Synthesis, crystal structures and electrochemical characterization of dinuclear paddlewheel copper(II) carboxylates

Article abstract of DOI:10.1016/j.poly.2013.04.020

Two new dinuclear copper(II) complexes 1 and 2, viz., [pyCu(μ ₂-OOCCH₂C₆H₄R)₄Cupy] where R = para-NO₂(1) and para-CH₃(2) and py = pyridine, have been synthesized and characterize

Full text of DOI:10.1016/j.poly.2013.04.020

Polyhedron 57 (2013) 83–93
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, crystal structures and electrochemical characterization of dinuclear
paddlewheel copper(II) carboxylates
b
c
Muhammad Iqbal a, Saqib Ali a, , Niaz Muhammad , Manzar Sohail
⇑
a Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan
b Department of Chemistry, Abdul Wali Khan University, Mardan, KPK, Pakistan
c Faculty of Science, Health, Education and Engineering, University of the Sunshine Coast, 90 Sippy Downs Drive, Sippy Downs, Queensland, 4556, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new dinuclear copper(II) complexes 1 and 2, viz., [pyCu(l2-OOCCH2C6H4R)4Cupy] where R = para-
Received 7 December 2012
Accepted 8 April 2013
Available online 19 April 2013
NO2(1) and para-CH3(2) and py = pyridine, have been synthesized and characterized using FT-IR, single
crystal XRD and electrochemical solution studies. In both complexes, the two copper centers are linked
by four carboxylate ligands in bridging bidentate bonding fashion. Each copper(II) ion is penta-coordi-
nated with distorted square pyramidal geometry. Both complexes have typical paddlewheel structures
with the apical positions occupied by pyridine molecules. The electrochemical behavior of the complexes
was investigated by cyclic voltammetry in which the complexes gave rise to metal centered single elec-
tron irreversible electro-activity, corresponding to Cu(II)/Cu(III) oxidation and Cu(II)/Cu(I) reduction pro-
cesses. A reduction wave in relatively higher negative potential range (ꢀ1.3 to ꢀ1.4 V) was observed for
Keywords:
Dinuclear Cu(II) complexes
Crystal structures
Electrochemistry
Heterogeneous kinetics
the nitro group of complex 1. From the slope value of log i versus ꢀlog
v, the redox processes were found
predominantly diffusion controlled. The order of diffusion coefficient and heterogeneous rate constant for
oxidation is: 1 > 2 owing to the more stable redox products in case of 1 while for reduction the order is:
2 > 1 due to overriding solvent effects and the larger molecular mass of 1 compared to 2. The charge
transfer coefficient was also calculated and found typical of the irreversible redox processes.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
gory needs ligands having relatively longer organic arms between
the donor sites in their structures [7,8]. In the latter case, some of
Copper carboxylates are interesting in their ease of synthesis,
structural diversity and diverse applications. The diversity in struc-
ture comes partly from the variable donating ability of oxygen
atoms of the carboxylate moiety and partly from the nature of
the attached ligands to the central copper(II) ion [1–4]. A paddle-
wheel or bridging dinuclear structure is resulted depending upon
the utilization of one or two lone electron pairs on oxygen of the
carboxylate group for bonding to copper(II) ion. The paddlewheel
the oxygen atoms of the bridging bidentate carboxylate groups of
the paddlewheel act as bridging bidentate atoms [9,10], bonding
simultaneously to two copper ions of a polymeric lattice. Thus
the properties of these two types of paddlewheel copper carboxyl-
ates will vary according to the nature of the attached ligands and
substituents on them [3,11,12].
The other geometry arising as a result of the versatile donor
ability of oxygen atoms of carboxylate moiety is
l2-O bridging
´
(l-O,O-carboxylate bridging) type arrangement is relatively more
square pyramidal. Here two carboxylate ligands are bonded to
two copper ions in an end on fashion through only one (out of
two) oxygen atom each. The oxygen atom bonding in this manner
is two electron pairs donor [5,6,13]. This type of structures also
contains monodentate carboxylate groups. One important conse-
symmetric compared to the bridging (carboxylato- 2-O) one and
l
the properties of the resulting central metal core of the complex
are less prone to variation in the ligand structure. However, varia-
tions from ideal paddlewheel structure do occur to allow some
special structural features such as polymeric behavior [5–7]. The
polymeric paddlewheel carboxylates may arise as a result of the li-
gands having trans donor sites in their structures, both suitable to
donate lone electron pairs to different copper(II) ions of the neigh-
boring ‘paddlewheels’ or through the multi-electrons donating
ability of oxygen of the carboxylate moiety itself. The former cate-
quence of the
l2-O bridging oxygen atoms in the copper carbox-
ylates is that the copper(II) ion becomes coordinatively less
saturated compared to the paddlewheel structure. Another prop-
erty owing probably to the above cited bonding nature of oxygen
atom is the absence of carboxylate bridging between two cop-
per(II) ions, conferring the important magnetic behavior of anti-
ferromagnetic coupling on the copper ions bonded in this way
[11,14]. The latter property may be due to the strain exerted by
the asymmetric coordination of the oxygen atoms bridging the
⇑
Corresponding author. Tel.: +92 51 90642130; fax: +92 51 90642241.
E-mail address: drsa54@yahoo.com (S. Ali).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.04.020

84
M. Iqbal et al. / Polyhedron 57 (2013) 83–93
two copper ions. Scheme 1 shows some most frequently encoun-
tered coordination modes of carboxylate ligands in metal com-
plexes. The relatively less saturated geometry around copper,
most commonly square pyramidal, has been found to possess
interesting applications in biological systems [12].
The apical ligand in both of the aforementioned groups plays
important decisive role in determining the final geometry of the
dinuclear or polynuclear copper(II) carboxylates. Owing to the rel-
ative rigidity of the structure resulting from the carboxylate li-
gands arrangement in a paddlewheel, the apical position is less
prone to variation and usually able to accommodate a single steri-
cally less demanding monodentate ligand. On the other hand, the
For complex 2 diffraction data were collected at 100(2) K on
beamline MX1 at the Australian Synchrotron (k = 0.71703 Å)
[24]. The data reduction and indexing of diffraction pattern was
performed by XDS software [25]. The crystal structures were
solved by direct method followed by refinement against F2 with
full-matrix least-squares using the program SHELXL-97 [26]. All
non-hydrogen atoms were refined with anisotropic displacement
parameters.
2.3. Electrochemistry
Voltammetric experiments were performed using an SP-300
potentiostate, serial number 0134, BioLogic Scientific Instruments,
France. Measurements were carried out in aqueous DMSO (1:4)
solution containing 0.01 M KCl, under an N2 saturated environ-
ment in a conventional three-electrode cell with saturated silver/
silver chloride electrode (Ag/AgCl) as reference, a Platinum wire
having diameter of 0.6 mm as counter and a bare glassy carbon
electrode (GCE) with a surface area of 0.196 cm2 as the working
electrode. Prior to experiment the GCE was polished with alumina
(Al2O3) on a nylon buffing pad followed by washing with acetone
and finally with distilled water. Electrochemical measurements
were carried out at room temperature (25 0.5 °C).
structure having
l2-O atoms offer room for the attachment of
bidentate as well as monodentate ligands during synthesis. In this
regard, square pyramidal complexes having quite symmetrical
square base have been synthesized using C2-symmetric ligands
´
such as 2,2-bipyridine and 1,10-phenanthroline [15,16]. The
resulting complexes of different geometries have been found to un-
dergo a great variation in their properties and the consequent
applications. Recently, the applicability of paddlewheel complexes
in the synthesis of useful metal organic frameworks (MOFs) [17,18]
and as a quantitative water absorber [19] has been highlighted. In
connection to our previous work on bridging paddlewheel com-
plexes [20], this paper is devoted to the synthesis, structural eluci-
dation and electrochemical study of the dinuclear paddlewheel
copper(II) carboxylates and to see whether any change is resulted
in structure-based behavior in the solution study of the complexes
compared to the bridging ones.
2.4. General procedure for the synthesis of complexes 1 and 2
Sodium bicarbonate (0.504 g, 6 mmol) was treated with equi-
molar quantity of the corresponding phenyl acetic acid derivatives
(1.086 g of 4-nitro phenyl acetic acid for complex 1 and 0.90 g of 4-
methyl phenyl acetic acid for 2, 6 mmol each) in distilled water at
60 °C.
2. Experimental
After the complete neutralization of the acid with base, the
aqueous solution (blue) of copper sulfate (0.478 g, 3 mmol for each
of the complexes 1 and 2) was added drop wise. The reaction mix-
ture was stirred for 3 h at 60 °C and then pyridine (0.24 ml,
3 mmol) was added to it with constant stirring. Stirring was con-
tinued for another 3 h under the same reaction conditions as de-
picted in Scheme 2. The final product (green reaction mixture)
was filtered, washed thoroughly with distilled water and air dried.
The solid was dissolved in a mixture of chloroform and methanol
(1:1) and green crystals, suitable for single crystal analysis were
obtained after a week. These were then characterized using FT-IR
and X-ray single crystal analysis.
2.1. Materials and methods
Anhydrous CuSO4, 4-nitro and 4-methylphenyl acetic acids,
pyridine and NaHCO3 were obtained from Fluka, USA. Solvents
were obtained from Merck, Germany and used as such without
drying and further purification. Water used was singly distilled.
The melting points were obtained in a capillary tube using a Gal-
lenkamp, serial number C040281, UK, electro-thermal melting
point apparatus. FT-IR spectra were recorded on a Nicolet-6700
FT-IR with ATR (attenuated total reflectance) spectrophotometer,
Thermoscientific, USA, in the range from 4000 to 400 cmꢀ1 with
number of scans = 32.
Complex 1. Green crystals; m.p. 185 °C; yield (80%). FT-IR
(cmꢀ1): 1625
m
(OCO)asym, 1398
(Ar–H), 1074 (Ar–N), 1510
(Cu–O), 463 (Cu–N).
Complex 2. Light blue crystals; m.p. 150 °C; yield (70%). FT-IR
(cmꢀ1): 1624
(OCO)asym, 1417 (OCO)sym = 207, 2950 CH2,
2921 CH3, 3032 (Ar–H), 1497 Ar(C@C), 415 (Cu–O), 465
(Cu–N).
m
(OCO)sym
,
D
m
= 227, 2967
m
CH2,
2.2. X-ray crystallographic studies
3071
422
m
m
mAr(C = C), 1431, 1341
m(NO2),
m
m
For complex 1 crystallographic data were collected at 293 K
using an Oxford Diffraction Gemini Ultra S CCD diffractometer
using graphite-monochromated Mo K
Data reduction and empirical absorption corrections were accom-
plished using CrysAlisPro (Oxford Diffraction version 171.33.66).
Structure of 1 was solved by direct methods with SHELXS-86 and re-
fined by full-matrix least-squares analysis against F2 with SHELXL-97
[21] within the WINGX package [22]. The drawings of the complexes
were produced using ORTEP3 [23].
m
m
,
D
m
m
a radiation (k = 0.71073 Å).
m
m
m
m
m
3. Results and discussion
3.1. FT-IR data
FT-IR spectra of the copper(II) complexes revealed all the
characteristic bands which were in accordance with the results
of the X-ray single crystal analyses. The carboxylate moiety
showed two bands in 1624–1625 and 1398–1417 cmꢀ1 regions
corresponding to the antisymmetric and symmetric COO stretch-
ing vibrations, respectively. This was further supported by the
appearance of Cu–O absorption band in 415–422 cmꢀ1 range
for both complexes which confirmed the coordination of the
R
R
R
R
C
C
C
C
O
O
O
O
O
O
O
O
Cu
II
Cu
I
Cu
Cu
Cu
Cu
III
IV
Scheme 1. Carboxylate coordination modes, asymmetric (I) and symmetric (II)
chelation, asymmetric bridging (III) and bidentate bridging (IV).
carboxylate ligand through oxygen. The values of Dm = {masym

M. Iqbal et al. / Polyhedron 57 (2013) 83–93
85
H2
C
H2
C
ONa
OH
NaHCO3
60oC
O
O
R
R
H2
C
ONa
O
R
60oC
CuSO4
3 h stirring, followed by the
addition of pyridine(py), again
followed by 3 h stirring
pyCu(RC6H4CH2COO)4Cupy
Scheme 2. Synthesis of complexes 1 and 2 where R = 4-NO2(1) and 4-CH3(2).
Fig. 1. ORTEP diagram of complex 1, [tetrakis(4-nitrophenolacetato-l-O,Ó)bis(pyridine-N)dicopper(II)].
(OCO) ꢀ
m
sym(OCO)} calculated for complexes 1 and 2 were 227
served in 1510–1497 and 3071–3032 cmꢀ1 regions, respectively.
The presence of nitro group in 1 was confirmed from the two in-
tense bands observed in 1438–1341 cmꢀ1 region. Methylene C–H
stretching was observed in 2967–2950 cmꢀ1 region which was
supported by the presence of bands in 707–695 and 1410–
1398 cmꢀ1 regions corresponding to its rocking and bending
deformations, respectively. Methyl C–H stretch gave rise to
absorption band at 2921 cmꢀ1 in complex 2, supported by the
band at 1446 cmꢀ1 assignable to the bending vibration of this
functionality.
and 207 cmꢀ1, respectively, indicating a bridging bidentate coor-
dination mode of carboxylate groups in both complexes [27]. In
addition, the appearance of C@N stretching band at 1600–
1599 cmꢀ1 instead of its normally observed characteristic region
(1625–1610 cmꢀ1) [28,29] indicated the involvement of nitrogen
of pyridine in bonding with the copper(II) ion [30]. This was fur-
ther supported by the appearance of a new medium intensity
band in the region 465–463 cmꢀ1, attributable to Cu–N vibration
[31]. The aromatic C@C and C–H stretching vibrations were ob-

86
M. Iqbal et al. / Polyhedron 57 (2013) 83–93
Fig. 2. ORTEP diagram of complex 2 [tetrakis(4-methylphenylaeato-l-O,Ó)bis(pyridine-N)dicopper(II)].
3.2. Single crystal X-ray analysis
(1.9705(18) (1) and 1.9735(16) Å (2)) are close to each other as
well as to those observed for the structurally related dinuclear
complexes of Cu(II) ions with trifluoroacetate ligands [Cu2(CF3
CO2)4(CH3CN)2] (Cu–Cu = 2.766(1) and Cu–O = 1.969(5) Å) [32].
The difference in Cu–Cu distances of 1 and 2 from that of the above
cited complex is attributable to the relatively higher basic strength
of the N-donor ligands in 1 and 2, resulting in slight decrease in
their Cu–O bond ionic character, compared to that in the latter
complex [19]. Thus the properties of Cu(II)-complexes, arising as
a result of the inter-copper(II) ions separation can be tuned by
changing the attached ligand and its donor atoms. The Cu–N dis-
tances (2.157(2) (1) and 2.1455(18) Å (2)) are comparable to those
Complexes 1 and 2
Single crystal analyses afforded the structures and the relevant
information of both the complexes presented in Figs. 1 and 2 and
Tables 1 and 2, respectively.
Both compounds display the classical paddlewheel structures
having four carboxylate ligands bonded in a syn–syn configuration,
bridging the two Cu(II) ions, while the pyridine molecules occupy
axial coordination sites resulting in a distorted square pyramidal
coordination geometry for each Cu(II) ion. The Cu–Cu distances
(2.6502(7) (1) and 2.6563(6) Å (2)) and Cu–O average distances
Table 1
Crystal data and structure refinement parameters for the complexes.
Complex
1
2
Chemical formula
Formula weight (g molꢀ1
Temperature (K)
Wavelength (Å)
Crystal system
Space group
C
42H34Cu2N6O16
C46H46Cu2N2O8
881.93
100(2)
0.71073
triclinic
)
1005.85
293(2)
0.71073
triclinic
ꢀ
ꢀ
P1
P1
Unit cell dimensions
a (Å)
b (Å)
8.1242(6)
9.9550(6)
13.8752(7)
85.367(5)
74.121(5)
10.3480(12)
10.9390(5)
11.3790(4)
78.681(4)
69.070(4)
61.996(5)
c (Å)
a
(°)
b (°)
(°)
c
76.464(6)
V (Å3)
1049.21(11)
1
1.592
1.096
514
1061.58(14)
1
1.380
1.057
458
Z
Dcalc (g cmꢀ3
)
Absorption coefficient (mmꢀ1
F(000)
)
Crystal size (mm)
h (°)
Index ranges
0.4 ꢁ 0.4 ꢁ 0.3
3.01 to 25.00
ꢀ9 6 h 6 9
ꢀ8 6 k 6 11
ꢀ16 6 l 6 16
6865
0.27 ꢁ 0.24 ꢁ 0.21
2.37 to 25.00
ꢀ12 6 h 6 11
ꢀ13 6 k 6 13
ꢀ13 6 l 6 13
6529
Reflections collected
Independent reflections (Rint
Refinement method
Completeness to h = 25.00° (%)
Data/restraints/parameters
Goodness-of-fit on F2
)
3687 (0.0319)
Full-matrix L.S on F2
99.8
3687/0/298
0.882
3454 (0.0185)
Full-matrix L.S on F2
92.3
3454/18/354
1.048
Final R indices [I > 2
R indies (all data)
r(I)]
R1 = 0.0360, wR2 = 0.0632
R1 = 0.0539, wR2 = 0.0659
R1 = 0.0291, wR2 = 0.0746
R1 = 0.0316, wR2 = 0.0759

M. Iqbal et al. / Polyhedron 57 (2013) 83–93
87
Table 2
ence in O–Cu–N angle value of this complex compared to those of 1
and 2, although all the three complexes have typical paddlewheel
central cores. The wider range of O–Cu–N angles {105.87(6)–
Selected bond lengths and angles of the complexes 1 and 2.
1
2
86.77(6)°} of the cited complex, compared to those of
1
Cu(1)–O(2)#1
Cu(1)–O(1)
Cu(1)–O(6)#1
Cu(1)–O(5)
Cu(1)–N(3)
Cu(1)–Cu(1)#1
1.9661(19)
1.9666(17)
1.9751(18)
1.9768(18)
2.157(2)
1.9773(16)
1.9691(16)
1.9689(16)
1.9784(16)
2.1455(18)
2.6563(6)
{94.68(8)– 97.76(8)°} and 2 {94.68(7)–97.51(7)°} is attributed to
the compromise between the two cross-linking O and N-donor li-
gands to allow the polymeric structure of the former complex. That
is, the polymeric structure is made possible at the expense of O–Cu–
N angle deviation from the value possessed by a typical paddle-
wheel central core. Here both the O and N-donor ligands are trans
bidentate and the overall structure is a 3-D network coordination
polymer but the central paddlewheel units have the same struc-
tural characteristics as those of the dimers 1, 2 and other reported
dimeric complexes [19,37].
The most important feature responsible for the paddlewheel
structure in such complexes comes out to be the contraction of
the O–Cuꢂ ꢂ ꢂCu angles and the concomitant expansion of O–Cu–N
angle from 90°. The result is the deviation of the {CuO4} square
base from ideal planarity and a substantial disparity from square
pyramidal geometry around penta-coordinated Cu(II) ion. The
N(1)–Cu(1)ꢂ ꢂ ꢂCu(1) bond angle is 177.75(6) and 176.82(5)° in com-
plexes 1 and 2, respectively. Each dcomplex possesses a center of
symmetry located in the midpoint of the Cuꢂ ꢂ ꢂCu ions and the
geometry around each Cu(II) in both complexes is distorted square
pyramidal.
2.6502(7)
167.52(7)
O(2)#1–Cu(1)–O(1)
O(2)#1–Cu(1)–O(6)#1
O(1)–Cu(1)–O(6)#1
O(2)#1–Cu(1)–O(5)
O(1)–Cu(1)–O(5)
O(6)#1–Cu(1)–O(5)
O(2)#1–Cu(1)–N(3)
O(1)–Cu(1)–N(3)
O(6)#1–Cu(1)–N(3)
O(5)–Cu(1)–N(3)
O(2)#1–Cu(1)–Cu(1)#1
O(1)–Cu(1)–Cu(1)#1
O(6)#1–Cu(1)–Cu(1)#1
O(5)–Cu(1)–Cu(1)#1
N(3)–Cu(1)–Cu(1)#1
167.74(6)
90.80(7)
91.41(7)
87.46(7)
87.73(7)
167.80(6)
95.90(7)
96.35(7)
94.68(7)
97.51(7)
82.24(5)
85.69(6)
85.57(5)
83.21(5)
176.82(5)
90.10(8)
88.95(8)
89.41(8)
88.84(8)
167.54(7)
96.12(8)
96.35(8)
94.68(8)
97.76(8)
81.84(6)
81.84(6)
84.38(6)
83.22(6)
177.75(6)
found for the apical N-donor acetonitrile ligand (2.114(2) Å) [32]
but longer than those found for imidazole N-donor ligand
(1.9815(15) Å) in mono-nuclear octahedral carboxylate complex
[33]. However the Cu–N bond length has been found to vary with
the basic strength of the lone pair on nitrogen atom of the N-donor
moiety. Varying the substituents on pyridine ring, Cu–N bond
length has been found to increase in the following order: 4-
NMe2 < H < 3-NH2 < 2-NH2 < (NH2)2 < NH2,CH3 [34–36].
ꢀ
Both dinuclear complexes crystallize in the same triclinic P1
space group. The crystal packing is formed by a large number of in-
ter-molecular associations in which oxygen and nitrogen atoms
take part. However, both complexes have different packing
arrangement of the dinuclear units. An oxygen atom and a methy-
lene hydrogen of the same carboxylate group of one dinuclear mol-
ecule associates with methylene hydrogen and oxygen atom,
respectively, of the adjacent complex in 1. In this way both ligands
of the trans-lying carboxylates pair of one molecule are H-bonded
with the adjacent carboxylate group of the neighboring molecule,
while the other pair of trans-carboxylate ligands is unable to do
so, resulting in 1D chain along a axis as shown in Fig. 3(a). The
crystal packing consists of infinite number of such chains lying side
by side forming 2D sheet as shown in Fig. 3(b). The overall packing,
especially the inter-chain association, is assisted by interaction of
oxygen atoms of the terminal nitro groups with H-atoms of pyri-
dine molecule and those of the aromatic ring of the carboxylate
moiety. Looking parallel to a axis, the molecules lie one above
the other in a symmetrical way as shown in Fig. 3(c).
Despite the similar structures of the two complexes, 2 has dif-
ferent packing pattern than 1. Here an oxygen atom of a carboxyl-
ate group is closer enough to the hydrogen at para position to
nitrogen of the pyridine molecule of the neighboring molecule.
Thus each molecule has two suitably oriented oxygen atoms capa-
ble of interaction with the trans H-atoms of the pyridine molecules
of the neighboring molecules. This arrangement is repeated indef-
initely, resulting in a 1D chain along a axis as shown in Fig. 4(a).
A comparison of the Cu–Cu, Cu–O and Cu–N distances of the
synthesized complexes with other related complexes is given in
Table 3.
The coordination environment around each copper of both dinu-
clear complexes is a {CuNO4} square pyramid. The matching struc-
tural parameters (bond lengths and angles) of the two complexes is
reflective of the similar coordination environment around Cu(II)
ions in both complexes. The cis angles (\O–Cu–O) of the square
base, consisting of four O atoms, in complexes 1 and 2 range from
88.84(8) to 90.10(8)° and 87.46(7) to 91.41(7)° while the respective
trans angles around copper are {\O(1)–Cu(1)–O(3)#1 = 167.52(7)°,
O(4)#1–Cu(1)–O(2) = 167.74(6)°} and {O(5)–Cu(1)–O(1) = 167.74(6)°,
O(4)–Cu(1)-O(2) = 167.80(6)°}, respectively. Moreover, all the O–
Cu–Cu bond angles in complexes 1 and 2 range from 81.84(6) to
84.38(6)° and 82.24(5) to 85.69(6)° while the O–Cu–N bond angles
are in the range 94.68(8)–97.76(8)° and 94.68(7)–97.51(7)°, respec-
tively. These angle values (and also the corresponding bond
lengths) are similar to those found for copper(II) complex where
the N-atom comes from the long tethering bis(4-pyridyl-
methyl)piperazine and the carboxylate oxygen atoms from 1,3-
phenylenediacetate [7]. However, noteworthy is the striking differ-
Table 3
Comparison of Cu–ligand bond lengths (Å) of 1 and 2 with structurally related complexes.
Complex
Cu–Cu
Cu–N
Cu–O
Reference
1
2
2.6502(7)
2.6563(6)
2.6307(9)
2.6360(18)
–
2.664(3)
2.6543(9)
2.730(1)
2.762(1)
2.157(2)
2.1455(18)
2.163(5)
2.155(10)
2.00(2)
2.138(10)
2.223(3)
2.296(3)
2.243(3)
1.9705(18)
1.9735(16)
this work
this work
[34]
[34]
[34]
((2-NH2)C5H4N)2Cu2(
((3-NH2)C5H4N)2Cu2(
l
l
-OOCCMe3)4
-OOCCMe3)4
1.942(5)–1.990(5)
1.937(8)–1.989(8)
1.965(16)
1.961(10)–1.999(9)
1.956(3)–1.985(2)
1.963(2)–1.974(2)
1.922(6)–2.019(5)
Cu(
((4-NMe2)C5H4N)2Cu2(
(C9H7N)2Cu2( -OOCCMe3)4
((2-NH2)(6-CH3)C5H3N)2Cu2(
((NH2)2C5H3N)2Cu2( -OOCCMe3)4.C6H6
g
2-OOCCMe3)2((4-NMe2)C5H4N)2
l
-OOCCMe3)4
[34]
l
[34,36]
[34,35]
[34,35]
l-OOCCMe3)4
l

88
M. Iqbal et al. / Polyhedron 57 (2013) 83–93
Fig. 3. (a) 1D chain of 1 along a axis (viewed along the c axis). (b) 2D arrangement of chains of compound 1 in oac plane. (c) Packing of compound 1 as viewed along the a axis.
These chains constitute 2D sheet in oac plane. Both complexes
have 1D chains along a axis and 2D sheets in oac plane but the mol-
ecules are arranged differently owing to the different atoms in-
volved in the inter-molecular interactions. This difference is
reflected in the relative orientation of their molecules in both 1D
and 2D arrangements (compare Fig. 3(a and b) and 4(a and b)).
The difference in supramolecular structures comes partly from
the difference in cis and trans angles around Cu(II) ions (see Table 2)
and partly from the different substituents at para-position of the
phenyl ring of the carboxylate ligands (nitro in 1 and methyl in

M. Iqbal et al. / Polyhedron 57 (2013) 83–93
89
Fig. 4. (a) 1D chain of complex 2 along a axis (viewed through c axis). (b) Arrangement of parallel chains of complex 2, resulting in 2D sheet in oac plane. (c) Packing of
compound 2 as viewed along a axis.
2) in 1 and 2. The former factor has rendered it difficult for the oxy-
gen of the carboxylate moiety to come closer and interact with the
methylene hydrogen atoms of the neighboring molecule in 2 while
the latter factor provides additional sites for the inter-molecular
interactions between two neighboring 1D chains affecting the
resulting 2D sheet of 1.
3.3. Electrochemical studies
The electrochemical studies of complexes 1 and 2 gave rise to
cyclic voltammograms shown with respect to a blank and at differ-
ent scan rates in Figs. 5 and 6(a) and (b), respectively. These volt-
ammogams consist of an oxidation and reduction peak each,
lying quite far away from each other, corresponding to the metal
Fig. 5. Cyclic voltammograms of blank and 2.6 mM of compounds 1 and 2 at scan
rate of 100 mV sꢀ1
.
centered irreversible electroactivity. The irreversible nature of
the redox processes is attributed to the instability of the redox
products in DMSO [38].
Cu(III) process each [19,39,40]. The linear plot of ꢀlog ipa versus
, shown in Fig. 7A, revealed a predominantly diffusion con-
ꢀlog
v
3.4. Oxidation
trolled process with slope values of 0.45 for complex 1 and 0.28
for 2 on the surface of glassy carbon electrode [41].
The peak current for such a process is given by Randles–Sevcik
model (Eq. (1)) [42]:
The oxidation signals of the complexes observed in the range
0.6–0.8 V were ascribed to an irreversible one electron Cu(II)/

90
M. Iqbal et al. / Polyhedron 57 (2013) 83–93
(a)
(b)
Fig. 6. Cyclic voltammograms at scan rates of 60, 80, 100, 120, 140 and 160 mV sꢀ1 of complexes 1 (a) and 2 (b).
Ip ¼ ð2:99 ꢁ 105Þnð
a
nÞ1=2ACꢃD1o=2v1=2
ð1Þ
and
where ip is the peak current in amperes,
a
is the charge transfer
EðVÞ ¼ ꢀ0:23391 log
According to Bard and Falkner [47] the value of
lated as
v
½Vsꢀ1ꢄ þ 1:11822
coefficient, n is the number of electrons involved in the oxidation
process, Do is the diffusion coefficient in cm2 sꢀ1, C⁄ is the bulk con-
centration of the compound in mol cmꢀ3, A is the surface area of the
a
n can be calcu-
working electrode in cm2 and
Calculating
the slope of the anodic peak current (ipa) versus potential scan rate
1/2) plot (shown in Fig. 7B) in Eq. (1) the values of Do for 1 and 2
v .
is the potential scan rate in Vsꢀ1
n from Eq. (5), assuming n equal to 1 and putting
an ¼ 47:7=½Ep ꢀ Ep=2ꢄ mV
ð3Þ
a
where Epa/2 is the potential where the current is at half the peak va-
lue. The value of
a
n thus calculated are given in Table 4. The values
(v
0
of formal potential (Eꢅ ) for complexes 1 and 2 were calculated from
were calculated to be 4.68 ꢁ 10ꢀ8 and 3.49 ꢁ 10ꢀ8 cm2 sꢀ1, respec-
tively. Here the order of Do is 1 > 2. Owing to the relative complex-
ity of electrochemical processes the values of diffusion coefficients
could not be explained on the basis of molecular weight or solva-
tion effects. However, the order coincides with that of heteroge-
neous rate constant as well as formal potential. This shows the
involvement of other factors such as the ease of oxidation and rel-
ative stability of the redox products. The peak shape, higher peak
current and the appearance of a relatively less pronounced peak
in ꢀ0.4 to ꢀ0.5 V potential range corresponding to Cu(0)/Cu(I) oxi-
dation process [43] provide ample evidence to conclude that the
product resulting as a result of oxidation process in 1 is more stable
as compared to that of 2. The irreversible nature of the complexes
is inferred from the graphical representation of the data according
to Eq. (1) and is further supported by the shift in the anodic peak
potential towards more positive values with increase in scan rate
as shown in Fig. 6 [42].
the intercept of oxidation peak potential versus scan rate curve
(shown in Fig. 7C) by extrapolating the curve to the potential axis
0
at
v
= 0 [48]. The values of Eꢅ for oxidation processes are listed in
Table 4 which exhibit the ease of electron removal from the central
metal ion in that order. The values of heterogeneous rate constant
(k°) were determined from the intercept of potential versus negative
logarithm of scan rate plot (shown in Fig. 7D) and putting various
parameters in Eq. (2). The k° values are 1.87 and 1.38 sꢀ1 for com-
plexes 1 and 2, respectively, as listed in Table 4. The order coincides
with that of Eꢅ0 confirming that easily oxidisable complex has higher
rate of oxidation process. The order also supports that of Do.
3.5. Reduction
Cyclic voltammograms given in Fig. 5 reveal an irreversible
reduction process for the complexes as no oxidation peak was ob-
served for the reduction products during anodic scan. The shifting
of peak current to more negative values with scan rate, character-
istic of an irreversible reduction process is clear from Fig. 6(a) and
(b).
The cathodic peaks of the complexes lying in potential ranging
from –0.60 to –0.850 V are attributed to the reduction of copper(II)
into copper(I) ion which is typical of the structurally related dinu-
clear copper(II) complexes [34,38,49] where the reduction of the
copper center in the first step gives rise to an unstable mixed-va-
lence complex such as:
The linear regression equations for the relation between log ipa
and log
v are given for complexes 1 and 2, respectively, as follows:
ꢀ log ipa ½mAꢄ ¼ 0:45523 log
and
v
½Vsꢀ1ꢄ ꢀ 4:89494
ꢀ log ipa ½mAꢄ ¼ 0:28439 log
v
½Vsꢀ1ꢄ ꢀ 5:18534
For irreversible electrode processes the value of peak potential
(E) is given by the Eq. (2) [44–46]:
2:303 RT Kꢅ 2:303 RT
E ¼ Eꢅ0 þ
ꢀ
log
v
ð2Þ
½pyCuðIIÞðl2-OOCCH2C6H4R ꢀ Þ4CuðIIÞpyꢄ þ e
! ½pyCuðIIÞðl2-OOCCH2C6H4RÞ4CuðIÞpyꢄ
an F
an F
where Eꢅ is the formal redox potential, k° is the standard heteroge-
neous rate constant, R is gas constant, T is the absolute temperature
and F is the Faraday constant. According to Eq. (2), there is a linear
0
Such mixed-valence complexes [Cu(II)ꢂ ꢂ ꢂCu(I)] have been ob-
served in the electrochemical reduction study of structurally re-
lated dinuclear Cu(II) tetraacetate complexes as well [50,51]. As
stated earlier, the redox products are unstable and may undergo
various changes such as disproportionation, elimination of a ligand
and a change in the coordination type of the central copper ion
[52]. A support to the structural reorganization of the mixed-va-
relation between potential (E) and logarithm of scan rate (log v) and
the linear regression equation thus obtained for complexes 1 and 2,
respectively, as given below:
EðVÞ ¼ ꢀ0:42522 log
v
½Vsꢀ1ꢄ þ 1:22453

M. Iqbal et al. / Polyhedron 57 (2013) 83–93
91
Fig. 7. Plots of ꢀlog of anodic peak current vs. ꢀlog of square root of the scan rate (A), anodic peak current vs. square root of the scan rate (B), anodic peak potential vs. scan
rate (C) and ꢀlog of the scan rate (D), cathodic peak current vs. square root of the scan rate (E), log of cathodic peak current vs. ꢀlog of scan rate (F) and cathodic peak
potential vs. ꢀlog of the scan rate (G) and scan rate (H) for complexes 1 and 2.
Table 4
Table 5
Thermodynamic and kinetic parameters for oxidation of the complexes obtained from
cyclic voltammetry.
Thermodynamic and kinetic parameters for reduction of the complexes obtained from
cyclic voltammetry.
D ꢁ 108 (cm2 sꢀ1
)
k° (sꢀ1
)
Complex
a
n
D ꢁ 10ꢀ8 (cm2 sꢀ1
)
k° (sꢀ1
)
0
0
Eꢅ (V)
Eꢅ (V)
Complex
a
n
1
2
0.58
0.76
0.18
0.14
4.68 0.1
3.49 0.1
1.87
1.38
1
2
–0.70
–0.79
0.15
0.16
5.40
15.00
0.36
0.38
sponding to the Cu(0)/Cu(I) oxidation process [43]. This peak tends
to disappear at higher scan rates and is more pronounced in 1 com-
pared to 2 owing probably to the relative stability of the redox
products in the former complex. Based on all these observations
the following scheme is likely for the redox processes of the com-
plexes 1 and 2 [34].
lence complex following the reduction process in 1 and 2 is pro-
vided by the broadening of the reduction peak especially in 2
and the absence of the reoxidation peak in the reverse scan of
the cyclic voltammogramms. Another support is the appearance
of weak oxidation peak in potential range ꢀ0.4 to ꢀ0.5 V corre-

92
M. Iqbal et al. / Polyhedron 57 (2013) 83–93
2½CuðIIÞ ꢂ ꢂ ꢂ CuðIÞꢄ ! ½CuðIIÞ ꢂ ꢂ ꢂ CuðIIÞꢄ þ ½CuðIÞ ꢂ ꢂ ꢂ CuðIÞꢄ
½CuðIÞ ꢂ ꢂ ꢂ CuðIÞꢄ ! 1=2½CuðIIÞ ꢂ ꢂ ꢂ CuðIIÞꢄ þ Cuð0Þ
ð4Þ
agreement with those of single crystal X-ray analysis. The cyclic
voltammetric studies in aqueous DMSO have shown that the com-
plexes exhibit metal centered irreversible one electron transfer
process in the potential range 1.5 V. The nitro group in 1 also gave
rise to a separate reduction peak in its typical potential region. The
oxidation and reduction processes occur independent of each other
and were found to be diffusion controlled. The kinetic and thermo-
dynamic parameters for the redox processes were calculated. The
order of diffusion coefficient and heterogeneous rate constant for
oxidation is: 1 > 2 indicating a facile oxidation process for 1 while
for reduction the order is reversed i.e., 2 > 1 due to overriding sol-
vent effect and the larger molecular mass of 1 compared to 2. From
the values of formal potential it is seen that 1 undergoes redox pro-
cess at relatively lower potential values than 2. The values of
charge transfer coefficient were also calculated and found typical
of the irreversible redox processes. The electron transfer processes
in oxidation and reduction were found to depend principally upon
the stability of the redox products and solvation effects,
respectively.
ð5Þ
To further confirm the nature of the reduction process in solu-
tion, the diffusion coefficient values were calculated using Eq. (1)
by putting the slope of the cathodic peak current (ipc) versus poten-
tial scan rate (v
1/2) plot (shown in Fig. 7E) and the values of various
reduction parameters in it. The Do values turned out to be
5.40 ꢁ 10ꢀ8 and 1.50 ꢁ 10ꢀ7 cm2 sꢀ1, respectively, for 1 and 2.
The higher value of Do for 2 is in accordance to its lower molec-
ular mass and smaller size both enhancing its mobility in solution.
Moreover, the non-polar methyl groups attached to the phenyl ring
have no affinity for the polar solvent molecules and thus the sol-
vent effect is minimized. Similarly, the nitro groups in 1 are ex-
pected to have high polar interactions with the solvent molecules
reducing its mobility.
Fig. 7F shows the linear relation between log ipc versus ꢀlog
v
with a slope value of 0.49 and 0.38 for complexes 1 and 2, respec-
tively, indicating a predominantly diffusion controlled process on
the surface of glassy carbon electrode [41].
The linear regression equation for this relation is given as fol-
lows for complexes 1 and 2, respectively:
Acknowledgment
M.I. is thankful to the Higher Education Commission of Pakistan
for financial support under Indigenous Ph.D. Fellowship Program
as well as International Research Support Initiative Program (IRSIP)
and the authors are thankful for crystal structure analysis per-
formed on the Australian synchrotron.
ꢀ log ipc ½mAꢄ ¼ 0:49606 log
and
v
½Vsꢀ1ꢄ þ 4:86949
ꢀ log ipc ½mAꢄ ¼ 0:37946 log
v
½Vsꢀ1ꢄ þ 5:13896
Fig. 7G shows the linear relation between potential (E) and neg-
ative logarithm of the scan rate for which the regression equation
is given, respectively, for complexes 1 and 2 as below:
Appendix A. Supplementary data
CCDC 900472 and 899284 contain the supplementary crystallo-
graphic data for compounds 1 and 2, respectively. These data can
be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic Data Cen-
tre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223 336
033; or e-mail: deposit@ccdc.cam.ac.uk.
EðVÞ ¼ 0:44658 log
and
v
½Vsꢀ1ꢄ ꢀ 1:2902
EðVÞ ¼ 0:38416 log
v
½Vsꢀ1ꢄ ꢀ 1:31993
Formal potential was calculated from the intercept of potential
versus scan rate curve (Fig. 7H) by extrapolating to the potential
axis at zero scan rate [48] and is given in Table 5.
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