Cadmium(II)-imidazolylazo dyes: Synthesis, structure and photochromism

Article abstract of DOI:10.1016/j.poly.2012.07.061

1-Alkyl-2-{(o-thioalkyl)phenylazo}imidazole acts as imidazolyl-N donor ligand to Cd(II) although they have potential three donor centres namely N(imidazole), N(azo) and -SR. Two series of complexes [Cd(SRaaiNR ^/)₂X₂] (X =

Full text of DOI:10.1016/j.poly.2012.07.061

Polyhedron 46 (2012) 81–89
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Cadmium(II)-imidazolylazo dyes: Synthesis, structure and photochromism
S. Saha (Halder) a,1, B.G. Chand d, J.-S. Wu b, T.-H. Lu b, P. Raghavaiah c, C. Sinha a,
⇑
a Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700 032, India
b Department of Physics, National Tsing Hua University, Hsinchu 300, Taiwan, ROC
c School of Chemistry, National Single Crystal X-ray Diffractometer Facility, University of Hyderabad, Hyderabad 500 046, India
d School of Education, Basantapur Education Society, Domkal, Murshidabad, India
a r t i c l e i n f o
a b s t r a c t
Article history:
1-Alkyl-2-{(o-thioalkyl)phenylazo}imidazole acts as imidazolyl-N donor ligand to Cd(II) although they
have potential three donor centres namely N(imidazole), N(azo) and –SR. Two series of complexes
[Cd(SRaaiNR/)2X2] (X = Cl, Br, I) and [Cd(SRaaiNR/)4](ClO4)2 are synthesized with these ligands. These
complexes are spectroscopically (IR, UV–Vis, 1H NMR) characterised and, the single crystal X-ray struc-
tures of [Cd(SEtaaiNEt)2I2] and [Cd(SEtaaiNEt)4](ClO4)2 (where SEtaaiNEt = 1-ethyl-2-{(o-thioalkyl)
phenylazo}imidazole) have confirmed the distorted tetrahedral structures. The UV light irradiation in
MeCN solution of the complexes shows E-to-Z (E and Z refer to trans and cis-configuration about –
N@N–, respectively) isomerisation of the coordinated azoimidazole. The rate of isomerisation follows
Received 28 April 2012
Accepted 16 July 2012
Available online 27 July 2012
Keywords:
Cd(II)-thioalkylphenyl-azoimidazole
Distorted Td-symmetry
Photochromism
the sequence
:
[Cd(SRaaiNR/)4](ClO4)2 < [Cd(SRaaiNR/)2Cl2] < [Cd(SRaaiNR/)2Br2] < [Cd(SRaaiNR/)2I2].
Activation energy
Quantum yields (/E?Z) and the activation energy (Ea) of the isomerisation of the complexes are lower
than that of free ligand data. The observation has been explained considering the molecular association
that increases the mass and rotor volume of the complexes.
Effect of halide
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
much recent interest because of their potential applications in dif-
ferent areas such as liquid crystal alignment [12], optical data
Cadmium and its compounds have some importance in social
perspective. Main application of cadmium is in rechargeable bat-
teries and photoconducting devices [1]. Cadmium may cause major
pathological effects on the kidneys. Because of borderline acidity
cadmium(II) can bind strongly imidazole and may hamper bio-
chemical reactions since most of the biomolecules have imidazole
unit [2]. This has created interest to study the coordination chem-
istry of Cd(II)-imidazole/imidazole containing ligands [3]. We have
synthesized series of ligands belong to azo dyes, 1-alkyl-2-(ary-
lazo)imidazoles (RaaiR/, 1) and have characterised many of their
metal complexes [4,5]. The functionalization to pendant aryl group
has been done by ortho –SR substitution to synthesize 1-alkyl-2-
{(o-thioalkyl)phenylazo}imidazoles (2) [6]. These ligands have
potentially three coordination centres – imidazole-N (N), azo-N
(N/) and thioether-S (–SR). The ligand shows both tridentate N,
N/, S [12] and bidentate N, N/ chelation [7].
storage [13], non-linear optics [14], photoswitching [15] and
molecular-photonic devices [16]. Photochromic compounds have
distinct absorption spectral change when excited at least in one
pathway by light. One of the important problems of organic
photochrome is ‘‘fatigue’’ and low durability where ‘‘fatigue’’ re-
fers to photodegradation, photobleaching, photooxidation, and
other chemical degradation and physical deformation or side reac-
tions. The attachment of a metal ion to photochromic organic
compounds such as azobenzene derivatives or diarylethens can
reduce fatigue and enhances the stability of the excited states.
In continuation of our effort, this work refers to the photoisomer-
isation of the cadmium(II)-SRaaiNR/ complexes. The ligand is
hooked to Cd(II) via imidazolyl-N coordination and the hanging
thioalkyl-arylazoimdazole undergoes reversible photoisomerisa-
tion. Ligand, SRaaiNR/, has three potential donor centres – N(imid-
azole), N(azo) and –S-R and can serve as tridentate-N,N/,S [6] and
bidentate-N,N/ [7] donor agents. The single crystal X-ray structure
determination of representative complexes show monodentate
ligating behaviour of SRaaiNR/ to Cd(II) and may be more flexible
to isomerisation by light irradiation relative to the chelated sys-
tem [6,7]. The effect of Cd-X (X = Cl, Br, I) and number of coordi-
nated SRaaiNR/ on photoisomerisation rate and quantum yields
have been discussed along with the addition of excess
cadmium(II) salt.
Arylazoimidazoles and some of their coordination complexes
show reversible transformation between trans-form and cis-form
upon UV light irradiation [8–11]. The phenomenon is defined as
photochromism. The photochromic compounds have attracted
⇑
Corresponding author. Fax: +91 2414 6584.
E-mail address: c_r_sinha@yahoo.com (C. Sinha).
Present address: Department of Chemistry, Kalyani Govt. Engg. College, India
1
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.07.061

82
S. Saha (Halder) et al. / Polyhedron 46 (2012) 81–89
m
(C@N), 1646 cmꢀ1
.
UV–Vis spectroscopic data in CH3CN
2. Experimental
2.1. Material
(kmax(nm)(10ꢀ3 2 (dm3 molꢀ1cmꢀ1): 371 (22.15), 414 (19.06)). 1H
NMR (300 MHz, CDCl3), d(ppm), (J(Hz)): 7.40 (bs, 4-H), 7.23 (bs,
5-H), 7.36 (d, 7.00 Hz, 11-H), 7.55 (m, 9- and 10-H), 7.84 (d,
8.00 Hz), 4. 72 (s, 1-CH3), 2.55 (s, S-CH3). [Cd(SMeaaiNEt)2Br2]
(4b) Found: C, 37.69; H, 3.67; N, 14.62%. Anal. Calc. for C24H28N8S2-
CdCl2, CdBr2 and CdI2 were obtained from Loba Chemicals,
Bombay, India 1-alkyl-2-{(o-thio alkyl)phenyl azo}imidazole were
synthesized by reported procedure [6]. All other chemicals and sol-
vents were reagent grade as received. Cd(ClO4)2 6H2O was pre-
pared from CdCO3 and perchloric acid.
Br2Cd C, 37.68; H, 3.66; N, 14.65. FT-IR (KBr disc, cmꢀ1),
1416;
m(N@N),
m
(C@N), 1645 cmꢀ1. UV–Vis spectroscopic data in CH3CN
(kmax(nm)(10ꢀ3 2 (dm3 molꢀ1cmꢀ1): 372 (28), 414 (22.01). 1H
NMR (300 MHz, CDCl3), d(ppm), (J(Hz)): 7.41 (bs, 4-H), 7.20 (bs,
5-H), 7.33 (d, 7.00 Hz, 8-H), 7.52 (m, 9- and 10-H), 7.88 (d,
8.00 Hz,11-H), 4.47 (q, 7.00 Hz, 1-CH2), 1.69 (t, 7.29 Hz, (1-CH2)-
CH3)), 2.64 (s, S-CH3). [Cd(SEtaaiNMe)2Br2] (4c) Found: C, 37.67;
H, 3.65; N, 14.63%. Anal. Calc. for C24H28N8S2Br2Cd C, 37.68; H,
2.2. Physical measurements
Microanalytical data (C, H, N) were collected on Perkin-Elmer
2400 CHNS/O elemental analyzer. Spectroscopic data were
obtained using the following instruments: UV–Vis spectra from a
Perkin Elmer Lambda 25 spectrophotometer; IR spectra (KBr disk,
4000–200 cmꢀ1) from a Perkin Elmer RX-1 FTIR spectrophotome-
ter; photoexcitation has been carried out using a Perkin Elmer
LS-55 spectrofluorimeter and 1H NMR spectra from a Bruker (AC)
300 MHz FTNMR spectrometer.
3.66; N, 14.65. FT-IR (KBr disc, cmꢀ1),
1645 cmꢀ1
UV–Vis spectroscopic data in CH3CN (kmax(nm)
m(N@N), 1416; m(C@N),
.
(10ꢀ3 2 (dm3 molꢀ1cmꢀ1): 373 (27), 415 (22.3). 1H NMR
(300 MHz, CDCl3), d(ppm), (J(Hz)): 7.42 (bs, 4-H), 7.20 (bs, 5-H),
7.35 (d, 7.00 Hz, 8-H), 7.56 (m, 9- and 10-H), 7.83 (d, 8.05 Hz,
11-H), 4.30 (s, 1-CH3), 3.00 (q, 7.35 Hz, S-CH2), 1.30 (6.17 Hz ,
(S-CH2)-CH3). [Cd(SEtaaiNEt)2Br2] (4d) Found: C, 39.39; H, 4.0; N,
14.12%. Anal. Calc. for C26H32N8S2Br2Cd C, 39.38; H, 4.03; N,
2.3. Synthesis
14.13. FT-IR (KBr disc, cmꢀ1), (C@N), 1649 cmꢀ1
m(N@N), 1428; m .
UV–Vis spectroscopic data in CH3CN (kmax(nm)(10ꢀ3 2 (dm3
molꢀ1cmꢀ1): 373 (29.56), 415 (28.62). 1H NMR (300 MHz, CDCl3),
d(ppm), (J(Hz)): 7.41 (bs, 4-H), 7.22 (bs, 5-H), 7.32 (d, 7.00 Hz),
7.53 (m, 9- and 10-H), 7.86 (d, 8.00 Hz, 11-H), 4.52 (q, 7.25 Hz, 1-
CH2), 1.62 (t, 7.24 Hz, (1-CH2)-CH3), 3.04 (q, 7.30 Hz, S-CH2), 1.35
(t, 6.16 Hz, (S-CH2)-CH3). [Cd(SMeaaiNMe)2I2] (5a), Found: C,
31.75; H, 2.84; N, 13.52. Anal. Calc. for C22H24N8S2I2Cd: C, 31.79;
2.3.1. Synthesis of [Cd(SEtaaiNEt)2I2] (5d)
1-Ethyl-2-{(o-thioethyl)phenylazo}imidazole(43 mg, 0.17 mmol)
in CH3CN was added in drops to MeOH solution of CdI2 (30 mg,
0.08 mmol) was stirred for 2 h. The resultant reddish solution was
collected by filtration; slow evaporation of the solution gives the
red crystal. The yield was 46.46 mg (64%). Other complexes were
prepared under identical conditions from MeOH/MeCN solution
and the yield varied in the range 60–70%.
Microanalytical data of the complexes are as follows:
[Cd(SMeaaiNMe)2Cl2] (3a) Found: C, 40.8; H, 3.6; N, 17.31%. Anal.
Calc. for C22H24N8S2Cl2Cd C, 40.79; H, 3.7; N, 17.30. FT-IR (KBr disc,
H, 2.89; N, 13.49. FT-IR (KBr disc, cmꢀ1),
1619 cmꢀ1
m(N@N), 1417;
m(C@N),
.
UV–Vis spectroscopic data in CH3CN (kmax(nm)
(10ꢀ3 2 (dm3 molꢀ1cmꢀ1): 373 (18.26), 415 (16.92). 1H NMR
(300 MHz, CDCl3), d(ppm), (J(Hz)): 7.41 (bs, 4-H), 7.28 (bs, 5-H),
7.31 (d, 7.00 Hz, 11-H), 7.55 (m, 9- and 10-H), 7.80 (d, 8.00 Hz),
4.14 (s, 1-CH3), 2.52 (s, S-CH3). [Cd(SMeaaiNEt)2I2] (5b) Found: C,
33.57; H, 3.20; N, 13.1. Anal. Calc. for C24H28N8S2I2Cd C, 33.55; H,
cmꢀ1),
m(N@N), 1426; m
(C@N), 1660 cmꢀ1. UV–Vis spectroscopic
data in CH3CN (kmax(nm) (10ꢀ3 2 (dm3 molꢀ1cmꢀ1): 372 (23), 415
(20). 1H NMR (300 MHz, CDCl3), d(ppm), (J(Hz)): 7.41 (bs, 4-H),
7.24 (bs, 5-H), 7.35 (d, 7.00 Hz, 8-H), 7.44 (m, 9- and 10-H), 8.10
(d, 8.00 Hz, 11-H), 4.32 (s, 1-CH3), 2.60 (s, S-CH3). [Cd(SMeaaiNEt)2
Cl2] (3b) Found: C, 42.66; H, 4.15; N, 16.56%. Anal. Calc. for C24H28
N8S2Cl2Cd C, 42.65; H, 4.14; N, 16.58. FT-IR (KBr disc, cmꢀ1),
3.26; N, 13.05. FT-IR (KBr disc, cmꢀ1),
1622 cmꢀ1
m
(N@N), 1428;
m(C@N),
.
UV–Vis spectroscopic data in CH3CN (kmax(nm)
(10ꢀ3 2 (dm3 molꢀ1cmꢀ1): 372 (27.05), 416 (21.59). 1H NMR
(300 MHz, CDCl3), d(ppm), (J(Hz)): 7.40 (bs, 4-H), 7.23 (bs, 5-H),
7.34 (d, 7.00 Hz, 8-H), 7.56 (m, 9- and 10-H), 7.82 (d, 8.00 Hz,11-
H), 4.51 (q, 7.00 Hz, 1-CH2), 1.63 (t, 7.30 Hz, (1-CH2)-CH3)), 2.59
(s, S-CH3). [Cd(SEtaainMe)2I2] (5c) Found: C, 33.47; H, 3.0; N,
13.00%. Anal. Calc. C24H28N8S2I2Cd C, 33.49; H, 3.10; N, 13.05. FT-
m
(N@N), 1420; m
(C@N), 1662 cmꢀ1. UV–Vis spectroscopic data in
CH3CN (kmax(nm)(10ꢀ3 2 (dm3 molꢀ1cmꢀ1): 372 (26), 414 (22).
1H NMR (300 MHz, CDCl3), d(ppm), (J(Hz)): 7.42 (bs, 4-H), 7.23
(bs, 5-H), 7.32 (d, 7.00 Hz, 8-H), 7.43 (m, 9- and 10-H), 7.98 (d,
8.00 Hz,11-H), 4.48 (q, 6.62 Hz, 1-CH2), 1.65 (t, 7.27 Hz, (1-CH2)-
CH3)), 2.67 (s, S-CH3).. [Cd(SEtaaiNMe)2Cl2] (3c) Found: C, 42.67;
H, 4.13; N, 16.57%. Anal. Calc. for C24H28N8S2Cl2Cd C, 42.65; H,
IR (KBr disc, cmꢀ1),
m(N@N), 1427; m
(C@N), 1620 cmꢀ1. UV–Vis
spectroscopic data in CH3CN (kmax(nm)(10ꢀ3 2 (dm3 molꢀ1cmꢀ1):
371 (26.00), 414 (22.30). 1H NMR (300 MHz, CDCl3), d(ppm),
(J(Hz)): 7.43 (bs, 4-H), 7.27 (bs, 5-H), 7.32 (d, 7.00 Hz, 8-H), 7.55
(m, 9- and 10-H), 7.81 (d, 8.05 Hz, 11-H), 4.17 (s, 1-CH3), 3.00 (q,
7.30 Hz, S-CH2), 1.29 (t, 7.00 Hz, (S-CH2)-CH3). [Cd(SEtaaiNEt)2I2]
(5d) Found: C, 35.1; H, 3.62; N, 12.64%. Anal. Calc. for C26H32N8S2
4.14; N, 16.58. . FT-IR (KBr disc, cmꢀ1),
1660 cmꢀ1
UV–Vis spectroscopic data in CH3CN (kmax(nm)
m(N@N), 1427; m(C@N),
.
(10ꢀ3 2 (dm3 molꢀ1cmꢀ1): 378 (23.35), 414 (25.56). 1H NMR
(300 MHz, CDCl3), d(ppm), (J(Hz)): 7.40 (bs, 4-H), 7.23 (bs, 5-H),
7.34 (d, 7.00 Hz, 8-H), 7.46 (m, 9- and 10-H), 7.94 (d, 7.90 Hz,
11-H), 4.28 (s, 1-CH3), 3.01 (q, 7.20 Hz, S-CH2), 1.31 (t, 7.35 Hz,
(S-CH2)-CH3). [Cd(SEtaaiNEt)2Cl2] (3d) Found: C, 44.35; H, 4.55;
N, 15.93%. Anal. Calc. for C26H32N8S2Cl2Cd C, 44.36; H, 4.54; N,
I2Cd C, 35.20; H, 3.61; N, 12.63. FT-IR (KBr disc, cmꢀ1),
1427;
m(N@N),
m
(C@N), 1620 cmꢀ1. UV–Vis spectroscopic data in CH3CN
(kmax(nm)(10ꢀ3 2 (dm3 molꢀ1cmꢀ1): 372 (26.10), 412 (24.35). 1H
NMR (300 MHz, CDCl3), d(ppm), (J(Hz)): 7.42 (bs, 4-H), 7.22 (bs,
5-H), 7.33 (d, 7.00 Hz), 7.56 (m, 9- and 10-H), 7.80 (d, 8.10 Hz,
11-H), 4.44 (q, 7.25 Hz, 1-CH2), 1.61 (t, 7.25 Hz, (1-CH2)-CH3),
3.05 (q, 7.40 Hz, S-CH2), 1.30 (t, 5.25 Hz, (S-CH2)-CH3).
15.92. FT-IR (KBr disc, cmꢀ1), (C@N), 1661 cmꢀ1
m(N@N), 1428; m .
UV–Vis spectroscopic data in CH3CN (kmax(nm)(10ꢀ3 2 (dm3
molꢀ1cmꢀ1): 371 (32.58), 413 (28.86). 1H NMR (300 MHz, CDCl3),
d(ppm), (J(Hz)): 7.41 (bs, 4-H), 7.22 (bs, 5-H), 7.35 (d, 7.00 Hz),
7.45 (m, 9- and 10-H), 7.97 (d, 8.00 Hz, 11-H), 4.59 (q, 6.68 Hz, 1-
CH2), 1.60 (t, 7.23 Hz, (1-CH2)-CH3), 3.04 (q, 7.20 Hz, S-CH2), 1.39
(t, 7.36 Hz, (S-CH2)-CH3). [Cd(SMeaaiNMe)2Br2] (4a) Found: C,
35.86; H, 3.22; N, 15.20%. Anal. Calc. for C22H24N8S2Br2Cd C,
2.3.2. Synthesis of [Cd(SEtaaiNEt)4](ClO4)2 (6d)
1-Ethyl-2-{(o-thioethyl)phenylazo}imidazole (86 mg, 0.32 mmol)
in MeOH was added in drops to aqueous solution of Cd(ClO4)2, 6
H2O (30 mg, 0.08 mmol) was stirred for 2 h. The resultant reddish
solution was collected by filtration; slow evaporation of the solu-
tion gives the red crystal. The yield was 61.87 mg (64%). Other
35.85; H, 3.25; N, 15.21. FT-IR (KBr disc, cmꢀ1),
m(N'N), 1415;

S. Saha (Halder) et al. / Polyhedron 46 (2012) 81–89
83
1089–1114 cmꢀ1 UV–Vis spectroscopic data in CH3CN (kmax
(nm)(10ꢀ3 2 (dm3 molꢀ1cmꢀ1): 371 (49.26), 411 (39.76). 1H NMR
(300 MHz, CDCl3), d(ppm), (J(Hz)): 7.09 (bs, 4-H), 6.90 (bs, 5-H),
6.98 (d, 7.00 Hz), 7.43 (m, 9- and 10-H), 7.66 (d, 8.10 Hz, 11-H),
4.46 (q, 7.10 Hz, 1-CH2), 1.46 (t, 7.20 Hz, (1-CH2)-CH3), 2.96 (q,
7.30 Hz, S-CH2), 1.26 (t, 5.50 Hz, (S-CH2)-CH3).
Table 1
Summarized crystallographic data for [Cd(SEtaaiNEt)2I2] (5d) and [Cd(SEtaaiNEt)4]
(ClO4)2 (6d).
Parameters
[Cd(SEtaaiNEt)2I2] (5d)
26H32N8S2I2Cd
[Cd(SEtaaiNEt)4](ClO4)2 (6d)
Empirical
formula
Formula
weight
Temperature
(K)
C
C52H64N16O8S4Cl2Cd
886.92
100(2)
1352.79
295(2)
2.4. X-ray diffraction study
Crystal system
Space group
Crystal size
(mm)
monoclinic
P21/n
0.20 ꢁ 0.20 ꢁ 0.10
triclinic
Single crystals suitable for data collection were grown from slow
evaporation of DMF solution of the complexes in methanol. The
crystal data and details of the data collections are given in Table 1.
A suitable single crystal of the complexes (5d: 0.20 ꢁ 0.20 ꢁ
0.10 mm and 6d: 0.20 ꢁ 0.10 ꢁ 0.10 mm) was mounted on a Bruker
ꢀ
P1
0.20 ꢁ 0.10 ꢁ 0.10
Unit cell dimensions
a (Å)
b (Å)
c (Å)
15.598(3)
12.5810(9)
13.2964(9)
19.5999(13)
75.0500(10)
83.8280(10)
82.2740(10)
3129.9(4)
2
11.355(2)
19.700(4)
90.00
111.998(5)
90.00
3235.3(10)
4
0.71073
2.742
SMART APEX CCD diffractometer (graphite monochromated MoK
a
0
radiation, k = 0.71073 ÅA) and data were collected by use of
x scans.
a
(°)
Unit cell parameters were determined from least-squares refine-
ment of setting angles (h) within the range 1.43 6 h 6 26.37° (5d)
and 1.08 6 h 6 28.33° (6d). Data were corrected for Lorentz
polarisation effects and for linear decay. Semi-empirical absorption
b (°)
c
(°)
V (Å)3
Z
k (Å)
0.71073
0.631
corrections based on
W-scans were applied. The structures were
l
(Mo-K
a
)
)
(mmꢀ1
solved by direct method using SHELXS-97 [17] and successive
difference Fourier syntheses. All non-hydrogen atoms were refined
anisotropically. The hydrogen atoms were fixed geometrically and
refined using the riding model. All calculation was carried out using
Dcalc (mgmꢀ3
)
1.821
1.435
hkl Range
ꢀ15 6 h 6 19,
ꢀ14 6 k 6 14, ꢀ24 6 l 6 24
1.43–26.37
ꢀ16 6 h 6 16,
ꢀ17 6 k 6 17, ꢀ25 6 l 6 26
1.08–28.33
h Range (°)
Refine
SHELXL-97 [18], ORTEP-32 [19] and PLATON-99 [20] programs.
356
748
parameters
Unique data
6599
4517
2.5. Photometric measurements
[I > 2
r
r
(I)]
(I)]
a
R1 [I > 2
0.0420
0.1283
0.0456
0.1147
0.693
b
Absorption spectra were taken with a PerkinElmer Lambda 25
UV/VIS Spectrophotometer in a 1 ꢁ 1 cm quartz optical cell main-
tained at 25 °C with a Peltier thermostat. The light source of a Perk-
inElmer LS 55 spectrofluorimeter was used as an excitation light,
with a slit width of 10 nm. An optical filter was used to cut off
overtones when necessary. The absorption spectra of the cis iso-
mers were obtained by extrapolation of the absorption spectra of
a cis- rich mixture for which the composition is known from 1H
NMR integration. Quantum yields (/) were obtained by measuring
wR2
Goodness of fit 1.07
P
P
a
R = |F0 ꢀ Fc|/ F0.
P
P
b
2
4 1/2
wR = [ w(F0 ꢀ Fc2)/ wF0
]
are general but
w are different, w = 1/
[
r
2(F2) + (0.0527P)2 + 2.3556P] for (5d); w = 1/[
r
2(F2) + (0.0980P)2] for (6d) where
P = (F02 + 2Fc2)/3.
complexes were prepared under identical conditions from MeOH/
H2O solution and the yield varied in the range 60–70%.
initial trans-to-cis isomerization rates (
within the above instrument using the equation,
m) in a well-stirred solution
[Cd(SMeaaiNMe)4](ClO4)2 (6a) Found: C, 42.61; H, 3.86; N,
18.08%. Anal. Calc. for C44H48N16S4Cl2O8Cd C, 42.60; H, 3.87; N,
m
¼ ð/I0=VÞð1 ꢀ 10ꢀAbsÞ:
18.07. FT-IR (KBr disc, cmꢀ1), v(N@N), 1423; (C@N), 1651 cmꢀ1
m ,
m
(ClO4), 1088–1118 cmꢀ1 UV–Vis spectroscopic data in CH3CN
(kmax(nm)(10ꢀ3 2 (dm3 molꢀ1cmꢀ1): 371 (78.85), 413(61.55). 1H
NMR (300 MHz, CDCl3), d(ppm), (J(Hz)): 7.10 (bs, 4-H), 6.92 (bs,
5-H), 6.98 (d, 7.00 Hz, 11-H), 7.47 (m, 9- and 10-H), 7.73 (d,
8.00 Hz), 4.18 (s, 1-CH3), 2.54 (s, S-CH3). [Cd(SMeaaiNEt)4](ClO4)2
(6b) Found: C, 44.48; H, 4.33; N, 17.28%. Anal. Calc. for C48H56
N16S4Cl2O8Cd C, 44.47; H, 4.32; N, 17.29. FT-IR (KBr disc, cmꢀ1),
where I0 is the photon flux at the front of the cell, V is the volume of
the solution, and Abs is the initial absorbance at the irradiation
wavelength. The value of I0 was obtained by using azobenzene
(/ = 0.11 for
conditions.
p
ꢀ
p⁄ excitation [21]) under the same irradiation
The Z-to-E isomerisation has been carried out at temperature
range 298–321 K. The rates of thermal isomerisation were
obtained by monitoring absorption changes intermittently for a
cis-rich solution kept in the dark at constant temperatures (298–
321 K). The activation energy (Ea) and the frequency factor (A)
were obtained from the Arrhenius plot, lnk = lnA ꢀ Ea/RT where k
is the measured rate constant, R is the gas constant, and T is
m
(N@N), 1426; m , m
(C@N), 1655 cmꢀ1 (ClO4), 1087–1088 cmꢀ1
UV–Vis spectroscopic data in CH3CN (kmax(nm)(10ꢀ3 2 (dm3
molꢀ1cmꢀ1): 373 (68), 415 (62.21). 1H NMR (300 MHz, CDCl3),
d(ppm), (J(Hz)): 7.12 (bs, 4-H), 6.95 (bs, 5-H), 7.00 (d, 7.00 Hz, 8-
H), 7.46 (m, 9- and 10-H), 7.71 (d, 8.00 Hz,11-H), 4.481 (q,
7.00 Hz, 1-CH2), 1.52 (t, 7.20 Hz, (1-CH2)-CH3)), 2.52 (s, S-CH3).
[Cd(SEtaaiNMe)4](ClO4)2 (6c) Found: C, 44.46; H, 4.31; N, 17.3%.
Anal. Calc. for C48H56N16S4Cl2O8Cd C, 44.47; H, 4.32; N, 17.29. FT-
temperature. The values of activation free energy (D
G⁄) and activa-
tion entropy
D
and h are Boltzmann’s and Plank’s.
(D
S⁄) were obtained through the relationships,
G⁄ = Ea ꢀ RTꢀT
D
S⁄ and
D
S⁄ = [lnA ꢀ 1 ꢀ ln(kBT/h)/R where kB
IR (KBr disc, cmꢀ1),
m(N@N), 1425; m , m(ClO4),
(C@N), 1653 cmꢀ1
1088 cmꢀ1 UV–Vis spectroscopic data in CH3CN (kmax(nm)
(10ꢀ3 2 (dm3 molꢀ1cmꢀ1): 372 (65), 413 (66). 1H NMR (300 MHz,
CDCl3), d(ppm), (J(Hz)): 7.09 (bs, 4-H), 6.90 (bs, 5-H), 6.97 (d,
7.00 Hz, 8-H), 7.45 (m, 9- and 10-H), 7.67 (d, 8.00 Hz, 11-H), 4.15
(s, 1-CH3), 2.70 (q, 7.30 Hz, S-CH2), 1.23 (t, 7.00 Hz, (S-CH2)-CH3).
[Cd(SEtaaiNEt)4](ClO4)2 (6d) Found: C, 46.16; H, 4.74; N, 16.55%.
Anal. Calc. for C52H64N16S4Cl2O8Cd C, 46.17; H, 4.73; N, 16.57. FT-
3. Results and discussion
3.1. The formulation of the complexes
o-(Thioalkyl)phenyldiazonium ion is coupled with imidazole in
sodium carbonate solution to synthesise 2-{(o-thioalkyl)pheny-
IR (KBr disc, cmꢀ1),
m(N@N), 1427; m , m(ClO4),
(C@N), 1652 cmꢀ1

84
S. Saha (Halder) et al. / Polyhedron 46 (2012) 81–89
N
N
N
N
N
R'
R'
N
N
N
S
R''
R
1
2
12
9
2+
8
8
R
9
10
11
R
R'
10
S
S
R'
R'
11
R
S
N
N
N
N
N
N
N
N
5
N
X
N
5
N
N
Fig. 2. ORTEP diagram of [Cd(SEtaaiNEt)4](ClO4)2 (6d) with 30% atomic probability
(for clarity we have used to label all atoms of one of the four similar ligands and
hydrogen atoms are omitted).
Cd
N
4
4
Cd
4
X
N
N
N
N
N
N
N
5
R'
S
N
N
N
11
N
10
9
R'
Table 2
R'
Selected bond distances and bond angles of [Cd(SEtaaiEt)2I2] (5d).
S
R
S
8
R
Bond lengths (Å)
Bond angles (°)
R
Cd(1)–I(1)
Cd(1)–I(2)
Cd(1)–N(2)
Cd(1)–N(2/)
N(3)–N(4)
N(3/)–N(4/)
2.7670(9)
2.7554(7)
2.251(5)
2.253(5)
1.264(6)
1.278(7)
I(1)–Cd(1)–I(2)
I(1)–Cd(1)–N(2)
I(1)–Cd(1)–N(2/)
I(2)–Cd(1)–N(2)
I(2)–Cd(1)–N(2/)
N(2)–Cd(1)–N(2/)
109.85(2)
97.06(12)
110.12(13)
106.24(12)
99.15(13)
133.52(17)
Scheme 1. The ligands and the complexes.
Table 3
Selected bond distances (Å) and angles (°) for [Cd(SEtaaiNEt)4](ClO4)2 (6d).
Bond distances (Å)
Bond angles (°)
Cd–N(1)
Cd–N(5)
Cd–N(9)
2.250(5)
2.279(5)
2.241(4)
2.269(4)
1.261(5)
1.270(6)
1.254(5)
1.169(4)
1.355(6)
1.308(7)
1.273(8)
1.383(9)
1.421(12)
N(1)–Cd–N(9)
N(1)–Cd–N(5)
N(1)–Cd–N(13)
N(1)–Cd–N(16)
N(9)–Cd–N(16)
N(5)–Cd–N(13)
N(9)–Cd–N(5)
N(9)–Cd–N(13)
N(1)–Cd–N(8)
N(13)–Cd–N(8)
N(5)–Cd–N(8)
N(16)–Cd–N(8)
N(13)–Cd–N(16)
98.50(17)
130.22(18)
103.67 (16)
83.34(17)
79.70(16)
100.32(16)
96.85(16)
131.84(16)
80.62(15)
78.50(15)
62.36(15)
131.49(15)
61.45(17)
Cd–N(13)
N(3)–N(4)
N(7)–N(8)
N(11)–N(12)
N(15)–N(16)
Cl(1)–O(1)
Cl(1)–O(2)
Cl(1)–O(3)
Cl(2)–O(5)
Cl(2)–O(6)
The reaction of CdX2 (X = Cl, Br, I) with SRaaiNR/ in CH3CN
(1:2 mol proportion) isolates dark coloured crystalline compounds
of composition [Cd(SRaaiNR/)2X2] (3–5). The reaction of Cd(ClO4)2
6H2O with SRaaiNR/ in CH3CN has synthesized [Cd(SRaaiNR/)4]
(ClO4)2 (6). The composition of the complexes has been confirmed
by microanalytical data and by other spectroscopic information.
The structures have been established in representative cases by
single-crystal X-ray diffraction studies.
Fig. 1. Crystal structure of [Cd(SEtaaiNEt)2I2] (5d).
The IR spectra of the complexes in KBr disc show moderately
lazo}imidazole. It is then undergone alkylation by adding alkyl
iodide (MeI, EtI) in dry THF solution in presence of NaH to
synthesise 1-alkyl-2-{(o-thioalkyl)phenylazo}imidazoles [SRaaiNR/
(R = R/ = Me (2a); R = Me, R/ = Et (2b); R = Et, R/ = Me (2c);
R = R/ = Et (2d)]. The compounds are then purified by column
chromatography.
intense stretching at 1620–1650 and 1415–1430 cmꢀ1 and are
assigned to m(C@N) and m(N@N), respectively. The m(ClO4) of 6 ap-
pears at 1110–1100 and 1080–1085 cmꢀ1 defining less symmetric
ꢀ
tetrahedral symmetry of hydrogen bonded ClO4 ion [22]. Other
vibrations are shifted to lower frequency in the complexes com-
pared to the free ligand values [23].

S. Saha (Halder) et al. / Polyhedron 46 (2012) 81–89
85
N
N
N
N
N
322
N
N
N
N
N
N
R
Cd
Cd
S
R
R
R
N
N
N
N
N
S
S
S
R
R
R
R
E- or trans-
Z- or cis-
Free SRaaiNR/
Coordinated SRaaiNR/
Scheme 2. Photochromism of free and coordinated SRaaiNR.
decreasing
decreasing
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
trans
cis
trans
0.75
0.60
0.45
0.30
0.15
0.00
342
342
335
335nm
0.24
0.20
0.16
0.12
0.08
0.04
0.00
cis
0.2
0.1
0.0
300
500
400
300
400
500
600
Wavelength (nm)
Wavelength (nm)
300
400
500
600
Wavelength (nm)
300
350
400
450
500
550
Wavelength (nm)
Fig. 4. Spectral changes of E (trans) ? Z (cis) isomerisation of coordinated SEtaaiNEt
at [Cd(SEtaaiNEt)2I2] (5d) in MeCN upon repeated irradiation at 372 nm at 3 min
interval at 25 °C.
Fig. 3. Spectral changes due to E ? Z isomerisation of SEtaaiNEt in MeCN upon
repeated irradiation at 357 nm at 3 min interval at 25 °C.
tively. The bond parameters are given in Tables 2 and 3. The struc-
ture of 5d shows the CdN2I2 coordination sphere and that of 6d is
CdN4 those are arranged in a distorted tetrahedral geometry.
1-Ethyl-2-{(o-thioethyl)phenylazo}imidazole (SEtaaiNEt) has three
donor centres imidazolyl-N, azo-N and –S-Et. The structures (Figs. 1
and 2) show that SEtaaiNEt hooks to Cd(II) by imidazolyl-N donor
centre and two other centres are freely hanging. The examples of
metal complexes where SRaaiNR/ act as tridentate [6] and
didentate [7] are known; however, monodentate coordination
complexes are scarce [4]. Uncoordinated donor centres in the mul-
tidentate ligands are important for the execution of metallo-ligand
activity and synthesis of multimetallic complexes [24] for different
reasons like catalyses [25]. Four Cd-N(imidazole) bond lengths in
6d are unequal and spanning 2.241–2.279 Å. Forced chelation of
N(azo) to Cd(II) shows appreciably long bond distance in the
complexes; the Cdꢂ ꢂ ꢂN(azo) lengths in 5d are Cd-N(4), 2.831(2);
Cd-N(4/) 2.958(4) Å and in 6d are Cdꢂ ꢂ ꢂN(4), 2.988; Cdꢂ ꢂ ꢂN(12),
3.009 Å. The sum of the van der Waals radii, Cdꢂ ꢂ ꢂN(sp2) is 3.13 Å
[26] and the chelate angle, N(imidazole)–Cd–N(azo) [N(2)–Cd–
N(4), 68.72(1); N(2/)–Cd–N(4/) 61.21(4)o] is smaller than previous
report [10] which has supposed to conclude the no chelation of
N(azo)/-SEt. The preferential bonding of imidazolyl-N to Cd(II) sup-
ports the strong toxicity of Cd(II) [27] because imidazole is an inte-
gral component of most of the biomolecules. The imidazolyl group
is deviated by 24° from the plane of phenyl-thioalkyl group. Two
ligands of coordinated SEtaaiNEt in 5d make a dihedral angle
78.4(6)°. The coordination environment about Cd in 6d is com-
pressed along the crystallographic c-axis in such a way that two
of the N–Cd–N angles are lower than tetrahedral angle with a value
of N(1)–Cd–N(9), 98.50(17) and N(5)–Cd–N(13), 100.32(16)°. Two
other N–Cd–N angles show higher value [N(1)–Cd–N(5),
The 1H NMR spectra of complexes are recorded in CDCl3 and the
signals are assigned unambiguously via spin–spin interaction, the
effect of substitutions therein, and on comparing the reported
cases [6,7]. The atom numbering pattern is shown in the Scheme 1.
Imidazolyl 4- and 5-H show broad singlet at 7.4 and 7.2 ppm,
respectively for the complexes [Cd(SRaaiNR/)2X2] (3–5) and are
downfield shifted by 0.1–0.2 ppm compared to free ligand data
[6]. The broadening may be considered of rapid proton exchange
between these imidazole protons and/or exchange with solvent
proton. The complexes [Cd(SRaaiNR/)4](ClO4)2 (6) show d(4-H)
and d(5-H) at 7.1 and 6.9 ppm, respectively. The coordination of
four bulky SRaaiNR/ to Cd(II) may reduce the bonding interaction
between metal ion and imidazole-N and causes insufficient down-
field shift. Phenyl protons (8- to 11-H) of –S-(R)-C6H4 remain al-
most unperturbed. This spectral information supports the single
crystal structures given in Figs. 1 and 2 where SRaaiNR/ act as
monodentate imidazolyl-N donor ligand. The N(1)–Me appears as
singlet at ca. 4.2 ppm; N–CH2–CH3 shows a quartet for –CH2– at
ca. 4.5 (7 Hz) and a triplet at ca. 1.6 (7 Hz) ppm. The thioalkyl group
S–R also exhibits a singlet signal at 2.6 ppm for S–Me of SMeaaiNR/
(3a,b–6a,b). SEtaaiNEt (3d–6d) shows two quartets [4.5 (7 Hz) and
3.00 (7 Hz) ppm for –CH2– protons of N–CH2–(CH3) and S–CH2–
CH3), respectively] and two triplets [at 1.6 (7 Hz) and 1.3 (6 Hz)
ppm for –(N–CH2)-CH3 and –(S–CH2)CH3, respectively].
3.2. Molecular structures of [Cd(SEtaaiNEt)2I2] (5d) and
[Cd(SEtaaiNEt)4](ClO4)2 (6d)
The molecular structures of [Cd(SEtaaiNEt)2I2] (5d) and
[Cd(SEtaaiNEt)4](ClO4)2 (6d) are shown in Figs. 1 and 2, respec-

86
S. Saha (Halder) et al. / Polyhedron 46 (2012) 81–89
Table 4
Excitation wavelength (k ), rate of E (trans) ? Z (cis) conversion and quantum yield (UE?Z).
p,p⁄
Compound
k
(nm)
p, p⁄
Isobastic points (nm)
Rate of E ? Z conversion ꢁ 108(sꢀ1
)
(UE?Z).
SMeaaiNMe (2a)a
SMeaaiNEt (2b)a
SEtaaiNMe (2c)a
SEtaaiNEt (2d)a
357
358
357
356
372
372
373
371
371
372
373
373
373
372
371
372
371
373
372
371
337
337
336
335
344
342
343
339
338
339
336
335
342
350
341
342
340
337
338
336
4.908
3.108
4.67
0.317
0.232
0.290
0.197
0.060
0.054
0.055
0.049
0.072
0.063
0.067
0.057
0.158
0.121
0.133
0.076
0.047
0.037
0.040
0.027
2.948
0.687
0.631
0.639
0.568
0.728
0.678
0.681
0.625
1.416
1.221
1.255
1.012
0.582
0.532
0.541
0.487
[Cd(SMeaaiNMe)2Cl2] (3a)
[Cd(SMeaaiNEt)2Cl2](3b)
[Cd(SEtaaiNMe)2Cl2] (3c)
[Cd(SEtaaiNEt)2Cl2] (3d)
[Cd(SMeaaiNMe)2Br2](4a)
[Cd(SMeaaiNEt)2Br2] (4b)
[Cd(SEtaaiNMe)2Br2] (4c)
[Cd(SEtaaiNEt)2Br2] (4d)
[Cd(SMeaaiNMe)2I2] (5a)
[Cd(SMeaaiNEt)2I2] (5b)
[Cd(SEtaaiNMe)2I2] (5c)
[Cd(SEtaaiNEt)2I2] (5d)
[Cd(SMeaaiNMe)4](ClO4)2 (6a)
[Cd(SMeaaiNEt)4] (ClO4)2 (6b)
[Cd(SEtaaiNMe)4](ClO4)2](6c)
[Cd(SEtaaiNEt)4] (ClO4)2 (6d)
a
Data are obtained from Ref. [17].
increasing
trans
rate
quantum yield
I
cis
0.75
0.50
0.25
0.00
0.75
1.0
336
Br
Cl
0.50
0.25
0.00
0.5
0.0
250 300 350 400 450 500 550
Wavelength (nm)
250 300 350 400 450 500 550 600
Wave length (nm)
-0.5
2.2
2.3
2.4
2.5
2.6
2.7
2.8
Fig. 6. Spectral changes during Z (cis) ? E (trans) isomerisation of [Cd(SEtaaiNEt)2-
I2] (5d) in MeCN solution at 298 K. Z-isomer is generated by irradiation of UV light
for 45 min and allowed to thermal isomerisation. Inset picture shows spectral
change of individual isomer.
Eletronegetivity (eV)
Fig. 5. Effect of electronegativity of X in [Cd(SEtaaiNEt)2X2] on rate and quantum
yield of photoisomerisation.
130.22(18) and N(9)–Cd–N(13), 131.84(16)°]. The angular distor-
tion from regular tetrahedron geometry may be due to steric
crowding among the pendant thioethylphenylazo groups. Out of
four –N@N– distances of 6d three lengths (N(3)–N(4), 1.261(5);
N(7)–N(8), 1.270(6); N(11)–N(12), 1.254(5) Å) are comparable to
the published results [8–11]. The N(15)–N(16) is 1.169(6) Å but
not unusual since similar observation is reported in case of ionic
compounds such as [MeaaiH2(H2O)+]2[PtCl6]2ꢀ:N@N 1.214(8),
1.197(11) Å [28].
3.3. UV–Vis spectra and photochromism
The solution electronic spectra of the compounds were re-
corded in CH3CN in 200–600 nm. There are two bands in the UV–
Vis region at 360–380 and 410–416 nm. The E-to-Z (trans-to-cis)
isomerisation of the complexes (Scheme 2) has been investigated
by irradiation of UV light in acetonitrile solution (Fig. 3) and has
also compared with free ligand spectral changes (Fig. 4). Absorp-
tion spectra of the E-configuration of coordinated SRaaiNR in the
complexes, 3–6, in CH3CN are changing with isosbestic point upon
excitation to the cis configuration of the ligand. The complexes
show little degradation upon repeated irradiation at least up to
15 cycles in each case that has been verified by measuring absor-
The packing of crystal (5d) shows recognisable weak interac-
tions such as Cg(1)ꢂ ꢂ ꢂCg(1/), 3.386(2) Å (where Cg(1): N(1), C(1),
N(2), C(2), C(3); and Cg(2): N(1/), C(1/), N(2/), C(2/), C(3’); symmetry
code: 2-x, 1-y, 2-z)and C(5)–H(5)ꢂ ꢂ ꢂCg(1/) (H(5)ꢂ ꢂ ꢂCg(1/), 2.979
(2) Å;
C(5)ꢂ ꢂ ꢂCg(1/),
3.336(4) Å
and
\C(5)–H(5)ꢂ ꢂ ꢂCg(1/),
bance before and after irradiation at p ?
p⁄ band. The quantum
ꢀ
103.7(8)°). In 6d, ClO4 bridges two complex units of {Cd(SEtaai-
NEt)4]2+ via hydrogen bonds to imidazolyl group (C–H(30)) and
imidazolyl-N-CH2-CH3 (C–H(42a)) of neighbouring molecule [C–
H(30)ꢂ ꢂ ꢂO(ClO2)Oꢂ ꢂ ꢂH(42a)–C: H(30)ꢂ ꢂ ꢂO, 2.51; C(30)ꢂ ꢂ ꢂO, 3.4083;
\C–H(30)ꢂ ꢂ ꢂO, 162°; H(42a)ꢂ ꢂ ꢂO, 2.54; C(42)ꢂ ꢂ ꢂO, 3.2425; \C–
H(42a)ꢂ ꢂ ꢂO, 130°)
yields were measured for the E-to-Z (UE?Z) photoisomerisation
of these compounds in CH3CN on irradiation of UV wavelength
(Table 4). The UE?Z values are significantly dependent on the nat-
ure of the substituents, halide type and molecular weight [29]. In
presence of –Me and –Et group quantum yield values reduce in
both cases compare to free ligands.

S. Saha (Halder) et al. / Polyhedron 46 (2012) 81–89
87
Table 5
Rate and activation parameters for Z (cis) ? E (trans) thermal isomerisation.
Compd
Temp (K)
Rate of thermal Z ? E conversion ꢁ 104 (sꢀ1
)
Ea (kJ molꢀ1
)
D
H⁄ (kJ molꢀ1
)
D
S⁄ (J molꢀ1Kꢀ1
)
D )
G⁄ (kJ molꢀ1
2aa
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
306
311
316
321
306
311
316
321
306
311
316
321
306
311
316
321
306
311
316
321
306
311
316
321
306
311
316
321
306
311
316
321
306
311
316
321
306
311
316
321
306
311
316
321
306
311
316
321
306
311
316
321
306
311
316
321
306
311
4.329
5.295
6.191
7.091
4.614
5.598
6.293
7.269
4.425
5.123
5.985
6.729
4.625
5.123
5.985
6.589
5.801
6.599
7.31
8.232
6.081
6.789
7.322
8.296
6.121
6.809
7.47
8.436
6.416
7.088
7.512
8.322
5.99
6.798
7.688
8.612
6.151
6.852
7.719
8.701
6.171
6.899
7.749
8.751
6.457
7.312
7.65
25.42
22.88
20.45
19.38
16.34
16.22
13.84
14.62
11.09
17.2
ꢀ232.41
ꢀ240.02
ꢀ244.04
ꢀ253.96
ꢀ254.06
ꢀ261.48
ꢀ258.90
ꢀ270.01
ꢀ250.61
ꢀ253.29
ꢀ253.01
ꢀ266.33
ꢀ249.41
ꢀ251.21
ꢀ250.14
ꢀ264.49
ꢀ257.90
ꢀ266.09
ꢀ265.26
92.13
93.29
94.45
95.62
91.97
93.17
94.37
95.57
92.11
93.33
94.55
95.77
92.02
93.29
94.56
95.83
77.76
79.03
80.23
81.57
80.026
81.33
82.64
83.95
79.24
80.53
81.83
83.12
82.63
83.98
85.33
86.68
76.71
77.96
79.21
80.46
77.52
78.79
80.06
81.32
77.44
78.70
79.97
81.23
81.51
82.84
84.17
85.50
76.34
77.59
78.83
80.08
76.89
78.14
79.4
2ba
22.99
21.92
18.88
18.82
16.44
17.22
13.69
19.80
18.93
19.01
14.78
21.83
19.599
19.92
15.37
17.61
14.50
15.24
2ca
2da
3a
3b
3c
3d
4a
4b
16.33
16.4
4c
4d
12.18
19.22
16.7
8.6
[Cd(SMeaaiNMe)2I2] (5a)
5.847
6.660
7.654
8.715
6.089
6.799
7.64
5b
5c
5d
6a
6b
6c
8.74
6.1
80.65
76.56
77.81
79.06
80.32
80.95
82.27
83.59
84.92
78.93
80.22
81.51
82.80
81.43
82.76
84.1
17.32
12.76
15.01
12.39
12.64
6.841
7.722
8.799
6.395
7.259
7.622
8.61
5.917
6.599
7.251
8.216
6.195
6.783
7.246
8.234
6.201
6.833
85.43
81.18
82.51
(continued on next page)

88
S. Saha (Halder) et al. / Polyhedron 46 (2012) 81–89
Table 5 (continued)
Compd
Temp (K)
Rate of thermal Z ? E conversion ꢁ 104 (sꢀ1
)
Ea (kJ molꢀ1
)
D
H⁄ (kJ molꢀ1
)
D
S⁄ (J molꢀ1Kꢀ1
)
D )
G⁄ (kJ molꢀ1
316
321
306
311
316
321
7.296
8.284
6.484
6.996
7.464
8.131
83.83
85.16
84.16
85.54
86.91
88.29
6d
12.14
9.54
ꢀ275.01
a
Data are obtained from Ref. [17].
4. Conclusion
13.4
13.3
13.2
13.1
13.0
12.9
12.8
12.7
Two series of Cd(II)-1-alkyl-2-{(o-thioalkyl)phenylazo}imidaz-
ole (SRaaiNR/) complexes are described: [Cd(SRaaiNR/)2X2]
(X = Cl, Br, I) and [Cd(SRaaiNR/)4](ClO4)2. The crystal structures of
the complexes have shown monodentate, N(imidazole), nature of
SRaaiNR/ although they have three donor centres N(imidazole),
N(azo) and –SR. Photochromism of the complexes are examined
by UV light irradiation in CH3CN solution. Rate of photoisomerisa-
tion and quantum yields are decreased in the complexes compared
with free ligands data. The decrease in electronegativity of X in-
creases the rate of E-to-Z photoisomerisation. The Z-to-E isomeri-
sation is thermally driven process. The activation energy (Eas) of
Z-to-E isomerisation has been calculated. The slow rate of isomeri-
sation in complexes may be due to higher rotor volume than that
of free ligands.
(a)
(b)
3.10
3.15
3.20
3.25
1/Tx1000, K-1
3.30
3.35
Fig. 7. The Eyring plots of rate constants of Z-to-E thermal isomerisation of
SEtaaiNEt (O) and [Cd(SEtaaiNEt)2I2] (5d) (j) at different temperatures.
Acknowledgements
Financial support from Department of Science & Technology,
New Delhi is thankfully acknowledged. Our sincere thanks due to
Mr. Rajat Saha, Department of Physics, Jadavpur University for help
in crystallographic work.
Upon UV light irradiation trans (E) structure of SRaaiNR/
changes to cis (Z) structure about the azo (–N@N–) function and
the cis molar ratio has reached to >80% (Scheme 2). UV-light irra-
diation to the solution of the photochrome may insist the rotation
of the azo-aryl group (–N@N–Ar) which isomerizes from trans (E)
to cis (Z) form. The rates of photoisomerisation (trans-to-cis (E–Z),
Appendix A. Supplementary data
/
E?Z) increases with decreasing electronegativity of X in the
CCDC 865742 and 654229 contain the supplementary crystallo-
graphic data for [Cd(SEtaaiNEt)2I2] (5d) and [Cd(SEtaaiNEt)4]
(ClO4)2 (6d), 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 Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
complexes (Fig. 5); rate follows [Cd(SRaaiNR/)2Cl2] (3) < [Cd(SRa-
aiNR/)2Br2] (4) < [Cd(SRaaiNR/)2I2] (5) although the molar mass
follows the order 3 < 4 < 5. Higher electronegativity of Cl in 3
may help to enhance association strength by electrostatic interac-
tion with neighbouring molecules and may effectively increase
rotor mass than that of 4 or 5.
The Z-to-E (cis-to-trans) isomerisation at thermal condition was
followed by UV–Vis spectroscopy (Fig. 6) in MeCN at varied tem-
peratures, 306–316 K and the activation energies were obtained
(Table 5) from the Eyring plots (Fig. 7). In the complexes the Eas
are quite closer to respective free ligands, which mean slow rate
of Z-to-E thermal isomerisation of the complexes. The entropies
References
[1] F.A. Cotton, G. Wilkinson, C.A. Murillo, M. Bochmann, Advanced Inorganic
Chemistry, sixth ed., Wiley-Interscience, New York, 1999.
[2] M.J. Scoullos, G.H. Vonkeman, I. Thornton, Z. Makuch, Mercury, Cadmium,
Lead: Handbook for Sustainable Heavy Metals Policy and Regulation, Springer,
2001.
[3] J.A. Christopher, A.K. Mukhtar, A. Guy Orpen, Inorg. Chem. 49 (2010) 10475.
[4] B.G. Chand, U.S. Ray, J. Cheng, J. Cheng, T.-H. Lu, C. Sinha, Inorg. Chim. Acta 358
(2005) 1927.
of activation (D
S⁄) are more negative in the complexes than that
of the free ligands. This is also defending the increase in rotor
volume in the complexes.
The complexes have available pendant N, S-coordination cen-
tres. The pendant arms bearing additional coordinating groups in
a metal complex have several advantages and are useful in many
different chemical applications such as catalysis [30], selective cat-
ion binding [30,31], mimicry of enzymes and siderophores [32].
The impact of metal coordination on the photoisomerisation of
the [Cd(SRaaiNR/)2X2] and [Cd(SRaaiNR/)4](ClO4)2 are examined.
The addition of Cd(ClO4)2 solution (in MeCN, 10ꢀ4 M) followed
by UV light irradiation has shown the quenching of photoisomeri-
sation. The coordination of Cd(II) to the pendant N,S-coordination
centres may increase rotor mass and may inhibit the photoisomer-
isation of the chromophore. The progress of the work is under
investigation.
[5] U.S. Ray, D. Banerjee, G. Mostafa, T.-H. Lu, C. Sinha, New J. Chem. 28 (2004)
1432.
[6] D. Banerjee, U.S. Ray, Sk. Jasimuddin, J.-C. Liou, T.-H. Lu, C. Sinha, Polyhedron
25 (2006) 1299.
[7] G. Saha, P. Datta, K.K. Sarkar, R. Saha, G. Mostafa, C. Sinha, Polyhedron 30
(2011) 614.
[8] J. Otsuki, K. Suwa, K. Narutaki, C. Sinha, I. Yoshikawa, K. Araki, J. Phys. Chem. A
109 (2005) 8064.
[9] J. Otsuki, K. Suwa, K.K. Sarker, C. Sinha, J. Phys. Chem. A 111 (2007) 1403.
[10] K.K. Sarker, D. Sardar, K. Suwa, J. Otsuki, C. Sinha, Inorg. Chem. 46 (2007) 8291.
[11] P. Pratihar, T.K. Mondal, A.K. Patra, C. Sinha, Inorg. Chem. 48 (2009) 2760.
[12] C.V. Yelamaggad, I. Shashikala, U.S. Hiremath, D.S.S. Rao, S.K. Prasad, Liq. Cryst
34 (2007) 153.
[13] S. Kawata, Y. Kawata, Chem. Rev 100 (2000) 1777.
[14] E. Katz, A.N. Shipway, I. Willner, in: Z. Sekkat, W. Knoll (Eds.), Photoreactive
Organic Films, Academic, San Diego, 2002, p. 220.

S. Saha (Halder) et al. / Polyhedron 46 (2012) 81–89
89
[15] M. Irie (Ed.), Photochromism: memories and switches, Chem. Rev. 100 100
(2000) 1683 (special issue).
[25] M.C. Das, S. Xiang, Z. Zhang, B. Chen, Angew. Chem., Int. Ed. Engl. 50 (2011)
10510.
[16] J.C. Crano, R.J. Guglielmetti (Eds.), Organic Photochromic and Thermochromic
Compounds, Kluwer Academic/Plenum Publishers, New York, NY, 1999.
[17] G.M. Sheldrick, SHELXS-97, University of Gottingen, Germany, 1997.
[18] G.M. Sheldrick, SHELXL 97, University of Gottingen, Germany, 1997.
[19] J. Farrugia, J. Appl. Cryst. 30 (1997) 565.
[26] <http://www.webelements.com/cadmium/atom_sizes.html>.
[27] J.E. Fergusson, The Heavy Elements: Chemistry, Environment Impact And
Health Effects, Pergamon, Oxford, 1990.
[28] S.S. Halder, G. Saha, B.G. Chand, A.D. Jana, G. Mostafa, J. Cheng, T.-H. Lu, C.
Sinha, Inorg. Chim. Acta 363 (2010) 2080.
[20] A.L. Spek, PLATON, The Netherlands (1999).
[29] H. Nishihara, Bull. Chem. Soc. Jpn. 77 (2004) 407.
[30] R. Reichenbach-Klinke, B. Knöig, Dalton Trans. (2002) 121.
[31] T. Koike, T. Abe, M. Takahashi, K. Ohtani, E. Kimura, M. Shiro, Dalton Trans.
(2002) 1764.
[21] G. Zimmerman, L. Chow, U. Paik, J. Am. Chem. Soc. 80 (1958) 3528.
[22] R. Gupta, S. Mukherjee, R. Mukherjee, J. Chem. Soc., Dalton Trans. (1999) 4025.
[23] T.K. Misra, Ph. D. thesis, (1999).
[24] S. Pal, T. Melton, R.N. Mukherjee, A.R. Chakravarty, M. Tomas, L.R. Falvello, A.
Chakravorty, Inorg. Chem. 24 (1985) 1250.
[32] T. Tanase, H. Inukai, T. Onaka, M. Kato, S. Yano, S.J. Lippard, Inorg. Chem. 40
(2001) 3943.