Synthesis, spectral characterization, thermal and photoluminescence properties of Zn(II) and Cd(II)-azido/thiocyanato complexes with thiazolylazo dye and 1,2-bis(diphenylphoshino)ethane

Article abstract of DOI:10.1016/j.saa.2010.09.004

A series of complexes of the type [M(L)(dppe)X₂]; where M = Zn(II) or Cd(II); L = 4-(2′-thiazolylazo)chlorobenzene (L₁), 4-(2′-thiazolylazo)bromobenzene (L₂) and 4-(2′- thiazolylazo) iodobenzene (L₃); dppe = 1,2-bis(diphenylphosphino) ethane; X = N₃^- or NCS^- have been prepared and characterized on the basis of their microanalysis, molar conductance, thermal, IR, UV-vis and ¹H NMR spectral studies. IR spectra show that the ligand L is coordinated to the metal atom in bidentate manner via azo nitrogen and thiazole nitrogen. An octahedral structure is proposed for all the complexes. The thermal behavior of the complexes revealed that the thiocyanato complexes are thermally more stable than the azido complexes. All the complexes exhibit blue-green emission with high quantum yield as the result of the fluorescence from the intraligand emission excited state.

Full text of DOI:10.1016/j.saa.2010.09.004

Spectrochimica Acta Part A 78 (2011) 102–106
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, spectral characterization, thermal and photoluminescence properties
of Zn(II) and Cd(II)-azido/thiocyanato complexes with thiazolylazo dye and
1,2-bis(diphenylphoshino)ethane
B.A. Yamgar, V.A. Sawant, B.G. Bharate, S.S. Chavan∗
Department of Chemistry, Shivaji University, Kolhapur (MS) 416004, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 April 2010
Received in revised form 6 August 2010
Accepted 8 September 2010
A
series of complexes of the type [M(L)(dppe)X2]; where M = Zn(II) or Cd(II); L = 4-(2ꢀ-
thiazolylazo)chlorobenzene (L1), 4-(2ꢀ-thiazolylazo)bromobenzene (L2) and 4-(2ꢀ-thiazolylazo)
−
iodobenzene (L3); dppe = 1,2-bis(diphenylphosphino)ethane; X = N3 or NCS− have been prepared
and characterized on the basis of their microanalysis, molar conductance, thermal, IR, UV–vis and 1H
NMR spectral studies. IR spectra show that the ligand L is coordinated to the metal atom in bidentate
manner via azo nitrogen and thiazole nitrogen. An octahedral structure is proposed for all the complexes.
The thermal behavior of the complexes revealed that the thiocyanato complexes are thermally more
stable than the azido complexes. All the complexes exhibit blue-green emission with high quantum
yield as the result of the fluorescence from the intraligand emission excited state.
Keywords:
Thiazolylazo dyes
Zn(II)/Cd(II) complexes
1,2-Bis(diphenylphosphino)ethane
Luminescence
© 2010 Elsevier B.V. All rights reserved.
Thermal properties
1. Introduction
acting as the charge equilibrium, but also influencing the final struc-
tures of the complexes. They can bind metal ions depending on
Considerable research efforts have been given to the prepara-
tion of transition metal complexes of hybrid ligands because they
can provide new materials with useful properties such as magnetic
exchange [1–4], electrical conductivity [5,6], photoluminescence
[7–9], non-linear optical property [10–12] and antimicrobial activ-
ity [13,14]. Among various ligands azo group containing ligands
have received much attention in recent years. Due to presence of
azo (–N N–) group, these compound possesses several distinctive
properties such as aggregation, optical data storage and tautomeri-
sation [15,16]. Thiazolylazo compounds particularly are important
because they can form different types of coordination compounds
with transition metals due to the several electron rich donor cen-
ters with unusual structural and chemical properties. Because of the
importance of thiazolylazo derivatives and its ability to act as poly-
functional ligand many studies on its metal complexes have been
carried out [17–21]. 1,2-Bis(diphenylphosphino)ethane (dppe) is
one of the most versatile phosphine ligand, capable of binding to the
metal atoms in variety of ways: monodentate, chelating or bridging.
They possess high covalency as well as ligand field effect to enforce
a drastic change in magnetic and other behaviou−r of the resulting
compounds [22]. The pseudohalides such as N3 and NCS− also
have distinct effects during the formation of complexes, not only
the nature and the stereochemical environment surrounding the
metal ion as well as the steric hindrance introduce by the other
coordinating ligands [23–26].
In this paper, we report the synthesis and characterization of
Zn(II) and Cd(II) complexes derived from some thiazolylazo ligands
(L1–3) with 1,2-bis(diphenylphosphino)ethane and N3− or NCS− as
coligands. The complexes were characterized particularly by ele-
mental analysis, molar conductance, spectral (IR, UV–vis and 1H
NMR) and thermal studies. The photoluminescence property of the
complexes has also been reported.
2. Experimental
2.1. Materials and methods
All the chemicals were used as received and solvents were
purified according to the literature methods [27]. Zn(NO3)2·6H2O
(E-Merk India), Cd(NO3)2·4H2O (E-Merck India), NaN3 (Aldrich,
USA), NH4NCS (Aldrich, USA), 1,2-bis(diphenylphosphino)ethane
(Aldrich) were purchased from the respective concerns and were
used as received. Microanalysis (C, H, N and S) was performed on a
Thermo Finnigan FLASH EA-112 CHNS analyzer. Electronic spectra
were recorded on a Shimadzu 3600 UV-Visible-NIR spectropho-
tometer. 1H NMR spectra of the samples were recorded on Bruker
300 MHz FT-NMR in DMSO-d6. Molar conductance (ꢀM) was mea-
sured on the ELICO (CM-185) conductivity bridge using ca. 10−3 M
∗
Corresponding author. Tel.: +91 231 2609164; fax: +91 231 2691533.
E-mail address: sanjaycha2@rediffmail.com (S.S. Chavan).
1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2010.09.004

B.A. Yamgar et al. / Spectrochimica Acta Part A 78 (2011) 102–106
103
solution in DMF. Infrared spectra were recorded on Perkin-Elmer
FT-IR spectrometer as KBr pellets in the 4000–400 cm−1 range.
Thermal analysis of the complexes was carried out on a Perkin-
Elmer thermal analyzer in nitrogen atmosphere at a heating rate
of 10 ◦C/min. The luminescence properties were measured using
a JASCO F.P. 750 fluorescence spectrophotometer at room tem-
perature (298 K) in DMF solution with 1 cm3 path length quartz
cell.
2.2. Synthesis of thiazolylazo ligands (L1–3
)
The thiazolylazo ligands 4(2ꢀ-thiazolylazo)chlorobenzene
(L1), (L2) and
4(2ꢀ-thiazolylazo)bromobenzene 4(2ꢀ-
thiazolylazo)iodobenzene (L3) were prepared by diazotization
of 2-aminothiazole by reported procedure [28] and were
characterized by elemental analysis, IR and UV–vis spectra.
2.3. Preparation of azido complexes (1a–6a)
To a methanolic solution of appropriate metal salt (1 mmol,
0.297 g Zn(NO3)2·6H2O) or 0.308 g Cd(NO3)2·4H2O), a methano-
lic solution of L (1 mmol, 0.223 g, L1 or 0.268 g, L2 or 0.315 g,
L3) was added while stirring. To this, a CH2Cl2 solution (5 ml) of
1,2-bis(diphenylphosphino)ethane (1 mmol, 0.398 g) was added,
followed by the addition of NaN3 (2 mmol, 0.130 g) in warm
methanol. The resultant mixture was stirred for 3 h at room tem-
perature. The solid product obtained was filtered, washed with
ethanol:water (1:1) mixture and dried under vacuum over CaCl2.
2.4. Preparation of thiocyanato complexes (1b–6b)
To a methanolic solution of appropriate metal salt (1 mmol,
0.297 g Zn(NO3)2·6H2O) or 0.308 g Cd(NO3)2·4H2O), a methanolic
solution of L (1 mmol, 0.223 g, L1 or 0.268 g, L2 or 0.315 g, L3) was
added while stirring. To this, a CH2Cl2 solution (5 ml) of 1,2-bis-
(diphenylphosphino)ethane (1 mmol, 0.398 g) was added, followed
by the addition of NH4NCS (2 mmol, 0.152 g) in warm methanol.
The resultant mixture was stirred for 3 h at room temperature. The
solid product obtained was filtered, washed with ethanol: water
(1:1) mixture and dried under vacuum over CaCl2.
3. Results and discussion
The reaction of thiazolylazo ligands L1–3 with Zn(II) and Cd(II)
salts in presence of dppe and NaN3 or NH4NCS in 1:1:1:2 molar
ratio yields mononuclear complexes of the type [M(L)(dppe)(N3)2]
(1a–6a) and [M(L)(dppe)(NCS)2] (1b–6b); where M = Zn(II),
Cd(II);
L = 4-(2ꢀ-thiazolylazo)chlorobenzene
(L1),
4-(2ꢀ-
thiazolylazo)bromobenzene (L2), 4-(2ꢀ-thiazolylazo)iodobenzene
(L3); dppe = 1,2-bis(diphenylphosphino)ethane (Fig. 1). The analyt-
ical and physical properties of the complexes are given in Table 1.
The air stable, moisture insensitive complexes are insoluble in
common organic solvents such as dichloromethane, acetonitrile,
chloroform except DMF and DMSO. The results of elemental
analysis support the composition of the complexes. The molar
conductances of the complexes in 10−3 M solution in DMF are in
the range of 15.93–27.09 ꢁ−1 cm2 mol−1 indicating that, although
some dissociation of these complexes seems to occurs in the
solvent, the conductance value support the non-electrolytic nature
of the complexes [29].
3.1. IR spectra
The IR spectra of ligands and their complexes are found to
be quite complex as they are in general exhibit large number of
bands of varying intensities. However, a strong band observed

104
B.A. Yamgar et al. / Spectrochimica Acta Part A 78 (2011) 102–106
Ph
Ph
Ph
X
M
X
S
N
N
P
P
N
Ph
R
Fig. 2. TG–DTA curve of [Zn(L1)(dppe)(N3)2] (1a).
M = Zn(II), Cd(II); R = Cl, Br, I; X = N 3, NCS
Fig. 1. Proposed structure of the complexes.
were carried out for complexes 1a–6a and 1b–6b between the
room temperature and 1000 ◦C under nitrogen atmosphere. Typical
TG–DTA curves of 1a and 1b are presented in Figs. 2 and 3.
The thermal decomposition process of Zn(II) and Cd(II) azido
complexes (1a–6a) involves three decomposition stages. The first
stage for both Zn(II) and Cd(II) azido complexes takes place
in the range 188–274 ◦C for 1a–3a and 190–288 ◦C for 4a–6a
with exothermic DTA peaks in the range 243–256 ◦C (1a–3a)
and 246–248 ◦C (4a–6a) corresponding to loss of two azide ions.
The second stage occurs in the range 265–390 ◦C for 1a–3a and
282–417 ◦C for 4a–6a which may be attributed to the decomposi-
tion of the ligand L1, L2 or L3. The DTA curve gives exothermic peaks
in the range 345–352 ◦C for 1a–3a and 340–352 ◦C for 4a–6a. The
third decomposition stage takes place in the range 376–874 ◦C for
1a–3a and 405–893 ◦C for 4a–6a corresponding to the decomposi-
tion of dppe molecule leaving anhydrous ZnO and CdO as a residue
at the end.
The Zn(II) and Cd(II) thiocyanato complexes 1b–6b show very
similar behavior to the azido complexes. However, the first decom-
position stage occurs in the range 198–300 ◦C for 1b–3b and
198–294 ◦C for 4b–6b with accompanying exothermic DTA peaks
in the range 340–346 ◦C (1b–3b) and 340–348 ◦C (4b–6b) which
may be attributed to the loss of two thiocyanate ions. The second
stage occurs in the range 286–418 ◦C for 1b–3b and 284–420 ◦C
for 4b–6b assigned to the decomposition of the ligand L1, L2 or L3.
The DTA curve gives exothermic peaks in the range 340–346 ◦C for
1b–3b and 340–348 ◦C for 4b–6b. The third stage takes place in the
range 410–882 ◦C (1b–3b) and 409–890 ◦C (4b–6b) corresponding
to the decomposition of dppe molecule leaving anhydrous ZnO and
CdO, respectively.
at 1618–1624 cm−1 in the spectra of all ligands (L1–3) is due to
ꢂ(C N) of thiazole nitrogen. This band shifted to lower frequencies
(1586–1595 cm−1) in the complexes indicates involvement of thi-
azole nitrogen in coordination [30]. Another band appeared at the
frequency range 1469–1480 cm−1 in the spectra of ligands assigned
to –N N– group shifted to lower frequency (1434–1440 cm−1) in
the complexes indicates involvement of azo nitrogen in coordina-
tion with metal ion [31]. This is also supported by the appearance
of ꢂ(M–N) band at 435–448 cm−1 in the complexes [32]. The band
at 744 cm−1 in the spectra of free ligands L1–3 remains unaltered
in the spectra of all the complexes, indicating non-involvement of
thiazole sulfur in coordination. The IR spectra of all the Zn(II) and
Cd(II) complexes exhibited the expected bands due to dppe ligand
at around 1483, 1384, 1172 and 738 cm−1. The azido complexes
1a–6a show strong bands at ∼2113 and ∼1343 cm−1. These are
assigned to ꢂas and ꢂs stretching vibrations of the coordinated azido
group [33]. The thiocyanato complexes 1b–6b exhibit a strong and
sharp band at ∼2096 cm−1, a weak band at ∼762 cm−1 and another
weak band at ∼488 cm−1, which can be attributed to ꢂ(CN), ꢂ(CS)
and ꢂ(NCS), respectively. These values are typical for N-bonded
thiocyanate complexes [34].
3.2. Electronic and NMR spectra
The electronic spectra for all the complexes were recorded
in DMF (10−4 M) in the range 200–900 nm. All the complexes
exhibit two bands in the region 278–292 and 330–342 nm. These
transitions are presumably due to intra-ligand ␲–␲* and n–␲* tran-
sitions, respectively. Another intense absorption observed in the
437–462 nm region at longer wavelength in the spectra of Zn(II)
and Cd(II) complexes which may be assigned to the characteristic
n–␲* transition of the coordinated azo ligand.
The 1H NMR spectra of all the complexes were recorded in
DMSO-d6. The 1H NMR spectra of the complexes show that the
resonances of phenyl protons of the coordinated dppe ligand over-
lap to some extent with those of phenyl hydrogen atoms of L in
the complexes. However, a broad multiplet observed in the range
ı 6.87–7.56 ppm for all the complexes were assigned to the phenyl
group of dppe together with ring proton of azo ligand L1–3. The spec-
tra of all the complexes exhibit a broad singlet at approximately ı
2.45 ppm is due to CH2 protons of the dppe ligand [35].
3.3. Thermal analysis
In order to examine the thermal stability of the complexes,
thermo gravimetric (TG) and differential thermal analysis (DTA)
Fig. 3. TG–DTA curve of [Zn(L1)(dppe)(NCS)2] (1b).

B.A. Yamgar et al. / Spectrochimica Acta Part A 78 (2011) 102–106
105
intraligand fluorescence and these are red shifted by >140 nm. The
intensity of emission in the Zn(II) and Cd(II) complexes is found
to be higher than that of free ligands. The chelation of the ligands
increases the rigidity and reduces the loss of energy by thermal
vibrational decay [36,37]. Significant differences in the intensities
of the emission of the complexes from that of the ligands may
be considered as an evidence of the metal–ligand complexation.
Introduction of substituents on the azo ligands (L1–3) has also an
interesting effect on the intensity of emission spectra.
The fluorescence quantum yield (˚) of the complexes was deter-
mined by using quinine sulfate as a reference with known ˚R of
0.52 and appeared at 0.014–0.069. The area of emission spectrum
was integrated using the software available in the instrument and
quantum yield was calculated according to the following equation.
AS
AR
(Abs)R
× ˚R
(Abs)S
˚
S
=
×
Here ˚S and ˚R are the fluorescence quantum yield of the sample
and reference, respectively. AS and AR are the area under the fluo-
rescence spectra of the sample and reference, respectively. (Abs)S
and (Abs)R are the respective optical densities of the sample and
the reference solution at the wavelength of excitation.
Fig. 4. Emission spectra of Zn(II) complexes (1a–3a and 1b–3b).
4. Conclusion
In the present study, the synthesis and spectroscopic char-
acterization of Zn(II) and Cd(II) complexes of thiazolylazo dye
with 1,2-bis(diphenylphosphino)ethane and N3 or NCS− coli-
−
gands have been carried out by microanalysis, IR, UV–vis, 1H NMR,
thermal analysis and fluorescence properties. IR spectra reveal that
the ligand is coordinated to the metal atom in bidentate manner
via azo nitrogen and thiazole nitrogen. Thiocyanato complexes are
thermally more stable than the azido complexes. All the complexes
exhibit blue-green emission with high quantum yield as the result
of the fluorescence from the intraligand emission excited state.
Acknowledgement
One of the authors (BAY) is thankful to UGC-SAP, New Delhi
for awarding UGC-Research Fellowship in Sciences for Meritorious
Students.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.saa.2010.09.004.
Fig. 5. Emission spectra of Cd(II) complexes (4a–6a and 4b–6b).
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