Coordination assembly of p-substituted aryl azo imidazole complexes: Influences of electron donating substitution and counter ions

Article abstract of DOI:10.1016/j.poly.2010.03.022

The formation of a series of Cu(II), Zn(II) and Cd(II) complexes using the p-substituted aryl azo system has been reported. Presence of electron donating -Me, -OMe groups impart some unusual coordination behaviors in Cu(II) complexes. Varying coordination mode and electron density in the ligand results in the formation of an unusual Cu-S bond in complex 1 and an unusual equatorial coordination of H₂O in Cu(II) complex 2 with a square-pyramidal geometry. The hydrogen bonding site along with the metal coordination site in the ligand results in the formation of higher dimensional self-assembled supramolecular architectures. Compatibility of the hard-soft nature of the metal ion and symbiosis effect of the ligand with metal is responsible for the formation of long range higher dimensional networks in the solid-state, which were confirmed by single crystal X-ray structure analysis. Furthermore, various physicochemical studies, viz. thermal behaviors, absorption spectra, electrochemistry and EPR studies, have been conducted to rationalize the structures in the solution phase.

Full text of DOI:10.1016/j.poly.2010.03.022

Polyhedron 29 (2010) 1980–1989
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Coordination assembly of p-substituted aryl azo imidazole complexes: Influences
of electron donating substitution and counter ions
*
Avijit Pramanik, Arghya Basu, Gopal Das
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The formation of a series of Cu(II), Zn(II) and Cd(II) complexes using the p-substituted aryl azo system has
been reported. Presence of electron donating –Me, –OMe groups impart some unusual coordination
behaviors in Cu(II) complexes. Varying coordination mode and electron density in the ligand results in
the formation of an unusual Cu–S bond in complex 1 and an unusual equatorial coordination of H2O in
Cu(II) complex 2 with a square-pyramidal geometry.
The hydrogen bonding site along with the metal coordination site in the ligand results in the formation
of higher dimensional self-assembled supramolecular architectures. Compatibility of the hard–soft nat-
ure of the metal ion and symbiosis effect of the ligand with metal is responsible for the formation of long
range higher dimensional networks in the solid-state, which were confirmed by single crystal X-ray
structure analysis. Furthermore, various physicochemical studies, viz. thermal behaviors, absorption
spectra, electrochemistry and EPR studies, have been conducted to rationalize the structures in the solu-
tion phase.
Received 3 February 2010
Accepted 12 March 2010
Available online 1 April 2010
Keywords:
Unusual Cu–S bond
Supramolecular architectures
Self-assembly
Electron rich dye
EPR
Thermal and electrochemical analysis
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
industries. In fact, a large majority of these compounds are medic-
inally important, [11,12] and their motifs are found in many natu-
Construction of metal–organic coordination frameworks via
metal coordination-directed self-assembly processes has proven
to be a fertile research field [1,2]. Metallosupramolecular chemis-
try, which involves the interplay between organic and metallic tec-
tons, can be used to design network materials with predefined
dimensional (1D, 2D or 3D) structural motifs [3–6]. The major
approaches to such materials that have been employed are
non-covalent interactions such as hydrogen-bonding, metal-ion
coordination, electrostatic and hydrophobic forces [7,8].
Formation of such supramolecular metal–organic coordination
structures [9] has attracted much attention in recent years due
to their applications in diverse areas such as catalysis, optoelec-
tronics, supramolecular storage of molecules, molecular recogni-
tion and magnetism. This approach allows the use of metal
ion-ligand combinations as the metal complex as a whole behaves
ral products. There has been a long-standing interest [13–16] in
the coordination chemistry of 2-(aryl azo) imidazole ligands due
to many reasons. The trans-isomer of this ligand forms stable com-
plexes with several d-block elements. Our interest lies in the de-
sign of azo compounds containing a heterocyclic system. The
presence of the –N@N– group can lead to the stabilization of low
valent metal oxidation states due to its
low lying azo-centered *-molecular orbitals [17,18]. The azo func-
p acidity and presence of
p
tion is photochromic, [19,20] redox active [21,22] pH responsive
[23,24] and its complexes act as a molecular switch [25,26]. Our re-
cent work has involved incorporating some electron donating
group in aryl azo systems and comparative studies upon the coor-
dinations which form extended H-bonding network structures.
Herein we have explored the coordination assembly of Cu(II), Zn(II)
and Cd(II) metal complexes with two different p-substituted aryl
azo imidazole ligands (Scheme 1). The complexes are [Cu(II)(L1)2-
(NCS)(SCN)] 1, [Cu(II)(L2)(H2O)(NO3)2] 2, [Zn(II)(L1)2(NCS)2] 3,
[Zn(II)(L2)(NCS)2(H2O)]H2O 4, [Cd(II)(L1)2Cl2] 5 and [Cd(II)(L2)2
(NO3)2] 6. We have compared the formation of coordination
assemblies by considering the stereo-electronic effect of
the substitutent on the phenyl ring in the aniline-imidazole
dyes (Scheme 1). We also studied their comparative absorption
spectral behaviours, electrochemical analysis, EPR and thermal
properties.
as
a supramolecular synthon generating higher dimensional
architectures.
Nitrogen containing heteroaromatic systems have been among
the most researched kinds of organic compounds, primarily be-
cause of their importance [10] in pharmaceutical and chemical
* Corresponding author. Tel.: +91 3612582313; fax: +91 3612582349.
E-mail address: gdas@iitg.ernet.in (G. Das).
0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.03.022

A. Pramanik et al. / Polyhedron 29 (2010) 1980–1989
1981
da-25 UV–Vis spectrometer at 298 K. NMR spectra was recorded
on a Varian FT-400 MHz instrument. The chemical shifts were re-
corded in parts per million (ppm) using tetramethylsilane (TMS)
as a reference. Elemental analyses were carried out on a Perkin–El-
mer 2400 automatic carbon, hydrogen and nitrogen analyzer. The
solid-state magnetic susceptibility of the complexes at room tem-
perature were recorded using a Sherwood Scientific balance MSB-
1. Solution electrical conductivity measurements were made with
a Systronics Conductivity meter 306. The thermo gravimetric anal-
yses (TGA) of the compounds were performed using an SDTA 851 e
TGA thermal analyzer (Mettler Toledo) with a heating rate of 2 °C
per min in a N2 atmosphere. Powder X-ray diffraction data were re-
corded with a Seifert powder X-ray diffractometer (XRD 3003T)
Complex 1 (1:2)
Complex 3 (1:2) Complex 5 (1:2)
N
N
X
N
N
H
X= -Me (L1), -OMe (L2)
with a Cu Ka source (k = 1.54 Å) on a glass surface of an air-dried
sample. Cyclic voltammetric measurements were carried out using
a CH Instruments make CHI660C electrochemistry system. The cell
contained a glassy carbon working electrode, a Pt wire auxiliary
electrode and a saturated calomel electrode (SCE) as the reference
electrode. A salt bridge (containing supporting electrolyte, tetra-n-
butylammonium perchlorate (TBAP) dissolved in dry MeCN) was
used to connect the SCE with the electrochemistry solution [27].
All experiments were carried out under a dinitrogen atmosphere
at RT and were uncorrected for junction potentials. Under our
experimental conditions, the E1/2 value (in volts) for the couple
Fc+/Fc was 0.45 in MeCN vs. SCE. X-Band EPR spectra were re-
corded with a Jeol JES-FA series spectrometer fitted with a quartz
dewar for measurements at liquid nitrogen temperature. The spec-
tra were calibrated with an internal manganese marker.
Complex 2 (1:1)
Complex 4 (1:1)
Complex 6 (1:2)
Scheme 1. The influence of electron donating substituents and counter ions for the
self assembly of metal complexes assisted by the aryl azo system.
2. Experimental
2.1. Chemicals and reagents
All reagents were obtained from commercial sources and were
used as received. Solvents were distilled freshly following standard
procedures. Acetonitrile (MeCN) was dried by distillation from
anhydrous CaH2 and stored over 4 Å molecular sieves for electro-
chemical analysis.
2.2. Physical measurements
2.3. X-ray crystallography
The IR spectra were recorded on a Perkin–Elmer-Spectrum One
The intensity data were collected using a Bruker SMART APEX-II
CCD diffractometer, equipped with fine focus 1.75 kW sealed tube
FT-IR spectrometer with KBr disks in the range 4000–400 cmÀ1
.
The absorption spectra were recorded on a Perkin–Elmer Lamb-
Mo Ka radiation (k = 0.71073 Å) at 278(3) K, with increasing x
Table 1
Crystal data and structure refinement for complexes 1–6.
Complex 1
Complex 2
Complex 3
Complex 4
Complex 5
Complex 6
CCDC no.
Empirical formula
Formula weight
T (K)
653805
739712
C10H12CuN6O8
407.81
739711
C22H20N10S2Zn
554.01
739713
C12H14N6O3S2Zn
419.82
653804
C20H22CdCl2N8
557.77
739716
C20H20CdN10O8
640.87
C
22H20CuN10S2
552.17
298(2)
298(2)
298(2)
298(2)
298(2)
298(2)
Radiation
Wavelength
Crystal system
Space group
a (Å)
Mo K
a
Mo K
a
Mo K
a
Mo K
a
Mo K
a
Mo Ka
0.71073 Å
orthorhombic
Pca21
11.8175(3)
12.7372(3)
17.0007(4)
90.00
0.71073 Å
monoclinic
P21/c
9.7920(1)
6.9041(1)
23.3863(3)
90
0.71073 Å
monoclinic
P21/n
11.0016(2)
15.0798(2)
15.5549(2)
90
0.71073 Å
monoclinic
P21/c
10.2949(2)
13.2491(2)
13.1432(2)
90
0.71073 Å
monoclinic
C2/c
11.7834(14)
12.9682(15)
15.504(2)
90
0.71073 Å
monoclinic
C2/c
15.4614(3)
10.9049(3)
16.3198(4)
90
b (Å)
c (Å)
a
(°)
b (°)
90.00
90.00
2558.98(11)
4
97.985(1)
90
1565.70(3)
4
92.6070(10)
90
2577.92(7)
4
91.698(1)
90
1791.92(5)
4
101.697(8)
90
2320.0(5)
4
116.295(3)
90
2466.88(12)
4
c
(°)
V (Å3)
Z
l
1.044
1.455
1.146
1.631
1.196
0.953
F(0 0 0)
1156
694
1136
692
1120
1288
hkl ranges
À15 ꢀ h ꢀ 12
À16 ꢀ k ꢀ 16
À22 ꢀ l ꢀ 19
5984/3858
0.786/0.711
5984/1/318
0.492/À0.320
0.843
À13 ꢀ h ꢀ 13
À8 ꢀ k ꢀ 9
À31 ꢀ l ꢀ 30
3817/3290
0.812/0.735
3817/0/275
0.297/À0.264
1.055
À14 ꢀ h ꢀ 13
À19 ꢀ k ꢀ 19
À20 ꢀ l ꢀ 20
6300/4541
0.874/0.829
6300/0/397
0.510/À0.477
1.010
À13 ꢀ h ꢀ 13
À14 ꢀ k ꢀ 17
À17 ꢀ l ꢀ 17
4448/3670
0.629/0.521
4448/0/274
0.619/À0.432
0.979
À15 ꢀ h ꢀ 15
À17 ꢀ k ꢀ 16
À19 ꢀ l ꢀ 20
2887/2695
0.861/0.789
2887/0/141
0.320/À0.366
1.067
À20 ꢀ h ꢀ 20
À14 ꢀ k ꢀ 14
À16 ꢀ l ꢀ 21
3045/2628
0.761/0.687
3045/0/179
0.379/À0.224
1.018
Reflections
Tmax/Tmin
Data/parameters
Maximum/minimum residual (e Å3)
Goodness-of-fit (GOF) (S)
Rint
0.0304
0.0181
0.0230
0.0160
0.0359
0.0243
Final R indices
R1 = 0.0429
wR2 = 0.1171
R1 = 0.843
wR2 = 0.1432
R1 = 0.0260
wR2 = 0.0712
R1 = 0.0320
wR2 = 0.0744
R1 = 0.0332
wR2 = 0.0720
R1 = 0.0563
wR2 = 0.0808
R1 = 0.0295
wR2 = 0.0630
R1 = 0.0390
wR2 = 0.0674
R1 = 0.0230
wR2 = 0.0615
R1 = 0.0253
wR2 = 0.0629
R1 = 0.0242
wR2 = 0.0583
R1 = 0.0311
wR2 = 0.0615
[I > 2r(I)]
R indices (all data)

1982
A. Pramanik et al. / Polyhedron 29 (2010) 1980–1989
(width of 0.3° per frame) at a scan speed of 3 s/frame. SMART soft-
ware was used for data acquisition. Data integration and reduction
were undertaken with SAINT and XPREP [28] software. Multi-scan
empirical absorption corrections were applied to the data using
the program SADABS [29]. The structures were solved by direct meth-
ods using SHELXS-97 and refined with full-matrix least squares on F2
using SHELXL-97 [30]. All non-hydrogen atoms were refined aniso-
tropically. The hydrogen atoms were located from the difference
Fourier maps and refined. Structural illustrations have been drawn
with ORTEP-3 for Windows [31]. Selected crystallographic data are
summarized in Table 1.
2.5.5. Synthesis of [Cd(L1)2Cl2] (5)
To a solution of L1 (0.37 g, 2 mmol) in CH3OH (20 mL), solid
CdCl2 (0.20 g, 1 mmol) was added in portions, with constant stir-
ring. The light yellow precipitate that formed immediately was col-
lected by filtration, washed with ether (20 mL), and dried in
vacuum. Yield: 0.52 g, 90% based on the ligand L1. Single-crystals
suitable for X-ray diffraction were obtained from slow evaporation
of an EtOH:water (1:1) solution of the compound. Anal. Calc. (%) for
C20H22Cl2N8Cd: C, 43.01; H, 3.97; N, 20.07. Found: C, 43.11; H,
3.41; N, 20.01%. IR (KBr disk) (cmÀ1);
1407 (m).
m(C@N), 1606 (m); m(N@N),
2.4. Synthesis of the ligands
2.5.6. Synthesis of [Cd(L2)2(NO3)2] (6)
To a solution of L2 (0.404 g, 2 mmol) in CH3OH (20 mL), solid
Cd(NO3)2 (0.30 g, 1 mmol) was added in portions, with constant
stirring. The light yellow precipitate that formed immediately
was collected by filtration, washed with ether (20 mL), and dried
in vacuum. Yield: 0.52 g, 90% based on the ligand L2. Single-crystals
suitable for X-ray diffraction were obtained by slow evaporation of
EtOH:water (1:1) solution of the compound. Anal. Calc. (%) for
C20H20N10O8Cd: C, 37.38; H, 3.13; N, 21.80. Found: C, 37.42; H,
The p-substituted aryl azo imidazole ligands (L1 and L2) were
synthesized following the literature method [32,33].
2.5. Synthesis of complexes 1–6
2.5.1. Synthesis of [Cu(L1)2(NCS)(SCN)] (1)
To a magnetically stirred solution of L1 (0.372 g, 2 mmol) in
CH3OH (20 mL), was added solid CuCl2 (0.171 g, 1 mmol) in por-
tions. Then an aqueous solution of NH4SCN (0.152, 2 mmol) was
added to this mixture. The greenish-yellow precipitate that formed
was filtered, washed with ether, and dried under vacuum. Yield:
0.524 g (95%). Single-crystals suitable for X-ray diffraction were
obtained by slow evaporation of an EtOH:water (1:1) mixture of
the compound at RT. Anal. Calc. (%) for C22H20N10S2Cu: C, 47.91;
H, 3.66; N, 25.41. Found: C, 47.98; H, 3.60; N, 25.50%. IR (KBr disk)
3.15; N, 21.88%. IR (KBr disk) (cmÀ1);
1275 (m); (ONO), 980; (C@N), 1604 (m); m
m(N@O), 1420 (s);
m(N–O),
m
m
(N@N), 1403 (m).
3. Results and discussion
3.1. Crystal structure studies
In our earlier report we have shown that 2-(phenylazo) imidaz-
ole can act as a borderline and/or soft donor ligand [34,35]. Here
we also reported that the aryl azo imidazole system can form in-
ter-molecular H-bonding in the solid-state, which in turn forms a
3D supramolecular network in the crystal of a different pattern.
In the present studies, we have compared the solid-state structure
of 2-(p-tolylazo) imidazole (L1) with unsubstituted 2-(phenylazo)
imidazole. In the solid-state, L1 contains two symmetrically inde-
pendent molecules. Substitution in the para position with the elec-
tron donating group –CH3 makes the ligand more planar than the
unsubstituted one. The imidazole and phenyl rings are more planar
and the azo group is virtually coplanar with these rings. The dihe-
dral angle between the two least-squares planes is only 1.75°, com-
pared to 2.1° in the unsubstituted ligand. The N@N bond length is
1.256(3) Å, which is similar to the previous one, but smaller than in
similar systems [34,35]. The bond between the azo-N and the phe-
nyl, as well as the imidazole ring, is longer than the unsubstituted
system. However, the bond between the imidazole ring and azo-N
is stronger than the phenyl ring and azo-N, showing a similar trend
to known aryl azo systems [34,35]. However, the N@N distance is
almost the same in both ligands (1.254 vs. 1.256 Å in L1). The imid-
azole N–H forms inter-molecular strong hydrogen bonds with the
imidazole N atom of the other asymmetric unit (N1Á Á ÁN6 = 2.845 Å
and N2Á Á ÁN5 = 2.856 Å). It forms a 1D hydrogen bonded chain
along the bc plane. The phenyl ring of the molecule is arranged
in an alternative direction about the 1D-chain (Fig. 1). The solid-
state packing shows the formation of alternate hydrophilic and
hydrophobic layers, as depicted in Fig. 1. The stretching frequency
(cmÀ1);
(N@N), 1401 (m);
m
NCS), 2090 (s);
mSCN), 2070 (s); m(C@N), 1595 (m);
m
m
(C@S), 772 (m).
2.5.2. Synthesis of [Cu(L2)(H2O)(NO3)2] (2)
To a solution of L2 (0.20 g, 1 mmol) in CH3OH (3 mL), solid
Cu(NO3)2 (0.23 g, 1 mmol) was added in portions, with constant
stirring. The light green precipitate that formed immediately was
collected by filtration, washed with ether (5 mL), and dried in vac-
uum. Yield: 0.37 g (90%). X-ray quality crystals having the compo-
sition [Cu(II)(L2)(H2O)(NO3)2] were grown by diffusion of an
EtOH:water (1:1) solution of the complex. Desolvation of crystals
occurred when they were kept without the mother liquor. Anal.
Calc. (%) for C10H12N6O8Cu: C, 29.48; H, 2.97; N, 20.64. Found: C,
29.51; H, 2.95; N, 20.65%. IR (KBr disk) (cmÀ1);
(N–O), 1271 (m); (ONO), 992; (C@N), 1600 (m);
(m).
m
(N@O), 1497 (s);
m
m
m
m(N@N), 1406
2.5.3. Synthesis of [Zn(L1)2(NCS)2] (3)
To a magnetically stirred solution of L1 (0.37 g, 2 mmol) in
CH3OH (20 mL), was added solid ZnCl2 (0.13 g, 1 mmol) in portions.
An aqueous solution of NH4SCN (0.15, 2 mmol) was then added to
this mixture. The yellowish-red precipitate that formed was fil-
tered, washed with ether, and dried under vacuum. Yield: 0.47 g,
85% based on the ligand L1. Slow evaporation of an EtOH:water
(1:1) solution of the compound at RT produced single-crystals suit-
able for X-ray diffraction. Anal. Calc. (%) for C22H20N10S2Zn: C,
47.82; H, 3.65; N, 25.36. Found: C, 47.91; H, 3.58; N, 25.45%. IR
(KBr disk) (cmÀ1);
(N@N), 1398 (m);
m
(SCN), 2190 (s), 2140;
m(C@N), 1593 (m);
of the endocyclic mC
shifted by 40–80 cmÀ1 from that of the unsubstituted ligand. The
mode in L1 appears at 1230–1260 cmÀ1 and is red shifted
@
N bond of L1 appears at 1520 cmÀ1 and is red
m
m(C@S), 685 (m).
mN
@
N
2.5.4. Synthesis of [Zn(L2)(NCS)2(H2O)]H2O (4)
by 150–170 cmÀ1 from that of unsubstituted ligand. The imidaz-
ole-N forms strong N–HÁ Á ÁN hydrogen bonding in the free ligand.
However, in the complexes this N is coordinated to the metal cen-
ter. Hence, in the metal complexes N–HÁ Á ÁN hydrogen bonding is
absent. In the solid-state, the metal complexes of these azo ligands
form different types of coordinative assemblies which are pat-
Complex 4 was synthesized using the same procedure as for
complex 3. Yield: 0.37 g, 90% based on the ligand L2. Anal. Calc.
(%) for C12H14N6O3S2Zn: C, 34.45; H, 3.37; N, 20.10. Found: C,
34.55; H, 3.41; N, 20.02%. IR (KBr disk) (cmÀ1);
m(SCN), 2182 (s),
2130; m(C@N), 1598 (m); m(N@N), 1393 (m); m(C@S), 680 (m).

A. Pramanik et al. / Polyhedron 29 (2010) 1980–1989
1983
systems, Cu(II)Pc (plastocyanin) contains Cu(II) bonded with the
S atom of cystine thiolate or a neutral methodize thioether
[37,38]. However, in non-biological systems complexes of Cu(II)
with thiocyanate (SCNÀ) are not so favored energetically, and
hence are less common in the literature. One of the ligands is
bonded to the Cu(II) in a bidentate manner in the equatorial plane.
The Cu(II) centre lies exactly on the equatorial plane formed by the
four coordinating atoms. The bite angle of the ligand in the equa-
torial plane is much smaller (ꢁ74°) than the other angles in the
equatorial plane. In the solid-state it forms several C–HÁ Á ÁN, C–
HÁ Á ÁS, C–HÁ Á Á
One of the imidazole N–H groups forms a N–HÁ Á Á
with the -bond of the C@N group in the equatorial NCS ion. It
p weak interactions [39].
p
interaction
p
forms a double-stranded 1D polymeric hydrogen bonded chain
along the b-axis (Fig. 2b). All the metal ions are lined up along
the b-axis. Analysis of the packing diagram of complex 1 reveals
that in each complex only one of the two imidazole N–H groups
forms a strong conventional hydrogen bond (N–HÁ Á ÁN = 2.717 Å)
with the thiocyanate N of a neighbouring molecule in the same
c
a
layer. Moreover, there exist weak C–HÁ Á ÁS and C–HÁ Á Á
p interac-
tions between neighbouring molecules. The cumulative effects
of these non-covalent interactions lead to the formation of a 1D
helical architecture along the b-axis (Fig. 2c). Along these chains
two Cu(II) atoms are separated by 9.423 Å and the repeating per-
iod in the helical column is about 11.817 Å. In this system an in-
ner channel runs parallel to the helical axis. The IR spectrum of
b
Fig. 1. Formation of a 1D hydrogen bonded chain in solid-state L1.
terned by the counter anion and the metal coordination
environment.
The molecular structure of the complex [Cu(II)(L1)2(NCS)(SCN)]
(1) along with the numbering scheme is shown in Fig. 2a. It forms
an axially elongated neutral monomeric square-pyramidal com-
complex
1
exhibits
a
strong sharp stretch at 2090 and
of the SCN and NCS coor-
2070 cmÀ1, which corresponds to mC
@
N
dinating anions, respectively [38]. The medium intense peak at
plex (s
= 0.05) [36] with two ambident SCNÀ ligands bonded to
772 cmÀ1 corresponds to
m
C@S stretching of the coordinated anion
Cu(II). The anion SCNÀ is bonded with Cu(II) in an equatorial posi-
tion with the N donor (Cu–N = 1.998 Å) whereas in axial position it
bonds with the S donor (Cu–S = 2.559 Å). Normally in biological
[39]. The magnetic moment,
leff, is ꢁ1.78 BM at 300 K, as ex-
pected for an isolated S = 1/2 (d9) Cu(II) monomeric distorted
square-pyramidal complex.
(a)
(b)
N10
C21
S1
b
C2
C4
C3
C18
C19
N9
C20
C22
S2
Cu1
c
C13
C12
N3
C16
C15
C9
C8
C5
C6
C7
C10
(c)
Fig. 2. (a) ORTEP plot of complex 1. Thermal ellipsoids are set at the 50% probability level; (b) packing of complex 1 viewed along the a-axis and (c) view of inter-molecular
N–HÁ Á ÁN interactions between adjacent molecules that lead to the formation of a 1D helical chain in complex 1 along the b-axis.

1984
A. Pramanik et al. / Polyhedron 29 (2010) 1980–1989
C6
(a)
O1
C7
(b)
a
C5
N3
N1
N2
b
C4
C1
C10
C8
N4
C9
O8
C3
C2
O5
Cu1
O2
N6
N5
O6
O3
O4
O7
Fig. 3. (a) ORTEP plot of complex 2. Thermal ellipsoids are set at the 50% probability level and (b) packing of complex 2 viewed along the c-axis.
In the absence of NH4SCN, L2 forms the neutral monomeric
complex [Cu(II)(L2)(H2O)(NO3)2] (2) when treated with Cu(NO3)2
(Fig. 3a). The Cu(II) centre has a distorted square-pyramidal coor-
weak C–HÁ Á ÁS type non-covalent interactions in the solid-state. The
benzene ring of one ligand in the asymmetric unit forms -stack-
ing interactions ( , 3.667 Å) [39] with the nearest imidazole
p
pÁ Á Áp
dination geometry (
s
= 0.05). Both the nitrates are monodentate
ring of the same ligand. The other ligand in the asymmetric unit
does not form this type of interaction as it is oriented perpendicu-
larly with the neighboring ligand. It forms a 2D honeycomb net-
work structure along the bc plane in the crystal (Fig. 4b). The FT-
IR spectrum of complex 3 exhibits strong stretches at 2190, 2140
in nature. The Cu(II) atom lies exactly on the square plane. The
azo-N forms the axial bond (Cu–N4 = 2.3787). L2 act as a bidentate
chelating ligand. The equatorial bite angle is similar (ꢁ75°) to that
of the Cu(II) complex 1. The imidazole-N forms strong hydrogen
bonds with the oxygen atoms of the nitrate [39]. Metal bound
water forms strong O–HÁ Á ÁO type hydrogen bonds with neighbour-
ing oxygen atoms of the nitrate. The methoxy group is involved in a
and 685 cmÀ1, which are in agreement with
mC@N and mC@S, respec-
tively. [40,41] Under similar reaction conditions, but different crys-
tallization conditions, L2 forms a Zn(II) complex with a trigonal
C–HÁ Á ÁO type hydrogen bond. Additionally a
p
Á Á Á
p
interaction
bipyramidal (TBP) geometry [44] (s = 0.88).
plays a crucial role in forming a brick wall structure along the ab
plane (Fig. 3b) in the solid-state. Complex 2 shows strong sharp
peaks at 1497, 1271 and 992 cmÀ1, which can be defined as the
mN@O stretching frequencies of the unidentate bonding of the ni-
trate anion [40]. The magnetic moment of the complex is
ꢁ1.89 BM, and this is due to the existence of the Cu(II) ion in the
square-pyramidal environment [42,43].
In complex 4, the Zn(II) centre is surrounded by two thiocya-
nate linkages, one bidentate L2 ligand and one water molecule
(Fig. 5a). The Zn(II) atom lies exactly on the equatorial plane. The
azo-N and the water molecule are in the axial positions. The ligand
behaves as a bidentate ligand, in contrast to Zn(II) complex 3. The
molecular packing shows a 1D coordination polymer running along
the c-axis [39]. The distances between the two nearest Zn(II) ions
in the polymer chain is 6.673 Å. Ligands in adjacent 1D polymeric
chains are directed in opposite directions. The closest inter chain
distance is 3.698 Å. The water of crystallization forms one strong
N–HÁ Á ÁO hydrogen bond and three more weak hydrogen bonds
[39]. It forms a 1D water chain along the c-axis (Fig. 5c). The metal
bound water simultaneously forms weak hydrogen bonds with
two adjacent S atoms of SCN ions, and acts as bridge. It forms a
ten membered chair type hydrogen bonded ring (Fig. 5b). The IR
spectrum of complex 4 exhibits strong sharp stretches at 2182
In contrast to the Cu(II) complex 1 with the ligand L1, Zn(II)
forms a completely different coordination geometry with the same
ligand under the same reaction conditions. The Zn(II) complex 3
forms
a mononuclear, neutral distorted tetrahedral complex
(Fig. 4a), which is confirmed by the measurement of angles around
the Zn(II) center. Moreover, the ambidentate thiocyanate ions bind
to the Zn(II) center only through the N-atom, unlike in complex 1.
The difference in coordinating behavior of the thiocyanate ions in
complexes 1 and 3 can be explained on the basis of the HSAB prin-
ciple. Both ligands are unidentate and they are oriented in the cis
orientation relative to the metal center. The ligands are oriented
in opposite direction to each other. Careful analysis of the crystal
packing diagram [39] reveals that both imidazole-NH units form
N–HÁ Á ÁS type hydrogen bonds with the neighboring same thiocya-
nate-S atom. However, the other thiocyanate-S atom is involved in
and 2130 cmÀ1, which corresponds to the mC
@ bond, and one at
N
680 cmÀ1 which corresponds to mC@S [40,41].
The ligand L1 forms the neutral mononuclear complex 5
(Fig. 6a) with CdCl2, where Cd(II) is surrounded by two unidentate
L1 ligands and two ClÀ ions in a distorted tetrahedral fashion. Both
ligands are disposed in opposite directions with respect to the
(a)
(b)
S2
S1
C22
C22
N12
Zn1
N11
N1
C6
C9
C19
N5
C5
C18
C17
N3
C7
a
N4
N8
C8
C2
N2
c
C15
C20
C16
Fig. 4. (a) ORTEP plot of complex 3. Thermal ellipsoids are set at the 50% probability level and (b) packing of complex 3 along the bc plane.

A. Pramanik et al. / Polyhedron 29 (2010) 1980–1989
1985
(a)
(b)
C6
C7
O1
C8
C5
N3
N4
N2
C3
C4
C9
C12
S2
N6
C2
N1
Zn1
O2
S1
C1
N5
C11
O3
(c)
a
c
Fig. 5. (a) ORTEP plot of complex 4. Thermal ellipsoids are set at the 50% probability level; (b) the water bridge ten membered chair type hydrogen bonded architecture in
complex 4 and (c) the packing of complex 4 along the b-axis.
metal center. The imidazole-N–Cd–N-imidazole angle is 133.88°,
which is much higher than a tetrahedral angle. The torsion angle
between the two rings of the ligand is 4.86° i.e. the ligand is less
planar than L1 of the previous complex. Each imidazole N–H forms
a relatively strong hydrogen bond with the neighbouring coordi-
nated chloride ion. This has a significant role in the formation of
a 3D network of rhombohedra, where each of them share a corner
with the adjacent one [39]. The para-substituted methyl groups
Ligand L2 also forms the neutral mononuclear complex 6
(Fig. 7a) with Cd(NO3)2. The structure shows that the Cd(II) ions
adopt a distorted octahedral geometry. The metal center is sur-
rounded by two unidentate ligands and two bidentate chelating ni-
trate groups. Both the ligands are on the same side of the metal ion.
The imidazole N atoms form stronger bonds than the oxygen atoms
of the nitrate groups [45]. Each imidazole N–H forms a relatively
strong hydrogen bond with the O-atom of a neighboring coordi-
nated NO3À ion. The C–H moiety adjacent to the imidazole nitrogen
forms a C–HÁ Á ÁO type hydrogen bond with the O-atom of a neigh-
form weaker C–HÁ Á Á
p interactions. These interactions help to form
a 1D chain along the c-axis [39]. Complex 5 forms a flower mosaic
À
architecture (Fig. 6b) in the solid-state.
boring coordinated NO3 ion. This combination of interactions re-
(a)
(b)
a
Cl1
b
C2
C3
N2
Cd1
N1
N4
C9
N3
C5
C4
C7
C6
C10
Fig. 6. (a) ORTEP plot of complex 5. Thermal ellipsoids are set at the 50% probability level and (b) packing of complex 5 along the a-axis.

1986
A. Pramanik et al. / Polyhedron 29 (2010) 1980–1989
(b)
(a)
b
O4
O2
N5
O3
a
C2
C3
Cd1
N1
N4
N2
N3
C4
O1
C5
C6
C7
C10
Fig. 7. (a) ORTEP plot of complex 6. Thermal ellipsoids are set at the 50% probability level and (b) packing of complex 6 along the c-axis.
sults in the formation of a diamond shape higher dimensional
coordination assembly [39]. The packing diagram reveals that the
self-assembly of complex 6 forms a flower mosaic architecture
along the c-axis (Fig. 7b). The FT-IR spectrum of the cadmium ni-
trate complex exhibits strong stretches at 1420, 1275 and
980 cmÀ1 which are in agreement with coordinated O–NO2 moie-
ties [40,41].
plex 2 the observed
D
kmax, n ?
p
is ꢁ30 nm and
Dkmax, p ? p is
ꢁ26 nm. Additionally, in the Cu(II) complex, a third band is ob-
served in the visible region, at ꢁ434 nm for complex 1 and at
ꢁ443 nm for complex 2. This higher wavelength band originates
from the forbidden d–d transition of the metal center, which is
generally weak in nature. [47]. In the presence of Cu(II), Zn(II)
and Cd(II) metal ions, a bathochromic shift is observed due to
MLCT transitions (Fig. 8b). However, the ligands, as well as the
complexes, do not exhibit any significant solvatochromism. Similar
results have been found for other ligands. In both the Cu(II) com-
3.2. Absorption spectral studies
plexes,
a
weak spectral band is observed at ꢁ550–660 nm
= 825–85 MÀ1 cmÀ1), which is generally attributed to dxz,
yz ? dx2Ày2 transitions. This band probably appears due to the typ-
The electronic absorption spectrum of aniline-imidazole [46]
dye shows an intense broad absorption band in the visible region
at ca. 362 nm in MeOH. The para-substituted ligands L1–2 show a
bathochromic shift with respect to the unsubstituted ligand
(Fig. 8a). The maximum red-shift is found in case of L2 (–OMe
substituted) and the minimum effect is observed for L1 (–Me
substituted). The amount of shift is attributed to the electronic ef-
fect of the substituted group. These absorption bands, between 362
(e
d
ical penta-coordinated square-pyramid geometry of the Cu(II)
complexes [48]. Overall the features are from the effect of the elec-
tron releasing substituents which increase the electron density of
the ligand system, and as a result sharp changes are observed in
the absorption behavior.
and 385 nm, correspond to intra-ligand n ?
p as well as p ? p
electronic transitions [47]. In addition, L2 shows a weak shoulder
at 421 nm. In the Cu(II) complexes of L1 and L2, the absorption
bands in the visible region are shifted to higher wavelengths rela-
3.3. EPR spectra analysis
The EPR spectra were recorded for both the Cu(II) complexes.
The X-band EPR spectrum of complex 1 in MeOH glass (77 K) is dis-
played in Fig. 9a. The spectrum is typical for a monomeric Cu(II)
tive to the free ligands. In complex 1, the observed
Dkmax, n ? p is
ꢁ12 nm and
D
kmax
,
p
?
p
is ꢁ14 nm, whereas in the case of com-
0.8
L1
(a)
(b)
0.8
0.6
0.4
0.2
0
L1 + CuII
0.7
L1 + ZnII
L1
0.6
0.5
0.4
0.3
0.2
0.1
0
L1 + CdII
L2
300
350
400
450
500
300
350
400
450
500
550
Wavelength (nm)
Wavelength (nm)
Fig. 8. (a) Absorption spectra of L1 and L2 in MeOH and (b) absorption spectra of L2 in the presence of Cu(II), Zn(II) and Cd(II) metal ions.

A. Pramanik et al. / Polyhedron 29 (2010) 1980–1989
1987
Epc = À0.695 V,
D
Ep = 91 mV and E1/2 = À0.65 V) (Fig. 10b). The vol-
(b)
(a)
g
g11
g
tammogram of the Cu(II) complex 1 shows only a single reduction
peak of Cu(II)/Cu(I) at 0.508 V (Epc) during the anodic potential
scan, and just after the reduction peak a cathodic peak is observed
at 0.414 V (Epa) (Fig. 10c). The separation between the anodic and
g11
175G
182G
cathodic peak potentials (DEp) of 94 mV indicates a quasi-revers-
ible redox process assignable to the Cu(II)/Cu(I) couple and E1/2 is
equal to 0.46 V.
It is believed that this response arises from oxidation of the azo-
imine function. The role of the azoimine function in bringing about
the high Cu(II)/Cu(I) potential is well established [54,55]. Substitu-
tion of the para-H with electron donating –Me/–OMe groups in-
creases the overall electron density in the ligand frame, which in
turn increases the oxidation potential of the ligand compared to
the unsubstituted one [34,35]. In the case of L2, the oxidation peak
remains similar without any significant change (Epa = À0.156 V). In
the complex 2, however, one quasi-reversible couple peak is ob-
served which may be due to azo bond oxidation (Epa = À0.613 V,
Complex 1
Complex 2
Fig. 9. X-band EPR spectra measured in methanol at 77 K of (a) complex 1 and (b)
complex 2.
Epc = À0.705 V,
complex 2 shows the quasi-reversible CV response (Epa = 0.441 V,
Epc = 0.534 V, Ep = (92 mV and E1/2 = 0.66 V) corresponding to
D
Ep = 91 mV and E1/2 = À0.65 V) and the Cu(II)
complex in a distorted square-pyramidal geometry with (g\ = 2.06,
g11 = 2.33, A11 = 175 G) with a dx2 À y2 ground state. Thus, the
stronger effect of the axial S donor as compared with a N donor
is indicative of a slightly stronger perturbation to the Cu(II) centre
[49,50]. This may be attributed to a marked deviation of the axial
Cu–S bond from the unique axis of the tetragonal square basal
plane, as has been demonstrated for a violet thiocyanate S–Cu(II)
complex with a distorted square-pyramidal structure. Also in the
solid-state, complex 1 exhibits a broad EPR signal around g = 2
with no visible hyperfine interactions at 300 K. In contrast, for
complex 2 we have shown that the monomeric Cu(II) is sur-
rounded by three O-donors and one N-donor in the basal plane
and a weak azo-N donor is in the axial position. The EPR spectrum
of complex 2 in methanol glass at 77 K resembles the monomeric
Cu(II) EPR spectrum (g\ = 2.05, g11 = 2.20, A11 = 182.3 G) in solution
(Fig. 9b) [51].
D
the Cu(II)/Cu(I) redox process [39].
3.5. Thermal studies
The interaction between the complexes and the solvent mole-
cules in the solid-state can be commented on with the help of ther-
mo gravimetric analysis. The onset temperature of weight loss and
decomposition temperature were 160 and 200 °C for L2, 220 and
250 °C for L1, respectively (Fig. 11a). The metal complexes of these
azo dyes were thermally more stable than the azo-dyes alone,
which is reflected in their higher thermal stability. All the com-
plexes containing solvent molecules in their crystal structure (viz.
complexes 2 and 4) decomposed thermally in a number of steps.
However, the other complexes showed either a single step or a
continuous weight loss in between 50 and 600 °C [39]. The robust-
ness of the solid-state framework is reflected in the TGA [56–59].
Complex 2 shows a thermal release of metal bound water at
3.4. Electrochemistry
ꢁ150 °C (130–165 °C;
Dm = 4.42% Calc. 4.18% found), followed by
The electrochemical properties of the Cu(II) complexes were
examined by cyclic voltammetry at a platinum working electrode
in acetonitrile (0.1 M TBAP) and the potentials are reported with
reference to SCE (at 50 mV sÀ1 scan rate). The voltammograms dis-
play a metal based redox couple on the positive side and a ligand
based redox couple on the negative side, with respect to the SCE
(Fig. 10). The oxidation peak of the ligand L1 shows one (Table 2)
oxidation response at a negative potential which may be attributed
to the redox couple irreversible peak (Epa = À0.133 V) (Fig. 10a)
[52,53]. In the complex one quasi-reversible couple peak is ob-
served, which may be due to azo bond oxidation (Epa = À0.604 V,
Table 2
Cyclic voltammetry data of L1 and L2 and with their corresponding Cu(II) salts.
Entry
Epc (V)
Epa (V)
D
Ep (mV)
E1/2 (V)
L1
L2
–
–
À0.133
À0.156
0.508
–
–
94
91
93
92
–
–
L1-Cu(II) complex
0.414
À0.695
0.441
À0.705
0.46
0.65
0.537
0.660
À0.604
L2-Cu(II) complex
0.534
À0.613
Fig. 10. (a) Cyclic voltammogram of L1 in MeCN; (b) reduction of the Cu(II)/Cu(I) couple in complex 1; (c) oxidation of the azo system in complex 1.

1988
A. Pramanik et al. / Polyhedron 29 (2010) 1980–1989
100
100
95
90
85
80
75
70
65
60
L1
90
L2
Complex 4
Complex 2
80
70
60
50
40
30
20
50
100
150
200
250
300
350
80
160 240 320 400 480 560
Temp (oC)
Temp (oC)
Fig. 11. (a) TGA analysis of ligands L1 and L2, (b) TGA analysis of complex 2 and 4.
Monodentate binding mode
Bidentate binding mode
M2+
M2+
N
N
X
N
N
X
N
N
N
N
H- bonding site
H- bonding site
H
H
-Me (L1), -OMe (L2)
X =
Zn2+(Complex 3)
Cd2+(Complex 5 and 6)
Cu2+(Complex 1 and 2)
Zn2+(Complex 4)
M2+
M2+
Scheme 2. Scheme of diverse coordination modes with different metal ions in aryl azo systems.
complete decomposition in the temperature range 190–340 °C
[37], whereas complex 4 shows successive thermal release of
water of crystallization and metal bound water followed by com-
plete decomposition of the complex (Fig. 11b). The TGA of complex
4 in N2 atmosphere shows the release of one crystal water mole-
salt resulted in the formation of a mononuclear (1:2) complex
whereas the Cu(II) and Zn(II) salts formed mononuclear (1:1) com-
plexes in the -p–OMe substituted system (Scheme 2).
The Cu(II) ion shows a marked preference for the N end of the
ambidentate thiocyanate ligand. However, CSD search shows that
only ꢁ6–7% of mononuclear Cu(II) complexes containing the SCNÀ
anion have the Cu(II) center bonded through the S end. In the com-
plex 1 we have both types of bonding at the same time. This can be
rationalized by Pearson’s hard–soft acid–base principle which re-
quires matching hardness of two species for effective bonding.
Bonding of SCNÀ through the S end to Cu(II) is not favoured ener-
getically. However, in complex 1, an inter-molecular H-bond be-
tween the N atom of the thiocyanate and the N–H proton of the
adjacent ligand (L1) favoured this type of bonding.
cule (100–125 °C;
bound water molecule at a relatively higher temperature range
(190–220 °C; m = 4.50% Calc. 4.76% found) and complete decom-
Dm = 4.30% Calc. 4.48% found) and one metal
D
position is achieved at ꢁ310 °C, which is in good agreement with
the X-ray crystal structure.
4. Conclusions
À
This work describes the structural similarities and dissimilari-
ties of Cu(II), Zn(II) and Cd(II) metal complexes with some electron
rich p-substituted (–Me, –OMe) 2-phenylazo imidazole ligands,
where counter ions also play a vital role in the higher dimensional
structures obtained. Though the -p- substitutions are away from
the coordination site, they still have a strong influence in the
self-assembly processes of the metal complexes in the solid-state.
The counter anions and metal ions tune the coordination environ-
ment of the complexes. A change in the coordination sphere has
been reflected in their crystal structures and other physico-chem-
ical properties. We have seen that the Cu(II), Zn(II) and Cd(II) salts
show mononuclear (1:2) complexes in unsubstituted as well as -p–
Me substituted systems. On the other hand, addition of the Cd(II)
The NO3 ligand shows both monodentate (complex 2) and
bidentate bridging (complex 6) coordination modes. The single
crystal of the different complexes also shows the formation of mul-
tifaceted networks in the resulting complexes. Solvent induced
changes in the solid-state structures have also been observed in
complexes 2 and 4. Both the Cu(II) complexes 1 and 2 have similar
geometries, but different coordination environments. In complex 1
two ligands are attached to the metal center, while in 2 only one
ligand is coordinated to Cu(II). More than 75% of the coordination
sphere of 1 is hydrophobic while in 2 less than 25% of the surface
is hydrophobic. This is clearly reflected in their solid-state packing.
The presence of an electron donating group in the ligand in-
creases the electron density in the ring which in turn reduces the

A. Pramanik et al. / Polyhedron 29 (2010) 1980–1989
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C–H bonds attached to the OMe group are more polarized in L2
than L1 where they are attached to a –Me group. Moreover, the
presence of electronegative O atoms in the neighboring position
in the solid-state helps to form C–HÁ Á ÁO type hydrogen bonds in
complex 2. On the other hand, the C–H bonds of the para –Me
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p type interactions with the imidazole
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The results of this study not only illustrates the coordination
mode of ligands but also shows the nature of the neutral ligands,
playing an important role in the construction of a variety of coor-
dination networks. Thermal studies show that most of the metal-
azo dyes have good thermal stability up to 300 °C, which is an
important criterion to be a recording material for DVD-R. We are
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