
0.31 ꢄ 0.31 ꢄ 0.30
3.1828–28.9516
3400/2044
29.02
Multi-scan
0.98345 and 1.00000
Theta range for data collection (°)
Reflections collected/unique [R(int)]
Completeness to theta
Absorption correction
Maximum and minimum
Transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F^2
1
[ZnCu
2
(L )(
l
2 2
-Cl) (H
2
O)
6
]ꢁ2H
2
O (1). Yield: 2.60 g (78%), M.p.:210 °C
ZnCl (%):Cu, 16.78; Zn, 8.63; C, 28.52;
H, 3.96; N, 7.39; Cl, 9.36. Found: Cu, 16.39; Zn, 8.79; C, 28.90; H,
Anal: Calc for C18
H
30
N
4
O
12Cu
2
2
3
.95; N, 7.72; Cl, 9.56. Mol. Wt. (theo.:(757.49), exp.: (980 ± 50).
Molar Conductance (DMSO,
ꢂ1
2
ꢂ1
Full-matrix least-squares on F2
3400/0/198
0.983
R1 = 0.0547, wR2 = 0.1330
R1 = 1067, wR2 = 0.1099
0.002 and 0.021
X
cm mol ): 1.7.
2
Final R indices [I > 2
r
(I)]
[ZnCu
2
(L )(
l
2 2
-Cl) (H
2
O)
6
]ꢁ2H
2
O (2). Yield: 2.23 g (72%), M.p.:290 °C
ZnCl (%): Cu, 14.82; Zn, 7.63;
C, 36.36; H, 3.97; N, 6.53; Cl, 8.27. Found: Cu, 15.13; Zn,7.46;
R indices (all data)
Largest diff. peak and hole
Anal: Calc for
C
26
H
34
N
4
O12Cu
2
2

R. Borthakur et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 94–101
97
Table 2
metal–ligand bond in these complexes have more ionic character
than those in the corresponding parent ligands. All of the com-
plexes are insoluble in water and common organic solvents but
are soluble in highly coordinating solvents such as DMSO and
DMF. All of the complexes show weight loss corresponding to
two water molecules at 110 °C, while six water molecules at
Selected interatomic distances (Å) and bond angles (°) for the compound.
C(1)–C(2)
C(11)–N(1)
N(1)–N(2)
N(2)–C(12)
C(12)–C(13)
C(2)–O(1)
C(12)–O(2)
N(1)–O(1)
1.382
1.273
1.375
1.350
1.507
1.348
1.221
2.555
C(1)–C(11)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(5)–C(6)
C(6)–C(7)
C(7)–C(8)
C(8)–C(9)
C(9)–C(10)
C(5)–C(10)
1.454
1.405
1.342
1.401
1.423
1.412
1.366
1.387
1.355
1.415
1
80 °C when heated in an electronic oven for 4 h. The weight loss
corresponding to two water molecules at 110 °C suggests that
these water molecules are present in the lattice structure of the
complexes. The six water molecules lost at 180 °C suggest that they
are present in the first coordination sphere around metal centre.
The vapours evolved at 110 °C and 180 °C in the complexes
were passed through a trap containing copper sulphate which
turned blue. This confirmed that the vapours in the complexes
originated from water molecules.
An effort was taken up to crystallize the complexes in various
solvent systems under different experimental conditions.
Unfortunately, only amorphous compounds precipitated in all
our efforts which prevented analysis of the complexes by X-ray
crystallography.
S(1)–C(15)
S(1)–O(3
C(1)–C(6)
1.775
1.505
1.439
C(1)–C(2)–O(1)
C(2)–C(1)–C(11)
O(1)–C(2)–C(3)
C(1)–C(2) –C(3)
C(2)–C(3)–C(4)
C(3)–C(4)–C(5)
C(4)–C(5)–C(6)
C(6)–C(1)–C(11)
C(11)–N(1)–N(2)
N(2)–C(12)–C(13)
C(12)–C(13)–C(13a)
C(14)–S(1)–C(15)
122.78
120.15
116.15
121.07
120.43
121.73
119.14
121.03
117.12
114.20
110.23
98.02
C(4)–C(5)–C(10)
C(5)–C(10)–C(9)
C(10)–C(9)–C(8)
C(9)–C(8)–C(7)
C(8)–C(7)–C(6)
C(7)–C(6)–C(5)
C(7)–C(6)–C(1)
C(1)–C(11)–N(1)
N(1)–N(2)–C(12)
C(13)–C(12)–O(2)
C(14)–S(1)–O(3)
C(15)–S(1)–O(3)
121.84
121.24
120.07
120.71
121.34
117.62
123.57
120.90
120.15
122.30
105.97
106.14
Mass spectral studies
Synthesis of complexes
All of the complexes have been characterized by mass spectros-
copy. The molecular ions along with their experimental and theo-
retical masses, for all the complexes have been given in (Table S1).
The mass spectra for the complexes 1 and 2 are shown in (Figs. S3
and S4).
Synthesis of the heterometallic complexes were achieved by
n
reaction of precursor zinc (II) complexes [Zn(H
copper (II) chloride in methanol keeping precursor complex:
CuCl .2H O molar ratio at 1:3. Green or brown solutions were
2 2 2
L )(H O) ] with
2
2
formed rapidly, resulting in compounds which were characterized
by elemental analysis, spectroscopic and electrochemical analysis.
On the basis of various analytical, thermo-analytical and mass
spectral data, the complexes have been suggested to have the
The complexes 1 to 3 show a peak at m/z value of 541.4, 717.9,
745.0, 1107.7 and 992.2, 1108.0 respectively. For the complex 1,
the peaks at m/z value of 541.4 and 717.9 are close to the mass
1
+
1
of molecular ions [Cu ZnL ] (542.49) and [Cu Zn(H L ) (DMSO) (-
2
2
2
2
n
n
1
2
+
1 +
stoichiometry [ZnCu
2
(L )(Cl
2
)(H
2
O)
6
]ꢁ2H
2
O (L = H
4
L , H
4
L
and
H
2
O)] (718.49), respectively. The molecular ion [Cu
2
ZnL ] arises
3
4
H L ). The complexes melt with decomposition in the temperature
from the loss of both the chloride ions and all water molecules
1
+
range 210–290 °C. It is imperative to mention that the melting
point of the complex 1 is less than that of the corresponding parent
ligand. This may be related to higher covalent character of
metal–ligand bond as compared to that of the ligand [20]. On the
other hand, the melting point of other complexes are higher than
those of the corresponding parent ligand. This suggests that the
2 2 2 2
while the molecular ion [Cu Zn(H L )](DMSO) (H O)] owes its
origin in the loss of two chloride ion and five water molecules from
the coordination sphere followed by coordination of two DMSO
molecules to the metal centre. The origin of different molecular
ion peaks in the mass spectra of the complexes 2 and 3 is under-
standable in the same way and hence their further discussion
Fig. 2. Crystal structure of H
4
L2 with thermal ellipsoids plotted at 50% probability level.

9
8
R. Borthakur et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 94–101
seems redundant. The mass spectral data and their discussion sug-
gest that all of the complexes are monomeric in character.
the staggered configuration in the present complexes is impossible,
because they would, at the most, give rise to binuclear complexes
only with the bivalent metal ions. If a trinuclear complex is to be
formed, the only option available to the ligands is to exist in the
cis-configuration and react with the metal ions [13,23]. Thus, the
stereochemical consideration of the ligands suggests that their
reaction with the metal ions in cis-configuration would give rise
to trinuclear complexes.
The complexes 2 and 3 show an isotropic spectra in the solid-
state at RT. The isotropic nature of the spectra of the complexes
without any copper hyperfine splitting in the solid state at RT is
due to intermolecular interaction [24]. However, when the com-
plexes are sufficiently cooled to low temperature, intermolecular
interactions are diminished in magnitude allowing the anisotropic
Molecular weight
The molecular weights of the complexes 1–3 have been deter-
mined in DMSO solution by freezing point depression method
due to insolubility of the complexes in non-coordinating solvents.
The experimental values of the molecular weights are (980 ± 50),
(
1150 ± 55) and (1250 ± 60), respectively. The experimental values
of the molecular weights are very close to the theoretical values of
57.49, 857.49 and 915.49 for the monomeric formulation rather
7
than the values 1514.98, 1714.98 and 1830.98 for the dimeric for-
mulation. This suggests that the complexes are monomeric in nat-
ure. However, the experimental value of the molecular weight is
considerably higher than the theoretical value calculated on the
basis of the monomer formulation. This may be related partly to
the break down of chlorido bridges in the complexes followed by
coordination of DMSO molecules to the metal ions and partly
due to substitution of some of the coordinated water molecules
by DMSO molecules in the solution in all of complexes.
k
spectra to appear with obvious hyperfine splitting in the g region
due to coupling of unpaired electron-spin with copper nucleus
(I = 3/2). The spectra for the complexes look like an isolated copper
(II) ion [25]. The hyperfine splitting constant for the complex 1 is
150 G. This value suggests either no interaction or very weak
Cuꢁ ꢁ ꢁCu interaction in the structural unit of the complex. The fact
that the intensity of the signals decreased upon cooling from room
temperature to 77 K confirms that the resonance occurs within the
triplet excited state. The spin–spin interaction between the copper
centres in the complexes can be considered to occur only via a
superexchange mechanism involving enolate oxo-bridging and
Molar conductance
The molar conductance values for the complexes in DMSO solu-
ꢂ3
ꢂ1
2
ꢂ1
10
tion at 10 M dilution are in the range 1.2–1.7 ohm cm mol
.
the chloride bridging via d orbitals of intervening zinc atom.
These values are consistent with their non-electrolytic nature in
this medium [21].
The large metal–metal separation and long molecular pathway
1
0
through zinc atom having d configuration between the copper
centres are consistent with very weak exchange. Such weak ex-
change is typified by the compounds in which the two copper cen-
tres are separated by large distances with intervening 1, 2, 4, 5-
tetrasubstituted benzene rings [26]. The weak exchange between
Cuꢁ ꢁ ꢁCu centres in these complexes is also corroborated by the
room temperature magnetic moment studies on the complexes.
At 77 K, dimethyl sulphoxide glass spectra for the complexes
show four weakly resolved peaks in the complexes 1 and 3 for
the low-field side of the perpendicular region while two for the
complex 2. At LNT also, the spectra appear like an isolated copper
Magnetic moment
The leff value for the complexes are 2.12, 2.25 and 2.35 BM per
molecular formula while per copper unit are 1.50, 1.59 and 1.66
BM, respectively. These values are less than the value of 2.44 BM
for dinuclear copper (II) complexes and 1.73 BM for mononuclear
complexes, respectively, involving no metal–metal interaction.
These leff values suggest considerably weak metal–metal interac-
tion in the structural unit of the complexes. Such metal–metal
interaction can be considered to occur only via superexchange
mechanism through ligand backbone by inductive effect involving
ligand bridging itself, the chloride bridging and enolate oxygen
atom bridging via filled d10-orbitals of the intervening zinc atom
k
(II) ion. The g value for the complexes falls in the region 120–147
G which shows either weak interaction or no interaction at all
between the two metal centres.
k k
The tendency of A to decrease with an increase of g is an index
[
22]. A minute examination of the magnetic moment value shows
of an increase in tetrahedral distortion in the square planar geom-
etry in the coordination sphere of copper. However, in the case of
octahedral or square pyramidal complexes, this has been related to
the increased distortion in the equatorial plane. In order to quan-
tify the degree of distortion in the copper (II) complexes, we have
that eff value increases in going from salicylaldehydato to 2-hy-
l
droxy-1-naphthaldehydato to 5-bromosalicylaldehydato com-
plexes. This suggests that magnetic exchange is stronger in
salicylaldehydato complex than that in 2-hydroxy-1-napnthalde-
hydato complex in which it is stronger than that in 5-bromosalicyl-
dehydato complex. This may be related to decreasing M–M
interaction with increasing steric crowding as the size of the aro-
matic ring increases in going from salicylaldimine to 2-hydrox-
ynaphthaldimine to 5-bromosalicylaldimine complex. On the
basis of the magnitude of magnetic moment, it may safely be con-
cluded that both oxido as well as chlorido bridges are expected to
be responsible for the lowering of the magnetic moment to the
same extent.
k k
selected the factor g /A obtained from EPR spectra which is con-
sidered as an empirical index of distortion in the equatorial plane
[27]. It ranges between 105 and 135 for square planar equatorial
configuration depending upon the nature of the coordinated
atoms, highly distorted structure in the equatorial plane can have
larger value [27]. The g
LNT in the solid state, while for all of the complexes in DMSO glass
at LNT, it lies in the range 171.2–200.4. The large value for g /A
k k
/A quotient for the complex 1 is 157.0 at
k
k
quotients for the complexes reflects the increased distortion in
the equatorial plane [28]. This clearly reflects a lower symmetry
for the complexes [28].
Electron paramagnetic resonance
The X-band EPR spectra for the three trinuclear complexes have
been recorded at RT and LNT in powder as well as DMSO glass at
LNT. The various EPR parameters for the complexes have been
set out in (Table S2). The EPR spectra for the complexes have been
given in (Figs. S5–S7). The present ligands can react with the metal
ions either in staggered configuration or syn-cis or anti-cis config-
uration [13,23]. The bonding of the ligands to the metal centre in
Electronic spectroscopy
The free dihydrazones show bands in the regions ꢃ292, 327–
340 and 330–382 nm, respectively. The bands in the regions
*
ꢃ292 and 327–340 nm are assigned to intra-ligand
p
?
p
p
*
transitions while the band in the region 330–382 nm to n ?

R. Borthakur et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 94–101
99
ꢂ1
transition. The band in the region 330–382 nm is characteristic of
1240–1269 cm in free dihydrazone ligand due to
m
(C–O)(pheno-
salicylaldimine/naphthaldimine part of the ligand [8e,8f].
lic/naphtholic). The shift of this band to higher frequency by
10–43 cm indicates bonding of phenolic/napntholic oxygen atom
ꢂ1
In the solid state, the ligand bands appear as a single broad band
having fine structure in the region 200–450 nm. However, they are
not resolved, most probably, because they are overlapped with one
another and are superimposed upon vibronic transitions. The com-
plexes display a broad band centred in the region 710–775 nm
to the metal centre via deprotonation [28]. The complexes show
ꢂ1
new bands in the region 500–546 cm
which are assigned to
m
(M–O)(phenolate/naphtholate) [31]. The weak bands appearing
ꢂ1
in the region 456–477 cm
are attributed to m(M–O)(enolic)
(
Table S2). This band is attributed to d–d transitions. The d–d tran-
stretching vibrations. The complexes show weak to medium inten-
sity new bands in the regions 669–710 and 447–490 cm , respec-
ꢂ1
sition occurs at around 800 nm in the octahedral complexes [29].
This band in octahedral complexes is considerably blue shifted
due to Jahn–Teller distortion. The position of the d–d band and
its shape suggests that the copper centres have distorted octahe-
dral geometry in complexes.
tively. These bands are not visible in the spectra of the free ligands.
Hence, they are assigned to arise due to oxo bridged metal atoms
[32]. The position of the bands in the complexes is consistent with
the involvement of enolate oxygen atoms in the bridge formation.
The position of the bands in the complexes suggests that they orig-
inate from the formation of dibridge [33]. The band in the region
The complexes display four well defined bands in the region
92–710 nm in solution (Table S2). The ligand band at ꢃ292 nm
2
ꢂ1
splits into two bands. While one band remains almost unshifted
in position, the other band shows red shift of about 20–29 nm in
the complexes and appears in the region 316–332 nm. Further,
the ligand band in the region 330–382 nm shows considerable
red shift of about 34–53 nm on complexation and appears in the
region 383–410 nm. Such a feature associated with the red shift
of the ligand bands provides a good evidence for the chelation of
dihydrazone to the metal centre. In DMSO solution, the complexes
have absorbance maxima in the region 700–710 nm with molar
extinction coefficient in the region 82–90 dm cm mol . The
d–d band in DMSO solution has slightly blue shifted as compared
to that in solid state in complexes. This indicates interaction of
DMSO molecule with metal centre in DMSO solution i.e. replace-
ment of water molecules by DMSO molecules. However, the essen-
tial feature of the band in DMSO solution remains almost same as
that in the solid state. This suggests that the stereochemistry of the
complexes in solution as well as in the solid state remains same i.e,
distorted octahedral (Fig. S11).
669–710 cm is assigned to antisymmetric vibrations while the
ꢂ1
band in the region 447–490 cm is assigned to symmetric vibra-
tions of (
) group respectively.
The complexes show a new weak to medium intensity band in
ꢂ1
the region 210–228 cm . The terminal metal-chloride stretching
ꢂ1
vibrations are observed in the region 253–333 cm in square pla-
nar chloride complexes while tetrahedral chlorido complexes show
ꢂ1
metal-chloride stretching vibrations in the region 306–355 cm
.
In the monomeric octahedral complexes, the terminal metal chlo-
3
ꢂ1
ꢂ1
ride stretching vibrations have been observed in the region 275–
ꢂ1
250 cm
[34]. The metal-chloride stretching vibration due to
bridging chloride group in the polymeric octahedral complexes of
first series transition metal complexes appears in the region
ꢂ1
170–195 cm . The position of the copper - chloride stretching
ꢂ1
absorption band in the region 210–228 cm in the present com-
plexes indicates that they have distorted octahedral stereochemis-
try and that the chloride group is involved in bridge formation.
ꢂ1
However,
mCu–Cl band appears in the region 210–228 cm
at
slightly higher frequency than that normally observed in polymeric
octahedral complexes of first series transition metal complexes
may be related to monomeric character of the complexes.
Infrared spectra
Some structurally significant IR bands for uncoordinated
dihydrazone and complexes have been set out in (Table S3). The
heterotrinuclear Cu(II) and Zn(II) complexes 1–3 under study show
characteristic bands due to ligation of ligands to the metal centre
Cyclic voltammetry
The cyclic voltammograms of a 2 mM solution of the complexes
have been recorded at a scan rate of 100 mV/s, 70 mV/s and 50 mV/
s by cyclic voltammetric method in DMSO solution because of the
in the KBr-phase IR spectra. The IR spectra of the ligands show
ꢂ1
strong broad bands in the region 3184–3235 cm
3
and
ꢂ1
421–3446 cm which are attributed to stretching vibrations of
solubility reasons in non-coordianting organic solvents (CH
3
CN
secondary –NH groups and phenolic/naphtholic –OH groups,
and CH Cl ) with a 0.1 M tetra-n-butyl ammonium perchlorate
2
2
respectively. In the IR spectra of the complexes, the band in the re-
(TBAP) as a supporting electrolyte. The potentials of the complexes
were scanned in the potential range +2.4 to ꢂ2.4. The cyclic vol-
tammetric data have been recorded in (Table S4). The cyclic vol-
tammograms for the complexes at scan rates of 100 mV/s have
been given in (Figs. S8–S10).
All of the complexes show three reductive and oxidative waves
each in the forward and return scan, respectively. The reductive
waves in the region ꢂ0.89 to ꢂ0.91 V and ꢂ1.37 to ꢂ1.56 V,
respectively, do not have their counterparts in the oxidative scan.
Hence, these waves are assigned to arise due to electron transfer
reactions centred on the ligand. With the highly negatively charged
dihydrazone ligand bonded to the metal centre, it is expected to
make the reduction of these metal centres unfavourable, leading
ꢂ1
gion 3184–3235 cm due to
mNH in free ligand is absent. Instead,
all of the complexes show a strong broad band in the region 3420–
ꢂ1
3
435 cm . The essential features of this band suggest it to be
either due to lattice or coordinated water molecules. This shows
that the dihydrazones coordinate to the metal centres in the enol
form in the complexes. Another important and most characteristic
feature of the IR spectra of the complexes is the absence of
ꢂ1
the band in the region 1659–1679 cm due to >C@O group in
the uncoordinated dihydrazones. This corroborates the fact that
the ligand is present in enol form in the complexes. The present li-
ꢂ1
gands show a very sharp band in the region 1615–1623 cm
which is assigned to stretching vibration of >C@N group. This band
ꢂ1
shifts to lower frequency by 15–27 cm suggesting that >C@N
to quite negative Epc values [35]. Further, an oxidative wave at
2
group is involved in bonding to the metal centre. Another impor-
tant feature of the IR spectra of the complexes is that they show
+0.75 V is observed in the complex [ZnCu
2
(L )(
l
2
-Cl)
(H
2 2
O)
6
].2H
2
O
2. This oxidative wave does not have its counterpart in the reduc-
tive scan. This suggests that the species produced corresponding to
this oxidative wave is unstable in DMSO solution and reverts back
to the original species. Hence, this oxidative wave is attributed to
the oxidation of >C@N group in the ligand [36]. The remaining
reductive and oxidative waves may be attributed to metal-centred
electron transfer reactions. The first redox couple (Epc = ꢂ0.41 V
ꢂ1
a new weak to strong band in the region 1500–1512 cm . This
band is characteristic of the presence of NCO– group [30].
The essential features associated with this band in the IR spectra
of the complexes suggest that the dihydrazone undergoes
enolization. The complexes show a band in the region 1267–
ꢂ1
1
305 cm which corresponds to the strong band in the region

1
00
R. Borthakur et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 94–101
and Epa = ꢂ0.13 V in complex 1), (Epc = ꢂ0.36 V and Epa = ꢂ0.24 V
in complex 2) and (Epc = ꢂ0.43 V and Epa = ꢂ0.21 V in complex 3)
is either irreversible or quasi-reversible in nature. The peak poten-
tial separation for the complexes 1 to 3 are 280, 120 and 220 mV,
mass spectra, the Head, SAIF, IIT Bombay, India for recording EPR
spectra. Authors express their heartfelt thanks to Prof. A.T. Khan,
Department of Chemistry, IIT Guwahati, Assam, India for helping
in recording the magnetic moment data.
II
II
II
I-
respectively. This wave may be assigned to (L) Cu Zn Cu /(L) Cu
II
II
Zn Cu , redox couple. The complex 2 shows another single oxida-
tive wave at +0.34 V while the complexes 1 and 3 show additional
oxidative waves at +0.10, +0.42 V and +0.11 and +0.38 V, respec-
tively. These oxidative waves do not have their counterparts in
the reductive waves. It appears that these oxidative waves arise,
Appendix A. Supplementary material
Crystallographic data (excluding structure factors) for the struc-
tures reported in this Article have been deposited with the Cam-
bridge Crystallographic Data Centre (CCDC) as deposition nos.
II
II
III 1+
most probably, either from the species [(L) Cu Zn Cu ] (in com-
2
CCDC 912816 (for H
4
L ). Copies of the data can be obtained, free
II
II
III 1+
III
II
plex 2) or from the species [(L) Cu Zn Cu ] and [(L)Cu Zn
of charge, on application to the CCDC, 12 Union Road, Cambridge
CB2 lEZ, UK (fax, 44 (1223) 336 033; e-mail, deposit@ccdc.cam.a-
c.uk). Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.saa.
III 2+
Cu ] (in complexes 1 and 3), respectively. However, non-appear-
ance of waves corresponding to these oxidative waves in the
reductive scan suggests that they are highly unstable in DMSO
solution and do not survive long. Hence, it is not possible to visu-
alize the presence of Cu (III) containing species in the complexes.
The reductive and oxidative waves for first redox couples are
separated from one another by 280 and 220 mV in complexes 1
and 3 respectively, suggesting that the redox processes are either
irreversible or quasi-reversible. The high peak separation, most
probably, originated from a slow heterogeneous electron exchange
rate rather than from intervening homogeneous reaction [36]. In
order to confirm that the electron transfer reaction in complexes
2
013.08.063.
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