Mononuclear and binuclear Cu(II) complexes of some tridentate aroyl hydrazones. X-ray crystal structures of a mononuclear and a binuclear complex

Article abstract of DOI:10.1016/j.ica.2012.12.018

Cu(II) complexes of eight N,N,O-donor hydrazone ligands, obtained by condensation of pyridine-2-carbaldehyde and 2-acetylpyridine with four aroyl hydrazides, are reported. Reactions of Cu(ClO₄)₂· 6H₂O with the pyridine-2-c

Full text of DOI:10.1016/j.ica.2012.12.018

Inorganica Chimica Acta 398 (2013) 98–105
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Inorganica Chimica Acta
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Mononuclear and binuclear Cu(II) complexes of some tridentate aroyl
hydrazones. X-ray crystal structures of a mononuclear and a binuclear complex
Satyajit Mondal a, Sumita Naskar a, Ayan Kumar Dey a, Ekkehard Sinn b, Carla Eribal b, Steven R. Herron c,
Shyamal Kumar Chattopadhyay a,
⇑
a Department of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah 711 103, India
b Chemistry Department, Western Michigan University, Kalamazoo, MI 49008-5413, USA
c 10687 Logan Canyon Road, South Jordan, UT 84095, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 August 2012
Received in revised form 28 November 2012
Accepted 1 December 2012
Available online 29 December 2012
Cu(II) complexes of eight N,N,O-donor hydrazone ligands, obtained by condensation of pyridine-2-carb-
aldehyde and 2-acetylpyridine with four aroyl hydrazides, are reported. Reactions of Cu(ClO4)2ꢀ6H2O with
the pyridine-2-carbaldehyde hydrazones (L1H, L2H, L3H, L4H) in presence of Et3N, in 1:2:2 molar ratio,
lead to isolation of mononuclear octahedral Cu(II) complexes of formula [Cu(L)2] (L = L1ꢁ, L2ꢁ, L3ꢁ, L4ꢁ).
However, for the 2-acetylpyridine hydrazones (L5H, L6H, L7H, L8H) reactions under similar conditions
yield dichloro bridged complexes of the type [Cu2(
ion is in a square-pyramidal geometry, the chloride ions coming from the in situ reduction of perchlorate
by the 2-acetylpyridine hydrazones. The X-ray crystal structures of [Cu(L1)2] and [Cu2( -Cl)2(L5)2] are
l
-Cl)2(L0)2] (L0 = L5ꢁ, L6ꢁ, L7ꢁ, L8ꢁ), where each Cu(II)
Keywords:
Mononuclear Cu(II) complexes
Binuclear Cu(II) complexes
l
reported. The spectroscopic and electrochemical behaviors of the complexes are also reported.
(l-Cl)2-Bridge
Ó 2013 Elsevier B.V. All rights reserved.
Pyridine-2-carbaldehyde hydrazone
2-Acetylpyridine hydrazone
X-ray crystal structure
1. Introduction
aroylhydrazides with pyridine-2-carbaldehyde and 2-acetyl pyri-
dine (Scheme 1).
Interest in the metal complexes of Schiff base ligands remains
unwavering because of their applications in varied fields such as
catalysis [1–3], synthesis of compounds possessing designed and
controllable magnetic properties [4,5], spin-crossover materials
[6], biological activities including radical scavenging ability [7],
synthons for crystal engineering and materials chemistry [8–14].
Aroyl hydrazones are an important class of Schiff base ligands,
coordinating through protonated/deprotonated amide oxygen
and the imine nitrogen of hydrazone moiety; very often an addi-
tional donor site (usually N or O) is provided by the aldehyde or ke-
tone forming the hydrazone Schiff base. Aroylhydrazones have
proved to be strong chelating agents for transition metals [10–
12,15–17] and lanthanides [18,19] as well as main group elements
[20,21]. In our laboratory we have been investigating the structural
diversity of aroylhydrazone complexes along with their redox and
magneto-structural properties [10–12,15,16]. In this paper we re-
port mononuclear and binuclear complexes of Cu(II) with triden-
tate hydrazone ligands obtained by the condensation of four
2. Experimental
2.1. Materials and methods
The hydrazone ligands were prepared following procedure de-
scribed in literature [15,22–25]. CuCl2ꢀ2H2O, Cu(ClO4)2ꢀ6H2O and
Cu(OAc)2ꢀH2O were obtained from Aldrich. The solvents for synthe-
sis were of AR grade obtained from SRL (India) or Spectrochem (In-
dia) and used without purification, while solvents for spectroscopic
and electrochemical work were of HPLC grade from Aldrich or SRL
(India). Elemental analyses were performed on a Perkin-Elmer
2400 C, H, N analyzer. Infrared spectra were recorded as KBr pellets
on a JASCO FT-IR-460 spectrophotometer. UV–Vis spectra were re-
corded using a JASCO V-530 spectrophotometer. Cyclic voltammo-
grams were recorded in DMF solutions (for compounds 1–4) or
DMSO solution (for compounds 5–8), containing 0.1 (M) TEAP as
supporting electrolyte, using a CH1120A potentiostat, with Pt
disc/glassy carbon working electrode, Pt wire as counter electrode
and Ag,AgCl/saturated KCl as reference electrode. The ferrocene/
⇑
ferrocenium couple was observed at E0(
DEp) = 0.48 V (100 mV) un-
der our experimental conditions.
Corresponding author. Tel.: +91 33 2668 0521; fax: +91 33 2668 2916.
E-mail address: shch20@hotmail.com (S.K. Chattopadhyay).
0020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.12.018

S. Mondal et al. / Inorganica Chimica Acta 398 (2013) 98–105
99
O
fused calcium chloride. The filtrate was allowed to evaporate
slowly at room temperature when greenish red colored square
shaped shiny crystals of complex [Cu(L1)2] (1), suitable for X-ray
diffraction studies, appeared after 4 days.
N
H
2
H
NH
H2
Y
Y
X
+
N
R
The other three mononuclear Cu(II) complexes [Cu(L2)2] (2),
[Cu(L3)2] (3), [Cu(L4)2] (4) were synthesized using a similar proce-
dure as that for 1, using appropriate ligands (Scheme 2).
[Cu(L1)2] (1): Yield: 230 mg (42%). Anal. Calc. for C26H20N6O4Cu:
C, 57.40; H, 3.71; N, 15.45. Found: C, 57.35; H, 3.65; N, 15.47%.
O
N
CH3OH Solvent
Stirring, r.t
e
max/Mꢁ1 cmꢁ1):
X
Electronic spectrum in DMF solution kmax/nm (
440(2400), 380(3744), 272(2625). Selected IR bands (cmꢁ1):
3440br( OH), 1599( C@N).
H
N
N
m
m
O
R
[Cu(L2)2] (2): Yield: 210 mg (41%). Anal. Calc. for C26H20N6O2Cu:
C, 60.99; H, 3.94; N, 16.42. Found: C, 60.94; H, 3.88; N, 16.48%.
Electronic spectrum in DMF solution kmax/nm (
708(86), 382(2323), 296(1780), 270(1882). Selected IR band
(cmꢁ1): 1595(
C@N).
e
max/Mꢁ1 cmꢁ1):
X
Y
R
Ligand
L1H
L2H
L3H
L4H
L5H
L6H
L7H
L8H
CH
CH
CH
N
H
OH
H
m
H
H
[Cu(L3)2] (3): Yield: 224 mg (41%). Anal. Calc. for C26H22N8O2Cu:
C, 57.62; H, 4.09; N, 20.68. Found: C, 57.55; H, 3.9; N, 20.72%. Elec-
NH2
H
tronic spectrum in DMF solution kmax/nm
417(2555), 380(2525), 356(2040), 272(2558). Selected IR band
(cmꢁ1): 1605(
C@N).
(e
max/Mꢁ1 cmꢁ1):
H
m
CH
CH
CH
N
CH3
CH3
CH3
CH3
OH
H
[Cu(L4)2] (4): Yield: 254 mg (49%). Anal. Calc. for C24H18N8O2Cu:
C, 56.08; H, 3.50; N, 21.81. Found: C, 56.01; H, 3.43; N, 21.86%.
Electronic spectrum in DMF solution kmax/nm (
702(66), 379(2175), 290(1334), 272(1448). Selected IR band
(cmꢁ1): 1573(
C@N).
e
max/Mꢁ1 cmꢁ1):
NH2
H
m
Scheme 1. Ligands used in this work.
2.2.2. Binuclear Cu (II) complexes
[Cu2(L5)2( -Cl)2] (5) To a vigorously stirred methanolic solution
2.2. Synthesis of the complexes
l
(15 ml) of the hydrazone ligand (L5H) (510 mg, 2 mmol) containing
Et3N (202 mg, 2 mmol) a solution of Cu(ClO4)2ꢀ6H2O (369 mg,
1 mmol) in methanol (10 ml), was added drop wise. The solution
turned light yellow to deep green. The mixture was stirred at room
temperature for three hours and then filtered. The green residue
was washed in methanol and then dried over fused calcium chlo-
ride. The filtrate was allowed to evaporate slowly at room temper-
ature, when green colored square shaped shiny crystals of complex
5, suitable for X-ray diffraction studies, appeared after 4 days. The
2.2.1. Mononuclear Cu(II) complexes
[Cu(L1)2] (1): To a vigorously stirred methanolic solution (15 ml)
of the hydrazone ligand (L1H) (482 mg, 2 mmol), Et3N (202 mg,
2 mmol) was added, when a clear yellow solution was obtained.
A solution of Cu(ClO4)2ꢀ6H2O (369 mg, 1 mmol) in methanol
(10 ml) was added drop wise to the above solution. The solution
turned deep yellow to greenish brown. The mixture was stirred
at room temperature for three hours and then filtered. The yel-
low-green residue was washed with methanol and then dried over
other three Cu (II) complexes [Cu2(L6)2(l-Cl)2] (6), [Cu2(L7)2(l-Cl)2]
H
X
N
N
Cu(ClO4)2.6H2O
Et3N,Stirring,r.t
O
N
R
Cu
N
O
N
(ligand:Cu2+ :Et3N)
2
:1
:2
N
X
Y
X
H
R
H
N
N
R
O
X
N
(ligand:Cu2+ :Et3N)
O
H3C
N
Cl
Cl
R
2
:1
:2
N
Cu
Cu
N
N
Cu(ClO4)2.6H2O
Et3N,Stirring,r.t
N
N
CH3
O
R
X
Scheme 2. Schematic representation of the synthesis of the mono- and binuclear complexes.

100
S. Mondal et al. / Inorganica Chimica Acta 398 (2013) 98–105
(7), [Cu2(L8)2(l-Cl)2] (8) were synthesized using similar procedure
displacement parameters. A summary of the crystallographic data
is presented in Table 1.
as that for 5, using appropriate ligands (Scheme 2).
[Cu2(L5)2( -Cl)2] (5): Yield: 343 mg (48%). Anal. Calc. for
28H24N6Cl2O4Cu2: C, 47.60; H, 3.42; N, 11.90. Found: C, 47.2; H,
3.32; N, 11.35%. Electronic spectrum in DMSO solution kmax/nm
max/Mꢁ1 cmꢁ1): 950(157), 818(159), 740(166), 610(151),
367(1492). Selected IR bands (cmꢁ1): 3443br(
OH), 1594( C@N).
[Cu2(L6)2( -Cl)2] (6): Yield: 321 mg (48%). Anal. Calc. for
28H24N6Cl2O2Cu2: C, 49.85; H, 3.59; N, 12.46. Found: C, 49.79;
H, 3.51; N, 12.52%. Electronic spectrum in DMSO solution kmax
nm(
max/Mꢁ1 cmꢁ1): 709(63), 378(2338), 360(1938)sh, 290(1242).
Selected IR band (cmꢁ1): 1590(
C@N).
[Cu2(L7)2( -Cl)2] (7): Yield: 336 mg (48%). Anal. Calc. for
C28H26N8Cl2O2Cu2: C, 47.72; H, 3.72; N, 15.90. Found: C, 47.60;
H, 3.61; N, 15.99%. Electronic spectrum in DMSO solution kmax
nm (
max/Mꢁ1 cmꢁ1): 686(124), 398(2667)sh, 290(1608). Selected
IR bands (cmꢁ1): 1610(
C@N).
[Cu2(L8)2( -Cl)2] (8): Yield: 357 mg (53%). Anal. Calc. for
C26H22N8 O2Cl2Cu2: C, 46.15; H, 3.28; N, 16.56. Found: C, 46.19;
H, 3.35; N, 16.50%. Electronic spectrum in DMSO solution kmax
nm (
max/Mꢁ1 cmꢁ1): 732(102), 379(2399), 290(1253). Selected
IR band (cmꢁ1): 1609(
C@N).
l
C
3. Results and discussion
(e
m
m
3.1. Synthesis
l
C
The Cu(II) complexes reported here were obtained by reaction
of Cu(ClO4)2ꢀ6H2O with the appropriate ligand, in presence of Et3N
in 1:2:2 molar ratio. For the pyridine-2-carbaldehyde Schiff base
ligands (L1H, L2H, L3H, L4H), mononuclear complexes, containing
two tridentate monoanionic ligands were obtained. Surprisingly,
under similar conditions, the Schiff bases of 2-acetylpyridine
(L5H, L6H, L7H, L8H) reduced perchlorate to chloride and we got
chloro-bridged binuclear complexes, where each Cu(II) ion is coor-
dinated to only one monoanioinic tridentate ligand. Perchlorates
are known to be explosively reduced by organic compounds,
though the reaction rate is often sluggish at room temperature
[27]. However, in our present work we find that the 2-acetylpyri-
dine aroyl hydrazone ligands smoothly convert perchlorate to
chloride even at room temperature, whereas the corresponding
Schiff bases of pyridine-2-aldehyde cannot do so. Cyclic voltamme-
try experiments show (vide infra) that the salicyloyl hydrazone of
pyridine-2-aldehyde (L1H) and 2-acetylpyridine (L5H) undergo oxi-
dations at almost similar potential, in both protonated form as well
as deprotonated form (generated in situ by addition of tetrabutyl
ammonium hydroxide in the electrochemical cell) and thus there
is not much difference in their reductive power. In spite of this,
the fact that perchlorate is reduced to chloride by 2-acetylpyridine
hydrazones but not by pyridine-2-aldehyde hydrazones, probably
indicates a favorable kinetic factor for the reduction of perchlorate
by 2-acetylpyridine hydrazones in presence of Cu2+. Semiemperical
ZINDO/1 calculations, performed using a Window based Hyper-
chem program [28], using default parameters of the program, sug-
gest that for both pyridine-2-aldehyde as well as 2-acetylpyridine
hydrazone ligands the bis(hydrazone) octahedral Cu(II) complexes
like 1–4 are much less stable than the binuclear complexes of the
type 5–8 (see Table S1 in Supplementary material). However, Pal
et al. have shown [29,30] that for pyridine-2-aldehyde aroyl hydra-
zone ligands, both mononuclear bis-(hydrazone) or dichloro-
bridged binuclear Cu(II) complexes can be obtained, by varying
metal: ligand stoichiometry from 1:2 to 1:1. Thus the formation
of the bis(hydrazone)Cu(II) complexes seemed to be kinetically
governed and it is the major product when metal: ligand stoichi-
ometry is 1:2 or more. So, it was thought, that if Cu(ClO4)2ꢀ6H2O
is reacted with excess of a 2-acetylpyridine hydrazone ligand, we
may get the corresponding bis(hydrazone) complex similar to 1–
4, as even after a part of the ligand is used up for reduction of per-
chlorate to chloride, still sufficient ligand will be present in the
reaction medium for formation of the bis(hydrazone) complex. In
fact, it was found that when Cu(ClO4)2ꢀ6H2O was reacted with
L5H and Et3N in 1:3:3 molar ratio we get the complex [Cu(L5)2];
the same product was also obtained by reaction of CuCl2ꢀ2H2O or
CuBr2ꢀ2H2O with L5H and Et3N in 1:2:2 molar ratio. Unfortunately,
we could not get single crystals of any bis-(hydrazone) complex of
2-acetylpyridine hydrazone ligands, so we have reported charac-
terization data of only the [Cu(L5)2] complex in the supplementary
material. Dang et al. were able to get single crystals of the same
compound by diffusion of solution of L5H (0.1 mmol) and Et3N
(0.1 mmol) in methanol to a solution of CuCl2ꢀ2H2O (0.1 mmol)
in DMF [31]; however, except for X-ray single crystal structure
no other characterization data was reported for the compound.
Koo et al. isolated the complex [Cu(L6)2] by reaction of Cu(OAc)2-
ꢀH2O with L6H in 1:2 molar ratio, but they also could not obtain sin-
gle crystals of the complex [32]. Sastry and Rao reported reaction
/
e
m
l
/
e
m
l
/
e
m
2.3. X-ray crystallography
For 1 data were collected on a Bruker smart CCD area diffrac-
tometer at 100(2) K, using Mo K radiation (0.71073 Å). Data were
corrected for absorption effects using the numerical method
SADABS). The ratio of minimum to maximum apparent transmission
a
(
was 0.822. For 5 data were collected from a single crystal at room
temperature on an APEX II CCD detector system using a Bruker FR-
591 rotating copper radiation source with X-ray mirrors. An entire
hemisphere of data was collected in multi-run mode with both phi
and omega as the rotation axis. The APEX2 software package (Bruker
AXS) was used for collecting data frames, indexing of reflections,
and determination of lattice parameters. The data was solved by
direct methods and refined by full matrix least squares on F2 using
SHELX-97 [26]. The non-hydrogen atoms were refined with aniso-
tropic displacement parameters. All hydrogen atoms were placed
at calculated positions and refined as riding atoms using isotropic
Table 1
Crystal data and refinement details of compounds 1 and 5.
Compounds
1
5
Formula
C
26H20CuN6O4
C28H24Cl2Cu2N6O4
706.51
Formula weight
Crystal size (mm)
Temperature (K)
Crystal system
Space group
a (Å)
544.02
0.15 ꢂ 0.34 ꢂ 0.35 0.1 ꢂ 0.06 ꢂ 0.03
100(2)
296(2)
orthorhombic
Aba2
12.57600(10)
17.82970(10)
10.76320(10)
90
90
90
1.497
0.951
triclinic
ꢀ
P1
8.3000(4)
9.1116(4)
10.8269(5)
71.369(2)
74.335(2)
68.143(2)
1.654
b (Å)
c (Å)
a
(°)
b (°)
(°)
dcalc (g cmꢁ3
(mmꢁ1
F(000)
c
)
l
)
3.98
358
1116
Total reflections
Unique reflections
Observed data [I > 2
Rint
41115
2564
2501
0.0340
11597
2394
2280
0.0392
r
(I)]
R1, wR2 (all data)
0.0288, 0.0882
2564/1/197
0.836
0.0349, 0.0951
2394/0/192
1.062
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
Largest difference in peak and hole 1.455, ꢁ0.584
0.392, ꢁ0.560
(e Åꢁ3
)

S. Mondal et al. / Inorganica Chimica Acta 398 (2013) 98–105
101
Fig. 1. ORTEP diagram for 1. H-atoms are omitted for clarity (Atoms with label ’A’ refers to symmetry code i).
Table 2
of CuCl2ꢀ2H2O with L8H in presence of KOH in 1:1:1 molar ratio,
Selected bond distances (Å) and bond angles (°) for 1 and 5.
but they probably wrongly formulated the compound as [Cu(L8-
H)Cl2(H2O)] [33]. Synthesis and X-ray crystal structures of com-
plexes 2 and 4 have been published by two groups in each case,
using Cu(OAc)2ꢀH2O or Cu(ClO4)2ꢀ6H2O as starting materials
[29,34,9,17]. However the detailed electrochemical study over
both positive and negative sides of the reference electrode were
lacking in these reports. Nardelli and co-workers reported crystal
structure of [Cu(L1)(L1H)]ClO4ꢀEtOH, prepared by reaction of
Cu(ClO4)2ꢀ6H2O with L1H in 1:2 molar ratio, without addition of
any base [35]. Similarly, the synthesis and X-ray crystal structure
of 6, obtained by reacting CuCl2ꢀ2H2O and L6H in 1:1 molar propor-
tion was reported [25], however, the electrochemical data reported
in the paper needs reexamination, as it appears, from the cyclic
voltammogram given in the paper, that it has been obtained
starting from a negative potential of ꢁ1.0 V, where the reduced
complex may undergo appreciable decomposition and these sec-
ondary products may be responsible for some of the electrochem-
ical responses reported therein.
1
5
Cu1–N2
Cu1–N1
Cu1–O2
1.9889(15)
2.1054(16)
2.1017(16)
Cu1–N2
Cu1–N1
Cu1–O1
Cu1–Cl1
N3–C8
1.9352 (19)
2.0102 (19)
1.9722 (17)
2.2505 (6)
1.337 (3)
N2–C6
N3–C7
O2–C7
O1–C9
N2–N3
N2i–Cu1–N2
N2–Cu1–O2
N2–Cu1–O2i
O2–Cu1–N1i
N2i–Cu1–N1
O2–Cu1–N1
N1i–Cu1–N1
O2–Cu1–O2i
N2–Cu1–N1
1.285(3)
1.345(2)
1.277(2)
1.351(2)
1.367(2)
N2–C6
1.290 (3)
O1–C8
O2–C10
N2–N3
1.277 (3)
1.343(4)
1.367 (3)
173.48(10)
76.69(6)
98.65(6)
96.90(6)
106.59(6)
154.67(6)
86.27(9)
90.90(9)
78.32(6)
N2–Cu1–O1
N2–Cu1–N1
O1–Cu1–N1
N2–Cu1–Cl1
O1–Cu1–Cl1
N1–Cu1–Cl1
N2–Cu1–Cl1ii
O1–Cu1–Cl1ii
N1–Cu1–Cl1ii
Cl1–Cu1–Cl1ii
Cu1–Cl1–Cu1ii
79.45 (7)
80.22 (8)
159.51 (8)
164.01 (6)
99.25 (5)
99.59 (6)
103.61 (6)
95.48 (5)
91.71 (5)
92.38 (2)
87.62 (2)
3.2. Description of X-ray crystal structure
Symmetry code: i = ꢁx + 2, ꢁy + 1, z; ii = ꢁx + 1, ꢁy, ꢁz + 1.
The ORTEP diagram along with atom numbering scheme for 1 is
given in Fig. 1, while important bond distances and angles for 1 as
well as 5 are collected in Table 2. In the structure of 1, Cu(II) is in a
distorted octahedral coordination environment, being coordinated
by two monoanioinic tridentate ligands through pyridine nitrogen,
azomethine nitrogen and enolate oxygen. The two ligands are
equivalent and related by a C2 axis passing through the copper cen-
ter. The metal–ligand bond distances follow the order Cu–N(azo-
methine) < Cu–O ꢃ Cu–N(py), which is similar to that observed
for the Ni(II) complex of the same ligand, which follow the order
Ni–N(azomethine) < Ni–O < Ni–N(py) [36]. The ligands are planar,
with the dihedral angle between two five membered chelate rings
formed by the same ligand being 2.99°, while the dihedral angles
between the chelate rings formed by two different ligands vary be-
tween 83.51° and 86.06°.The N(2)–N(3) (1.367(2) Å), C(7)–N(3)
(1.345(2) Å) and C(7)–O(2) (1.277(2) Å) distances bear testimony
to the delocalization over the deprotonated hydrazone backbone,
which leads to the planarity of the ligand. The Cu(II) center may
be considered to be in an axially compressed (along N(2)–Cu(1)–
N(2A)) Jahn–Teller distorted octahedral geometry. There is intra-
molecular hydrogen bonding between the salicyloyl –OH moiety
and the N(3) atom (O(1)–H(14)ꢀ ꢀ ꢀN(3)) of a given ligand.
A comparison of the structural parameters of the complex 1
with those of 2, 4 and [Cu(L1)(L1H)]ClO4 EtOH, as well as with that
of [Cu(L5)2], reported earlier, is given in Table 3. It is interesting to
note that while in the structure of 1 both the ligands are equivalent
and related by a C2 axis passing through the copper center, similar
to that found in [Cu(L5)2], in the structures of 2 and 4 the two

102
S. Mondal et al. / Inorganica Chimica Acta 398 (2013) 98–105
Table 3
Comparisons of the structural parameters of 1 with those of related compounds.
1
2 [29,34]
4 [9,17]
Cu(L5)2
[31]
[Cu(L1)(HL1)] ClO4 [35]
Cu–NPy (Å)
Cu–Nimine (Å)
Cu–O (Å)
2.1054(16) 2.130(3); 2.286(3) average = 2.208(3)
1.9889(15) 1.940(3); 1.972(3) average = 1.956(3)
2.1017(16) 2.099(2); 2.294(3) average = 2.196(3)
2.180(6); 2.234(7)
average = 2.207(7)
1.949(6); 1.956(6)
average = 1.952(6)
2.124(5); 2.173(5)
average = 2.148(5)
153.36(12); 148.58(11)
2.156(3)
1.966(3)
2.158(3)
152.95(10)
172.1(2)
2.277(4); 2.081(3)
average = 2.179(4)
2.043(4); 1.930(4)
average = 1.986(4)
2.334(4); 2.062(3)
average = 2.198(3)
NPy–Cu–O (°)
154.67(6)
155.01(10); 150.11(9)
average = 152.56(10)
Nimine–Cu–Nimine (°)
173.48(10) 172.20(11)
176.6(2)
Fig. 2. ORTEP diagram for 5 (Atoms with label ’a’ refers to symmetry code ii).
ligands were inequivalent and the Cu(II) were in an axially elon-
gated octahedral geometry, with the longest Cu–N(py) and Cu–O
distances being 2.286(3) and 2.294(3) Å, respectively for 2, while
for 4 the corresponding values are 2.234(7) and 2.173(5) Å, respec-
tively. However, the average Cu–N(imine) distance at 1.956(3) Å
for 2 and 1.952(6) Å for 4 are slightly shorter than that observed
for complex 1 (1.9889(15) Å).
The molecular structure and atom numbering scheme for the
complex 5 is shown in Fig. 2. It turned out to be a binuclear Cu(II)
complex, with two chloride ions bridging the Cu(II) centers in an
axial–equatorial manner. Each Cu(II) is in a square pyramidal envi-
unit are related by a center of inversion passing through the middle
of [Cu2( -Cl)2] rectangle. Expectedly the equatorial Cu–Cl distance
l
at 2.2505(6) Å is much shorter than the corresponding axial Cu–Cl
bond length at 2.6686(6) Å. The metal ligand distances involving
the tridentate hydrazone ligand follow the same trend as observed
for the mononuclear complex 1. However, each of the individual
bond distance in the binuclear complex is much shorter than the
corresponding bond distance in the mononuclear complex, proba-
bly due to lower coordination number of Cu(II) in the former. There
are weak Cuꢀ ꢀ ꢀN(3)iii complementary interactions (symmetry ele-
ment iii = 2 ꢁ x, ꢁy, 1 ꢁ z, Cuꢀ ꢀ ꢀN(3)iii distance being 3.626(2) Å),
which leads to an infinite one dimensional chain along the [100]
base vector. Like the mononuclear complex 1, in 5 also there are
intramolecular hydrogen bonding between the salicyloyl –OH moi-
ety and the N(3) atom (O(2)–H(2a)ꢀ ꢀ ꢀN(3)).
ronment (s = 0.08) [37], coordinated by pyridine nitrogen (N(1)),
imine nitrogen (N(2)) and enolate oxygen (O(1) of a deprotonated
tridentate mono-anionic hydrazone ligand, along with a chloride
ion (Cl(1)), forming the equatorial plane, whereas another chloride
(Cl(1a)) related by symmetry element 1 ꢁ x, ꢁy, 1 ꢁ z (symmetry
element ii), is coordinated axially. The two halves of the binuclear
A comparison of the structural parameters of 5 with those of
similar binuclear Cu(II) complexes are given in Table 4. Among

S. Mondal et al. / Inorganica Chimica Acta 398 (2013) 98–105
103
Table 4
Comparisons of structural parameters of 5 with those of related compounds.
5
6 [25]
Cu2(l
-Cl)2(L2)2 [30]
[Cu(
l
1,3-NCS)(L1)2]n [38]
Cu2(
l1,1-OAc)2(L2)2 [30]
Cu2(l1,1-N3)2(L2)2 [34]
Cu–NPy (Å)
Cu–Nimine (Å)
Cu–O (Å)
Cu–Xeq (Å)
Cu–Xax (Å)
Cu–X–Cu
1.9722(17)
1.9352(19)
2.0102(19)
2.2505(6) (X = Cl)
2.6686(6) (X = Cl)
87.62(2)
2.013(2)
1.928(2)
1.964(2)
2.2603(7) (X = Cl)
2.6434(7) (X = Cl)
86.80(2) (X = Cl)
93.20(2) (X = Cl)
2.028(2)
1.9299(19)
1.9853(17)
2.2440(9) (X = Cl)
2.6682(10) (X = Cl)
86.89(3) (X = Cl)
93.11(3)
2.038(3)
1.925(4)
1.990(3)
1.930(5) (Cu–Neq, X = NCS)
2.709(2) (Cu–Sax, X = NCS)
–
–
2.033(6)
1.924(6)
1.989(5)
1.935(5)
2.286(6) (X = OAc)
101.4(2) (X =
78.6(2) (X =
2.007(6); 2.022(6)
1.929(6); 1.935(6)
1.978(4); 1.965(4)
2.460(5); 1.970(6)
2.460(5); 2.386(5)
92.2(2), 94.8(2)
l
1,1-OAc)
X
eq–Cu–Xax
92.38(2)
l1,1-OAc)
85.6(2), 87.4(2)
200
150
100
50
4000
3500
3000
2500
2000
1500
1000
500
1
0
400
500
600
700
800
900
1000
1100
1200
Wavelength(nm)
2
4
3
0
300
400
500
600
Wavelength(nm)
Fig. 3. Electronic spectra of the mononuclear complexes. The d–d bands are shown in the inset. Color codes are same for both the graphs.
3000
250
2500
200
5
2000
1500
1000
500
0
7
150
100
50
8
6
0
500
600
700
800
900
1000
Wavelength (nm)
300
350
400
450
500
wavelength (nm)
Fig. 4. Electronic spectra of the binuclear complexes. The d–d bands are shown in the inset. The color codes are same in both the graphs.
the complexes whose structural data are given Table 4, all have
center of inversion at the middle of [Cu2( -X)2] rectangle, except
Cu2( 1,1-N3)2(L2)2, where the bridge is asymmetric [35]. It is seen
from Table 4 that in 5 the Cu–Npy distance is shortest while the
Cu–O and Cu–Nimine distances are slightly longer compared to
the other structures. The equatorial Cu-X distance is expectedly
l
l

104
S. Mondal et al. / Inorganica Chimica Acta 398 (2013) 98–105
Table 5
Electrochemical data for the Cu (II) complexes.
Compounds E1/2OX/V
E/mV)
E1/2red/V (
DE/mV)
(D
1
2
3
0.93a,b
1.20a,b
1.16a,b
ꢁ0.98a,b
ꢁ0.75c,d, ꢁ1.58c,d, ꢁ1.8c,d
ꢁ0.64(385)d, ꢁ1.37a,d, ꢁ1.84(120)dꢁ0.85b,c
ꢁ1.20b,c, ꢁ1.70b,c
,
ꢁ0.85a,b,ꢁ1.29a,b
ꢁ0.30(350)d,ꢁ0.90a,d, ꢁ1.56a,d
ꢁ0.20a,c, ꢁ0.75a,c, ꢁ1.55a,c,ꢁ1.80a,c
ꢁ0.54(179)d, ꢁ0.80a,d, ꢁ1.75(100)d
ꢁ0.50c,d, ꢁ0.75c,d,ꢁ1.7c,d, ꢁ1.95c,d
ꢁ0.57(250)b
4
5
6
1.25a,b
1.06b,c
1.04a,b
ꢁ0.30b,c, ꢁ0.55b,c
ꢁ0.22(270)b, ꢁ0.88(Ep.c)b,ꢁ0.78(Ep.a)b, ꢁ0.48
b
(Ep.a
)
ꢁ0.30a,c, ꢁ0.80a,c
7
8
1.08a,b
1.03a,b
ꢁ0.22 (220)b,ꢁ0.56a,b, ꢁ0.86a,b
ꢁ0.30b,c, ꢁ0.77b,c
ꢁ0.12 (150)b, ꢁ0.55(100)b
ꢁ0.10a,c, ꢁ0.30a,c, ꢁ0.50a,c, ꢁ0.80a,c
a
b
c
Fig. 6. Cyclic voltammogram of 4 at scan rate of 100 mV/s at a glassy carbon
electrode.
Irreversible response.
Pt-electrode.
Response obtained from DPV.
d
Glassy carbon working electrode.
Fig. 7. Cyclic voltammogram of 8 at scan rate of 100 mV/s at a Pt-electrode. The
inset shows the second reductive couple only.
Fig. 5. Cyclic voltammogram of 2 at scan rate of 100 mV/s at a glassy carbon
electrode.
3.4. Electronic spectra
The mononuclear complexes 2 and 4 and the binuclear com-
plexes 6 and 7 show one weak, broad band at 680–750 nm (Figs. 3
and 4). The binuclear complex 5 shows number of weak shoulders/
broad bands at 600–950 nm region, while the complex 8 shows
two overlapping broad and weak transitions at 677 and 732 nm.
These bands are typical of d–d band of Jahn–Teller distorted Cu(II)
complexes in distorted octahedral or square pyramidal geometry.
One or two strong bands at 380–440 nm for the mononuclear com-
plexes and 370–400 nm for the binuclear complexes are assigned
to ligand to metal charge transfer transitions. Bands at higher en-
ergy are due to intraligand transitions. Our spectral data is consis-
tent with those reported by earlier workers [17,29,30,32].
larger for X = Cl, compared to X = NCS, OAc or N3, whereas the axial
Cu–X distance follow the order Cu–SCN > Cu–Cl > Cu–N (azide) > -
Cu–OAc.
3.3. Infrared spectra
In the infrared spectra the disappearance of characteristic m(N–
H) band of free hydrazone ligand at 3190–3280 cmꢁ1 region, along
with bathochromic shift of the amide-I band from 1630–
1660 cmꢁ1 in the ligands to 1590–1610 cmꢁ1 in the complexes
suggest coordination via enol form, by deprotonation of the N–H
proton, during the complexation process. The amide-II vibration
at 1550–1570 cmꢁ1 of the free ligands shifts to 1480–1500 cmꢁ1
in the complexes. The azomethine (
m
C@N) band is often buried un-
3.5. Electrochemical studies
der the amide-II band in the ligands and amide-I band in the com-
plexes. The phenolic OH group of the free ligands (L1H and L5H)
appears as a broad band around 3470 cmꢁ1 region, which in the
The electrochemical data for the eight complexes are summa-
rized in Table 5. The free ligands in their deprotonated form were
found to show an irreversible oxidation at 0.8 V and a quasi-revers-
corresponding complexes is observed at around 3440 cmꢁ1
.

S. Mondal et al. / Inorganica Chimica Acta 398 (2013) 98–105
105
ible reductive response at ꢁ0.8 V (Supplementary Figs. S1 and S2).
The mononuclear complexes show a ligand based irreversible oxi-
dation at 0.9–1.2 V, for the binuclear complexes a similar irrevers-
ible oxidative response at 1.03–1.08 V region were observed. On
the negative side of Ag/AgCl electrode, the mononuclear complexes
show a quasi-reversible/irreversible response at ꢁ0.5 to ꢁ0.9 V,
which sometimes (e.g. for 3 and 4) resolves into two overlapping
peaks (Figs. 5 and 6 and Supplementary Figs. S3–S5). We tenta-
tary data associated with this article can be found, in the online
version, at http://dx.doi.org/10.1016/j.ica.2012.12.018.
References
[1] E. Tsuchida, K. Oyaizu, Coord. Chem. Rev. 237 (2003) 213.
[2] L. Canali, D.C. Sherrington, Chem. Soc. Rev. 28 (1999) 85.
[3] J. Tisato, F. Refosco, F. Bandoli, Coord. Chem. Rev. 135 (1994) 325.
[4] J.-M. Lehn, Supramolecular Chemistry, VCH, Weinheim, 1995. p113.
[5] R.E.P. Winpenny, Chem. Soc. Rev. (1998) 447.
[6] M.S. Shongwe, S.H. Al-Rahbi, M.A. Al-Azani, A.A. Al-Muharbi, F. Al-Mjeni, D.
Matoga, A. Gismelseed, I.A. Al-Omari, A. Yousif, H. Adams, M.J. Morris, M.
Mikuriya, A. Gismelseed, I.A. Al-Omari, A. Yousif, H. Adams, M.J. Morris, M.
Mikuriya, Dalton Trans. (2012) 2500–2514.
[7] P. Krsihnamoorthy, P. Sthyadevi, K. Senthilkumar, P.T. Muthiah, R. Ramesh, N.
Dharmaraj, Inorg. Chem. Commun. 14 (2011) 1318–1322.
[8] Q. Yu, L.G. Zhu, H.D. Bian, J.H. Deng, X.G. Bao, H. Liang, Inorg. Chem. Commun.
10 (2007) 437.
[9] W.X. Ni, M. Li, S.Z. Zhan, J.Z. Hou, D. Li, Inorg. Chem. 48 (2009) 1483.
[10] S. Naskar, D. Mishra, S.K. Chattopadhyay, M. Corbella, A.J. Blake, Dalton Trans.
12 (2005) 2428–2435.
tively assign the response at ꢁ0.5 to ꢁ0.9 V to overlapping Cu2+
/
Cu+ and a ligand based reduction processes [30,36,39,40]. Two
more reductive waves at ꢁ1.3 to ꢁ1.5 V and ꢁ1.8 to ꢁ2.0 V (Figs. 5
and 6 and Supplementary Figs. S3–S5) are assigned to ligand based
reductions [15]. For the binuclear complexes, there are often two
irreversible/quasi-reversible reductions at ꢁ0.2 to ꢁ0.4 V and
ꢁ0.5 to ꢁ0.8 V (Fig. 7 and Supplementary Figs. S7–S11). The first
reduction is assigned to Cu2+/Cu+ couple [30,41], while the reduc-
tive response at more negative potential is due to combined ligand
centered and a second metal centered reductions.
[11] S. Naskar, M. Corbella, A.J. Blake, S.K. Chattopadhyay, Dalton Trans. (2007)
1150.
[12] S. Naskar, D. Mishra, R.J. Butcher, S.K. Chattopadhyay, Polyhedron 26 (2007)
3703.
[13] M. Prabhakar, P.S. Zacharias, S.K. Das, Inorg. Chem. 44 (2005) 2585.
[14] Z. He, C. He, E.-Q. Gao, Z.-M. Wang, X.-F. Yang, C.-S. Liao, C.-H. Yan, Inorg.
Chem. 42 (2003) 2206.
4. Conclusions
The tridentate aroyl hydrazone ligands reported here, obtained
by condensation of pyridine-2-carbaldehyde and 2-acetyl pyridine
with four aroyl hydrazides, were found to form mononuclear octa-
hedral or dichloro bridged binuclear Cu(II) complexes with square
pyramidal geometry. It is found that aroyl hydrazones of pyridine-
2-aldehyde and 2-acetyl pyridine have very similar reducing
power and it is probably the kinetic factor that is responsible for
only the later being able to reduce perchlorate to chloride at ambi-
ent conditions. Presence of excess ligand in the reaction medium
always leads to bis-(hydrazone)Cu(II) complex, in spite of the fact
that the mononuclear octahedral complexes are thermodynami-
cally much less stable than the binuclear complexes. Thus the for-
mation of binuclear complexes with 2-acetylpyridine aroyl
hydrazone ligands on reaction with Cu(ClO4)2ꢀ6H2O in 2:1 M ratio
is due to ligand deficiency in the reaction medium, caused by par-
tial oxidative degradation of the ligands by perchlorate.
[15] S. Naskar, S. Biswas, D. Mishra, B. Adhikary, L.R. Falvello, T. Soler, C.H.
Schwalbe, S.K. Chattopadhyay, Inorg. Chim. Acta 357 (2004) 4257.
[16] D. Mishra, S. Naskar, A.J. Blake, S.K. Chattopadhyay, Inorg. Chim. Acta 360
(2007) 2291.
[17] C.M. Armstrong, P.V. Bernhardt, P. Chin, D.R. Richardson, Eur. J. Inorg. Chem.
(2003) 1145.
[18] Y.X. Ma, Z.Q. Ma, G. Zhao, Y. Ma, M. Yan, Polyhedron 8 (1989) 2105.
[19] R.K. Agarwal, R.K. Sarin, R. Prasad, Pol. J. Chem. 67 (1993) 1947.
[20] M. Carcelli, C. Pelizzi, G. Pelizzi, P. Mazza, F. Zani, J. Organomet. Chem. 488
(1995) 55.
[21] Z.Y. Yang, R.D. Yang, K.B. Yu, Polyhedron 15 (1996) 3749.
[22] A.A.R. Despaigne, L.F. Vieira, I.C. Mendes, F.B. da Costa, N.L. Speziali, H. Beraldo,
J. Braz. Chem. Soc. (2010) 1247–1257.
[23] D. Demertzi, D. Nicholls, Inorg. Chim. Acta 73 (1983) 37.
[24] P. Pelagatti, M. Carcelli, C. Pelizzi, M. Costa, Inorg. Chim. Acta 342 (2003) 323.
[25] J. Patole, U. Sandbhor, S. Padhye, D.N. Deobagkar, C.E. Anson, A. Powell, Bioorg.
Med. Chem. Lett. 13 (2003) 51–55.
[26] G.M. Sheldrick, Acta Crystallogr. Sect. A 64 (2008) 112–122.
[27] N.N. Greenwood, A.E. Earnshaw, Chemistry of the Elements, second ed.,
Butterworth-Heinnemann, Oxford, 2005, p. 868.
[28] Hyperchem for Windows, Release 7.01, Hypercube Inc., 115 NW, 4th Street,
Gainesville, FL 32601.
[29] S. Pal, J. Pushparaju, N.R. Sangeetha, S. Pal, Transition Met. Chem. 25 (2000)
529.
Acknowledgments
[30] N.R. Sangeetha, S. Pal, Polyhedron 19 (2000) 1593.
[31] D.B. Dang, Y. Bai, C.Y. Duan, Acta Crystallogr., Sect. E 62 (2006) m1567.
[32] Y.J. Jang, U.K. Lee, B.K. Koo, Bull. Korean Chem. Soc. 26 (2005) 925.
[33] P.S.J. Sastry, T.R. Rao, Proc. Indian. Acad. Sci. (Chem. Sci.) 107 (1995) 25–33.
[34] S. Banerjee, S. Mondal, W. Chakraborty, S. Sen, R. Gachhui, R.J. Butcher, A.M.Z.
Slawin, C. Mandal, S. Mitra, Polyhedron 28 (2009) 2785–2793.
[35] P. Domiano, A. Musatti, M. Nardelli, C. Pelizzi, G. Predieri, J. Chem. Soc., Dalton
Trans. (1979) 1266–1269.
Satayajit Mondal thanks CSIR for a NET JRF. S.K.C. acknowledges
AICTE for funding the purchase of CH1120A potentiostat. Infra-
structural facility created in our department through DST-FIST,
UGC-SAP and MHRD special grants are also thankfully acknowl-
edged. Steven R. Herron gratefully acknowledges the use of the
X-ray facility at Brigham Young University for determining the X-
ray crystal structure of 5.
[36] B. Samanta, J. Chakraborty, S. Shit, S.R. Batten, P. Jensen, J.D. Masuda, S. Mitra,
Inorg. Chim. Acta 360 (2007) 2471–2484.
[37] A.W. Addison, T.N. Rao, J. Reedijk, J. van Rijn, G.C. Verschoor, J. Chem. Soc.,
Dalton Trans. (1984) 1349–1356.
[38] P. Domiano, A. Musatti, M. Nardelli, C. Pelizzi, G. Predieri, J. Chem. Soc., Dalton
Trans. (1975) 2357–2360.
Appendix A. Supplementary material
[39] Z. Lu, W. Xiao, B.S. Kang, C.Y. Su, J. Liu, J. Mol. Struct. 523 (2000) 133.
[40] D. Matoga, J. Szklarzewicz, W. Nitek, Polyhedron 36 (2012) 120.
[41] S. Shit, S.K. Dey, C. Rizzoli, E. Zangrando, G. Pilet, C.J.G. Garcia, S. Mitra, Inorg.
Chim. Acta 370 (2011) 18–26.
CCDC 870577 and 870578 contain the supplementary crystallo-
graphic data for compounds 5 and 1, respectively. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplemen-