1-Alkyl-2-{(o-thioalkyl)phenylazo}imidazole complexes of mercury(II) and their photochromic properties

Article abstract of DOI:10.1016/j.poly.2012.07.063

The reaction of HgX₂ (X = Cl, Br, I) with 1-alkyl-2-{(o- thioalkyl)phenylazo}imidazole (SRaaiNR′) in MeOH-ethyleneglycol mixture has synthesised [Hg(SRaaiNR′)X₂]. The structure of the products has been established by microanalytical

Full text of DOI:10.1016/j.poly.2012.07.063

Polyhedron 46 (2012) 25–32
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1-Alkyl-2-{(o-thioalkyl)phenylazo}imidazole complexes of mercury(II) and their
photochromic properties
S. Saha (Halder) a,1, P. Raghavaiah b, C. Sinha a,
⇑
a Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700 032, India
b School of Chemistry, National Single Crystal X-ray Diffractometer Facility, University of Hyderabad, Hyderabad 500 046, India
a r t i c l e i n f o
a b s t r a c t
The reaction of HgX2 (X = Cl, Br, I) with 1-alkyl-2-{(o-thioalkyl)phenylazo}imidazole (SRaaiNR0) in MeOH-
ethyleneglycol mixture has synthesised [Hg(SRaaiNR0)X2]. The structure of the products has been
established by microanalytical and spectral (IR, UV–Vis, 1H NMR) data, and a distorted Td-geometry
has been confirmed by a single crystal X-ray diffraction study of [Hg(SMeaaiNEt)I2] (SMeaaiNEt =
1-ethyl-2-{(o-thiomethyl)phenylazo}imidazole). The UV light irradiation in CH3CN solution of the
complexes shows E-to-Z (E and Z refer to trans and cis-configurations) isomerisation of the coordinated
SRaaiNR0 ligand about the –N@N– bond. The rate of isomerisation follows: [Hg(SRaaiNR0)Cl2] < [Hg
(SRaaiNR0)Br2] < [Hg(SRaaiNR0)I2]. The quantum yields (/E?Z) of the isomerisation of the complexes are
lower than those of the free ligands. These may be due to increased mass and rotor volume of the
complexes. The electronegativity sequence of X (I < Br < Cl) may regulate the molecular association and
hence the effective mass of the rotor and the photoisomerisation rates. Thermal isomerisation determines
Article history:
Received 23 May 2012
Accepted 8 July 2012
Available online 27 July 2012
Keywords:
Hg(II)-thioalkylphenyl-azoimidazoles
X-ray structure
Photochromism
Activation energy
Effect of halide
the rate, activation energy (Ea), activation enthalpy (D D
H⁄) and activation entropy ( S⁄) of the Z ? E
transformation of the coordinated SRaaiNR0. The Eas of the thermal isomerisation of the complexes are
lower than those of the free ligands.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
been reported on derivatives of azobenzene, which are amongst
the best characterized photoswitches [3,9,14–18]. Brown and
The color change of molecules upon light irradiation has been
defined as ‘‘photochromism’’ by Hirshberg in 1950 [1] [from the
Greek words: phos (light) and chroma (color)]. This field has been
expanded since the 1960s [2–8]. Silver halide, iron, cobalt, manga-
nese, copper, gold and a mixture of lanthanide compounds have
been used as inorganic photochromes. The important problem of
inorganic photochromes is ‘‘fatigue’’ and low durability. Numerous
organic photochromes have been synthesized, which include spiro-
pyrans, diarylethenes, quinones, azobenzenes, azopyridines, etc.
However, organic photochromes suffer photodamage after several
cycles of UV light irradiation. Use of metal complexes as photo-
chromes gives increased durability, reversibility and resistance to
photodegradation, photobleaching, photooxidation, etc., and also
increased storing capacity. Azo compounds are important colour-
ing agents in the dye industry and have significant capacity to
coordinate to metal ions [9–11]. Hartley reported the cis–trans
isomerisation of aromatic azo dyes in 1937 upon irradiation with
UV light [12,13], and subsequently a large volume of work has
Granneman had reported in 1975 [19] the isomerisation of pheny-
lazopyridines and symmetrical azopyridines. The advantage of azo-
pyridine derivatives is their coordination to a metal ion and
hydrogen bonding through the pyridyl-N and the azo group itself.
The implantation of pyridyl-N on nano particles/composite sur-
faces brings enormous opportunity to light driven communication
in nanoscience and nanotechnology. It is expected that the coordi-
nation of ligands to metal ions drifts the charge from the ligand,
and hence influences the electron density. This affects the elec-
tronic, physical and chemical properties of the compounds, and
even induces some chemical transformations that are otherwise
impossible. The coordination of metal ions to organic photo-
chromes may affect the isomerisation properties, rates and conver-
sion efficiency [20,21]. The exploration of photoswitching of other
azoheterocycles is an exciting area of research. The first report of
photoisomerisation of phenylazoimidazole appeared in 2003 [22]
by Majima and co-workers and a subsequent publication appeared
in 2005 by Otsuki et al. [23]. Phenylazoimidazoles constitute an
interesting class of compounds in coordination chemistry that
are important in biological applications since imidazole is a ubiqui-
tous and essential group in biology, especially as a metal coordi-
nating site. The photochromism of 1-alkyl-2-(arylazo)imidazoles
(RaaiNR0) [23,24] and some of their metal complexes [25–28] have
⇑
Corresponding author.
E-mail address: c_r_sinha@yahoo.com (C. Sinha).
Present address: Kalyani Govt. Engg. College, Kalyani, Nadia, West Bengal 741
1
235, India.
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.07.063

26
S. Saha (Halder) et al. / Polyhedron 46 (2012) 25–32
Table 1
4
5
Summarized crystallographic data for [Hg(SMeaaiNEt)I2] (5b).
N
N
N
N
N
N
X
N
N
R
H
5b
R
Hg
11
N
X
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
Crystal size (mm)3
a (Å)
C
12H14N4I2SHg
N
N
N
9
700.73
100(2)
Monoclinic
P21/n
S
S
R
S
R
R
8
10
0.10 ꢁ 0.15 ꢁ 0.20
12.3065(14)
1
2
3 – 5
b (Å)
c (Å)
11.9461(17)
12.6300(18)
90.00
112.170(5)
90.00
1719.5(4)
Scheme 1. The ligands SRaaiNR0 and the mercury(II) complexes Hg(SRaaiNR’)X2
(X = Cl (3), Br (4), I (5) R = R0 = Me (a); R = Me, R0 = Et (b); R = Et, R0 = Me (c);
R = R0 = Et (d)).
a
(°)
b (°)
c
(°)
V (Å)3
Z
l
4
12.7
(Mo K
a
) (mmꢀ1
)
synthesized by a reported procedure [29]. All other chemicals
and solvents were reagent grade as received.
h range
1.97–28.71
hkl range
ꢀ16 < h < 15; ꢀ16 < k < 16; ꢀ17 < l < 17
Dcalc (mg mꢀ3
)
2.707
183
Refine parameters
Total reflections
2.2. Physical measurements
19575
4445
0.0332
0.0913
1.04
Unique reflections
Microanalytical data (C, H, N) were collected on a Perkin-Elmer
2400 CHNS/O elemental analyzer. Spectroscopic data were ob-
tained using the following instruments: UV–Vis spectra from a Per-
kin 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.
a
R1 [I > 2
r
(I)]
b
wR2
Goodness of fit
P
P
a
R = ||Fo| ꢀ |Fc||/ |Fo|.
P
P
wR2 = [ w(Fo ꢀ Fc2)2/ w(Fo2)2]1 2, w = 1/[
r
2(Fo)2 + (0.0356P)2] for 5b; where
b
2
0
P = (Fo2 + 2Fc2)/3.
inspired us to examine the photochromism of other transition and
non-transition metal complexes. In this work we have used 1-al-
kyl-2-{(o-thioalkyl)phenylazo}imidazole (SRaaiNR0), a flexidentate
ligand–tridentate (N,N0,S) [29]/bidentate (N,N0) [30]/monodentate
N [31] (where N, N0, S refer to N(imidazolyl), N(azo) and –S-R donor
centres, respectively), to synthesise Hg(II) complexes using differ-
ent halides (X = Cl, Br, I) as neutralizing charges. The complexes
have been spectroscopically (FT-IR, UV–Vis, 1H NMR) characterized
and in one case single crystal X-ray diffraction has been used for
the structural confirmation. The photoisomerisation of coordinated
SRaaiNR0 (E (trans) ? Z (cis)) has been monitored by UV light irra-
diation and the rates and quantum yields have been determined;
the effect of coordination of SRaaiNR0 to Hg(II), the influence of X
in the complexes etc. are accounted for in this work.
2.3. Synthesis of [Hg(SMeaaiNEt)I2] (5b)
1-Ethyl-2-{(o-thiomethyl)phenylazo}imidazole
(16.30 mg,
0.066 mmol) in CH3OH was added in drops to a MeOH–ethylene-
glycol (4:1, v/v) solution of HgI2 (30 mg, 0.066 mmol) and the
resulting mixture was stirred for 2 h. The resultant red solution
was collected after filtration. Slow evaporation of the solution crys-
tallized a red compound. The yield was 30.98 mg (67%). The other
complexes were prepared following identical reaction conditions
and the yield varied in the range 60–70%.
Data of the complexes are as follows: [Hg(SMeaaiNMe)Cl2] (3a)
Anal. Calc. for C11H12N4SCl2Hg: C, 26.22; H, 2.40; N, 11.12. Found:
C, 26.20; H, 2.42; N, 11.15%. FT-IR (KBr disc, cmꢀ1):
m
(N@N)
(C@N) 1505. UV–Vis spectroscopic data in CH3CN (kmax
(nm) (10ꢀ3 (dm3 molꢀ1 cmꢀ1))): 379 (13.83), 416 (10.71).
1424;
m
2. Experimental
2
[Hg(SMeaaiNEt)Cl2] (3b) Anal. Calc. for C12H14N4SCl2Hg: C, 27.83;
2.1. Material
H, 2.72; N, 10.81. Found: C, 27.78; H, 2.74; N, 10.84%. FT-IR (KBr
disc, cmꢀ1):
m(N@N) 1424; m(C@N) 1512. UV–Vis spectroscopic
HgCl2, HgBr2 and HgI2 were obtained from Loba Chemicals,
Bombay, India. 1-Alkyl-2-{(o-thioalkyl)phenylazo}imidazole were
data in CH3CN (kmax (nm) (10ꢀ3 2 (dm3 molꢀ1 cmꢀ1))): 380
(12.18), 417 (9.52). [Hg(SEtaaiNMe)Cl2] (3c) Anal. Calc. for C12H14-
Table 2
1H NMR spectral data of [Hg(SRaaiNR0)X2] in CDCl3.
d, ppm (J, Hz)
4-Hbs 5Hbs 8Hd
9,10-Hm 11-Hd
N(1)–CH3
7.95 (8.1) 4.20
7.97 (8.2)
N(1)–CH2
[N(1)CH2–] CH3
S–CH3
S–CH2
[S–CH2–] CH3
s
q
t
s
q
t
[Hg(SMeaaiNMe)Cl2] (3a)
[Hg(SMeaaiNEt)Cl2] (3b)
[Hg(SEtaaiNMe)Cl2] (3c)
Hg(SEtaaiNEt)Cl2] (3d)
7.38
7.40
7.35
7.39
7.27
7.22
7.20
7.27
7.23
7.21
7.23
7.20
7.19
7.20
7.23
7.20
7.35 (7.4)
7.34 (7.4)
7.30 (7.8)
7.31 (7.8)
7.34 (8.0)
7.33 (7.8)
7.30 (7.8)
7.35 (8.0)
7.33 (8.0)
7.37 (7.5)
7.37 (7.8)
7.59
7.58
7.58
7.59
7.58
7.56
7.55
7.58
7.57
7.54
7.53
2.68
2.67
4.66 (7.3)
4.58 (7.3)
4.57 (7.3)
4.55 (7.3)
4.55 (7.3)
4.52 (7.3)
1.69 (8.0)
1.65 (8.0)
1.63 (8.0)
1.64 (8.0)
1.66 (8.0)
1.62 (8.0)
7.97 (8.2) 4.19
7.96 (8.2)
7.94 (8.0) 4.17
7.96 (8.1)
7.95 (8.0) 4.16
7.96 (8.1)
7.93 (8.0) 4.16
7.90 (7.9)
3.19 (7.0) 1.45 (8.0)
3.18 (7.0) 1.46 (8.0)
[Hg(SMeaaiNMe)Br2] (4a) 7.38
2.66
2.62
[Hg(SMeaaiNEt)Br2] (4b)
[Hg(SEtaaiNMe)Br2] (4c)
[Hg(SEtaaiNEt)Br2] (4d)
[Hg(SMeaaiNMe)I2] (5a)
[Hg(SMeaaiNEt)I2] (5b)
[Hg(SEtaaiNMe)I2] (5c)
[Hg(SEtaaiNEt)I2] (5d)
7.39
7.41
7.39
7.36
7.37
7.38
7.37
3.19 (7.0) 1.45 (8.0)
3.18 (7.0) 1.44 (8.0)
2.63
2.61
7.87 (7.9) 4.14
7.90 (8.0)
3.18 (7.0) 1.46 (8.0)
3.24 (7.0) 1.43 (8.0)
7.38 (8.00) 7.58
bsBroad singlet; ddoublet; mmultiplet; ssinglet; ttriplet; qquartet.

S. Saha (Halder) et al. / Polyhedron 46 (2012) 25–32
27
Fig. 1. Crystal structure of (a) [Hg(SMeaaiNEt)I2] (5b) and (b) 1D chain structure constituted by C–Hꢂ ꢂ ꢂI hydrogen bonds and pꢂ ꢂ ꢂp interactions.
1.5
Table 3
Selected bond distances and bond angles of the compound Hg(SMeaaiNEt)I2 (5b).
Bond distance (Å)
Bond angle (°)
Hg(1)–I(1)
Hg(1)–I(2)
2.6826(6)
2.6467(6)
2.297(5)
1.271(6)
2.780(4)
4.330
I(1)–Hg(1)–I(2)
I(1)–Hg(1)–N(2)
I(2)–Hg(1)–N(2)
Hg(1)–N(2)–C(1)
Hg(1)–N(2)–C(2)
N(2)–Hg(1)–N(4)
I(2)–Hg(1)–N(4)
I(1)–Hg(1)–N(4)
N(3)–N(4)–Hg(1).
C(4)–N(4)–Hg(1).
128.475(16)
112.11(12)
119.21(12)
120.2(4)
134.3(4)
64.29(15)
96.98(9)
101.08(9)
112.5(3)
131.8(3)
1.0
0.5
0.0
Hg(1)–N(2)
N(3)–N(4)
Hg(1)–N(4)
Hg(1)ꢂ ꢂ ꢂHg(1a)
Hg(1)ꢂ ꢂ ꢂI(1a)
3.581
250 300 350 400 450 500 550 600
Wave length (nm)
N4SCl2Hg: C, 27.83; H, 2.72; N, 10.81. Found: C, 27.82; H, 2.73; N,
10.82%. FT-IR (KBr disc, cmꢀ1):
m(N@N) 1423; m(C@N) 1504. UV–
Fig. 2. UV–Vis spectrum of [Hg(SMeaaiNEt)I2] (5b) in CH3CN at 27 °C.
Vis spectroscopic data in CH3CN (kmax (nm) (10ꢀ3 2 (dm3 molꢀ1
cmꢀ1))): 379 (13.01), 418 (9.12). [Hg(SEtaaiNEt)Cl2] (3d) Anal. Calc.
for C13H16N4SCl2Hg: C, 29.36; H, 3.03; N, 10.53. Found: C, 29.37; H,
(8.34). [Hg(SEtaaiNMe)Br2] (4c) Anal. Calc. for C12H14N4SBr2Hg: C,
3.03; N, 10.50%. FT-IR (KBr disc, cmꢀ1):
m
(N@N) 1424;
m
(C@N)
23.75; H, 2.32; N, 9.23. Found: C, 23.72; H, 2.29; N, 9.22%. FT-IR
1511. UV–Vis spectroscopic data in CH3CN (kmax (nm) (10ꢀ3
2
(KBr disc, cmꢀ1):
m(N@N) 1421; m(C@N) 1499. UV–Vis spectro-
(dm3 molꢀ1 cmꢀ1))): 380 (12.91), 417 (9.22). [Hg(SMeaaiNMe)Br2]
(4a) Anal. Calc. for C11H12N4SBr2Hg: C, 22.29; H, 2.04; N, 9.45.
Found: C, 22.25; H, 2.05; N, 9.46%. FT-IR (KBr disc, cmꢀ1):
scopic data in CH3CN (kmax (nm) (10ꢀ3 2 (dm3 molꢀ1 cmꢀ1))):
369 (12.12), 418 (8.25). [Hg(SEtaaiNEt)Br2] (4d) Anal. Calc. for C13-
H16N4SBr2Hg: C, 25.15; H, 2.59; N, 9.02. Found: C, 25.10; H, 2.60; N,
m
(N@N) 1423;
m
(C@N) 1500. UV–Vis spectroscopic data in CH3CN
9.00%. FT-IR (KBr disc, cmꢀ1):
m(N@N) 1422; m(C@N) 1509. UV–Vis
(kmax (nm) (10ꢀ3 2 (dm3 molꢀ1 cmꢀ1))): 369 (12.31), 419 (9.52).
[Hg(SMeaaiNEt)Br2] (4b) Anal. Calc. for C12H14N4SBr2Hg: C, 23.75;
H, 2.32; N, 9.23. Found: C, 23.77; H, 2.31; N, 9.20%. FT-IR (KBr disc,
spectroscopic data in CH3CN (kmax (nm) (10ꢀ3 2 (dm3 molꢀ1
-
cmꢀ1))): 369 (12.90), 419 (8.36). [Hg(SMeaaiNMe)I2 ] (5a) Anal.
Calc. for C11H12N4SI2Hg: C, 19.24; H, 1.76; N, 8.16. Found: C,
cmꢀ1):
m(N@N) 1422;
m(C@N) 1508. UV–Vis spectroscopic data in
19.26; H, 1.70; N, 8.18%. FT-IR (KBr disc, cmꢀ1):
m
(N@N) 1416;
CH3CN (kmax (nm) (10ꢀ3 2 (dm3 molꢀ1 cmꢀ1))): 368 (13.11), 420
m(C@N) 1498. UV–Vis spectroscopic data in CH3CN (kmax (nm)

28
S. Saha (Halder) et al. / Polyhedron 46 (2012) 25–32
N
N
N
N
N
N
N
N
N
N
N
N
N
R
N
N
N
Hg
Hg
R
R
R
N
N
N
N
S
S
R
S
S
R
R
R
E- or trans-
Z- or cis-
Free SRaaiNR/
Coordinated SRaaiNR/
Scheme 2. Photochromism of free and coordinated SRaaiNR0.
Table 4
Excitation wavelength (k
(/E?Z).
0.5
0.4
0.3
0.2
0.1
0.0
Decreasing
), rate of E (trans) ? Z (cis) conversion and quantum yield
p,p⁄
Trans
342
342
0.4
0.2
0.0
Compound
k
Isosbestic points Rate of E ? Z conversion
/
E?Z
p,
p
⁄
Cis
(nm)
(nm)
(ꢁ108 sꢀ1
)
2aa
2ba
2ca
2da
3a
3b
3c
3d
4a
4b
4c
4d
5a
5b
5c
5d
357
358
357
356
360
361
362
362
361
362
363
362
361
364
363
362
337
337
336
335
337
340
341
339
338
341
340
339
338
342
343
340
4.908
3.108
4.67
2.948
3.61
2.30
3.1
2.12
3.89
2.51
3.39
2.42
4.11
2.79
3.75
2.62
0.317
0.232
0.290
0.197
0.235
0.173
0.187
0.121
0.258
0.19
300
375
450
525
Wavelength (nm)
0.22
0.151
0.288
0.202
0.261
0.168
300
375
450
525
600
Wavelength (nm)
Fig. 3. Spectral changes of [Hg(SMeaaiNEt)I2] (5b) in MeCN upon repeated light
irradiation at 364 nm at 4 min interval at 25 °C. Inset shows the trans (E) and cis (Z)
isomeric spectra.
a
Received from Ref. [25].
(10ꢀ3 2 (dm3 molꢀ1 cmꢀ1))): 364 (12.56), 428 (7.14). [Hg(SMeaai-
NEt)I2] (5b) Anal. Calc. for C12H14N4SI2Hg: C, 20.57; H, 2.01; N,
7.99. Found: C, 20.59; H, 2.03; N, 7.94%. FT-IR (KBr disc, cmꢀ1):
syntheses. All non-hydrogen atoms were refined anisotropically.
The hydrogen atoms were fixed geometrically and refined using
the riding model. All calculations were carried out using
S
HELXL-97 [33], ORTEP-32 [34] and PLATON-99 [35] programs.
m
(N@N) 1421; m(C@N) 1494. UV–Vis spectroscopic data in CH3CN
(kmax (nm) (10ꢀ3 2 (dm3 molꢀ1 cmꢀ1))): 363 (13.34), 431 (7.89).
[Hg(SEtaaiNMe)I2] (5c) Anal. Calc. for C12H14N4SI2Hg: C, 20.57; H,
2.01; N, 7.99. Found: C, 20.55; H, 2.03; N, 7.95%. FT-IR (KBr disc,
3. Results and discussion
cmꢀ1):
m(N@N) 1424; m(C@N) 1497. UV–Vis spectroscopic data in
3.1. Synthesis of the complexes
CH3CN (kmax (nm) (10ꢀ3 2 (dm3 molꢀ1 cmꢀ1))): 363 (12.054), 424
(7.12). [Hg(SEtaaiNEt)I2] (5d) Anal. Calc. for C13H16N4SI2Hg: C,
21.84; H, 2.25; N, 7.84. Found: C, 21.86; H, 2.22; N, 7.85%. FT-IR
The ligands were synthesized by coupling the o-(thi-
oalkyl)phenyldiazonium ion with imidazole in sodium carbonate
solution, and purified by solvent extraction and chromatographic
processes [29]. The alkylation of 2-[o-(thioalkyl)phenylazo]imidaz-
ole (1) has been carried out by adding alkyl iodide (MeI, EtI) in dry
THF solution in the presence of NaH and the 1-alkyl-2-{(o-thi-
oalkyl)phenylazo}imidazoles [SRaaiNR0 (R = R0 = Me (2a); R = Me,
R0 = Et (2b); R = Et, R0 = Me (2c); R = R0 = Et (2d))] were purified by
chromatography.
(KBr disc, cmꢀ1):
m(N@N) 1423; m(C@N) 1496. UV–Vis spectro-
scopic data in CH3CN (kmax (nm) (10ꢀ3 2 (dm3 molꢀ1 cmꢀ1))):
364 (12.84), 427 (7.45).
2.4. X-ray diffraction study
The single crystals suitable for data collection were grown from
slow evaporation of a CH3CN solution of the complexes. The crystal
data and details of the data collections are given in Table 1. A suit-
able single crystal of 5b was mounted on a Bruker SMART APEX
CCD diffract0ometer (graphite monochromated Mo K
k = 0.71073 ÅA) and data were collected by use of scans. Unit cell
The reaction of HgX2 (X = Cl, Br, I) and SRaaiNR0 in MeOH–ethyl-
eneglycol (4:1 v/v) isolates the crystalline compounds [Hg(SRa-
aiNR0)X2] (3–5). The composition of the complexes has been
confirmed by microanalytical data and by other spectroscopic
information. The structure has been established in the case of
[Hg(SMeaaiNEt)I2] (5b) by single-crystal X-ray diffraction studies.
a radiation,
x
parameters were determined from least-squares refinement of
setting angles (h) within the range 1.97° 6 h 6 28.71°. Out of
19575 collected data, 4445 with I > 2
r(I) were used for the struc-
3.2. Infrared and 1H NMR spectra
ture solution. The hkl ranges were ꢀ16 6 h 6 15, ꢀ16 6 k 6 16,
ꢀ17 6 l 6 17. Data were corrected for Lorentz polarization effects
and for linear decay. Semi-empirical absorption corrections based
The infrared spectra of the compounds in KBr discs show mod-
erately intense stretches at 1500–1515 and 1415–1425 cmꢀ1
on
w
-scans were applied. The structure were solved by direct
which are assigned to
m(C@N) and m(N@N), respectively. Other
method using SHELXS-97 [32] and successive difference Fourier
vibrations are shifted to lower frequency in the complexes

S. Saha (Halder) et al. / Polyhedron 46 (2012) 25–32
29
Fig. 4. 1H NMR spectra of the aliphatic region of [Hg(SMeaaiNMe)Cl2] (3a) in CD3CN (a) before irradiation and (b) after irradiation of UV light at 360 nm for 20 min.
compared to the free ligand values [29]. The 1H NMR spectra are
trans
recorded in CDCl3 (Table 2). The atom numbering pattern is shown
in Scheme 1. The N(1)–Me signal appears as a singlet at 4.14–
4.20 ppm; N–CH2–CH3 shows a quartet for –CH2– at ca. 4.52–
4.66 (7.3 Hz) and a triplet at 1.62–1.69 (8.0 Hz) ppm. The thioalkyl
group S–R also exhibits a singlet signal at 2.61–2.68 ppm for S–Me
of SMeaaiNR0 (3a,b–6a,b). SEtaaiNEt (3d–6d) shows quartets (4.5
(7 Hz) and 3.00 (7 Hz) ppm for the –CH2– protons of N–CH2–
(CH3) and S–CH2–(CH3), respectively) and triplets (at 1.6 and
1.4 ppm for –(N–CH2)–CH3 and –(S–CH2)CH3, respectively). Imid-
azole 4- and 5-H appears as a broad singlet at 7.35–7.41 and
7.19–7.27 ppm, respectively. Broadening may be due to rapid pro-
ton exchange between these imidazole protons. The aryl protons,
8-H to 11-H (7.3–7.9 ppm), reveal that the signals in the spectra
of the complexes are shifted downfield relative to the free ligand
values.
0.6
0.4
0.2
0.0
0.6
0.4
0.2
0.0
cis
250 300 350 400 450 500 550 600
Wavelength (nm)
250
300
350
400
450
500
550
600
3.3. Molecular structure of [Hg(SMeaaiNEt)I2] (5b)
Wavelength (nm)
Fig. 5. Thermal isomerisation, Z (cis) ? E (trans) of [Hg(SMeaaiNEt)I2] (5b) in MeCN
at 4 min interval at 25 °C (inset shows spectra of cis- and trans isomers).
The crystals of [Hg(SMeaaiNEt)I2] (5b) were obtained by slow
evaporation of CH3CN solution under ambient conditions. The
molecular structure is shown in Fig. 1 and selected bond parame-
ters are listed in Table 3. Each discrete molecular unit consists of
a HgN2I2 coordination sphere where SMeaaiNEt acts as N,N0-che-
lating agent (N and N0 refer to N(imidazole) and N(azo) donor cen-
tres), although it has three potential N,N0,S donor centres. The S-Me
moiety remains uncoordinated and stereochemically away from
the Hg(II) centre. The atomic arrangement of Hg(1), N(4), N(3),
C(1) and N(2) constitute the chelate plane with a maximum devi-
ation <0.04 Å. The pendant phenyl ring makes a small dihedral of
4.7° with the chelated azoimidazole ring. The small chelate angle
(64.29(15)°) may be one of the reasons for geometrical distortion.
The Hg–N(imidazole) distance (Hg(1)–N(1), 2.297(5) Å) is shorter
than that of Hg–N(azo) (Hg(1)–N(4), 2.780(4) Å), which reflects
the stronger interaction between Hg(II) and N(imidazole).
Although the Hg–N(azo) bond length is very long, it is less than
the sum of the van der Waals radii of Hg(II) (1.55 Å) and N(sp2)
(1.53 Å). This implies a significant bonding interaction between
these components. The stronger coordination of imidazole-N to
Hg(II) compared to azo-N has a significant biochemical implication
and explains the strong toxicity of Hg(II). Because of the long
Hg(II)–N(azo) distance, the molecule is useful for photo-activation
via cleavage of this bond followed by rotation to introduce photo-
isomerisation (vide infra). The N@N distance is 1.271(6) Å and is
longer than the free ligand data [24]. The bond parameters are in
good agreement with previously reported structures of Hg(II)-{1-
alkyl-(2-arylazo)imidazole}iodide [27]. The non-covalent interac-
tion of –SEt–C6H4– such as [Cg(2) (molecule A)ꢂ ꢂ ꢂCg(2a) (molecule
B) is 3.370 Å (Cg(2/2a)–C(4)–C(5)–C(6)–C(7)–C(8)–C(9)); symme-
try, 2 ꢀ x, 1 ꢀ y, 2 – z]. A weak hydrogen bonding is also observed
at C(7)–H(7)ꢂ ꢂ ꢂCg(1) [H(7)ꢂ ꢂ ꢂCg(1), C(7)ꢂ ꢂ ꢂCg(1) and \C(7)–
H(7)ꢂ ꢂ ꢂCg(1) are 2.768, 3.648 Å and 154.46°, respectively where
Cg(1) is N(1)–C(1)–N(2)–C(2)–C(3); symmetry ꢀ1/2+ x, 3/2 ꢀ y,
1/2 + z] and C(11)–H(11)ꢂ ꢂ ꢂCg(2) [H(11)ꢂ ꢂ ꢂCg(2), C(11)ꢂ ꢂ ꢂCg(2)
and \C(11)–H(11)ꢂ ꢂ ꢂCg(2) are 3.380, 4.353 Å and 171.95°, respec-
tively; symmetry ꢀ3/2 ꢀ x, 1/2+ y, 3/2 ꢀ z]. Iodine is also hydrogen
bonded to form a 1D chain, C(2)–H(2)ꢂ ꢂ ꢂI(1) where H(2)ꢂ ꢂ ꢂI(1),
3.08(2) Å; C(2)ꢂ ꢂ ꢂI(1), 3.77(9) Å and \C(2)–H(2)ꢂ ꢂ ꢂI(1), 131.54°
(Fig. 1b).
3.4. UV–Vis spectra and photochromism
The electronic spectra were taken in CH3CN solution. Two main
bands are observed in the UV –Vis region. On comparing with the
free ligands spectra [27] and that of Hg(II) complexes, we may con-
clude that these bands come from intramolecular charge-transfer

30
S. Saha (Halder) et al. / Polyhedron 46 (2012) 25–32
Table 5
Rate and activation parameters for Z (cis) ? E (trans) thermal isomerisation.
Compound
Temperature (K)
Rate thermal c ? t conversion (ꢁ104 sꢀ1
)
Ea (kJ molꢀ1
)
D
H⁄ (kJ molꢀ1
)
D
S⁄ (J molꢀ1 Kꢀ1
)
D )
G⁄ (kJ molꢀ1
2aa
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
304
309
314
319
304
309
314
319
304
309
314
319
304
309
314
319
304
309
314
319
304
309
314
319
304
309
314
319
304
309
314
319
304
309
314
319
304
309
314
319
304
309
314
319
304
309
314
319
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
7.82
8.33
8.855
10.6
8.18
8.7
25.42
22.88
20.45
19.38
16.34
13.09
11.07
11.16
9.21
ꢀ232.41
ꢀ240.02
ꢀ244.04
ꢀ253.96
ꢀ261.68
ꢀ267.91
ꢀ267.55
ꢀ273.54
ꢀ255.58
ꢀ258.91
ꢀ257.85
ꢀ264.53
ꢀ249.31
ꢀ254.31
ꢀ253.3
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
79.56
80.87
82.18
83.49
81.45
82.79
84.13
85.47
81.35
82.69
84.02
85.36
83.16
84.53
83.16
87.27
77.71
78.99
80.27
81.54
78.72
80.02
81.31
82.61
78.40
79.69
80.98
82.27
80.43
81.75
83.07
84.34
75.80
77.05
78.3
2ba
2ca
2da
3a
3b
3c
22.99
21.92
18.88
15.68
13.66
13.75
11.8
9.1
10.7
8.25
8.77
9.2
10.8
8.63
9.1
3d
4a
4b
4c
9.67
10.8
8.23
8.82
9.79
11.4
8.45
9.26
10.1
11.5
8.49
9.28
10.2
11.6
8.94
9.65
10.3
11.8
8.28
8.9
10.2
11.8
8.5
9.18
10.3
11.8
8.54
9.2
10.4
11.9
8.99
9.5
10.3
11.9
17.4
14.81
13.7
16.29
16.6
14.01
11.86
16.71
15.1
4d
5a
5b
5c
14.45
19.29
17.69
17.99
14.82
79.55
77.33
78.6
79.87
81.14
77.02
78.28
79.55
80.82
80.06
81.38
82.7
15.40
12.23
5d
ꢀ263.33
84.01
a
Received from Ref. [25].
transitions (n ? p⁄
,
p
?
p⁄). The complexes show a structured
360–380 nm corresponds to p–
p⁄ transitions, while the tail corre-
absorption band around 360–390 nm with a molar absorption
coefficient (2 ꢃ 103 Mꢀ1 cm2) and a weak band at 415–420 nm
(2 ꢃ 103 Mꢀ1 cm2) (Fig. 2). From the analogous absorption spectra
of the ligands, it is concluded that the absorption band around
sponds to an admixture of n–p⁄ and Hg(II) ? p⁄(azoimine)
transitions.
The UV-light irradiation at a fixed time interval at 360–390 nm
to a CH3CN solution of the ligands shows changes in the absorption

S. Saha (Halder) et al. / Polyhedron 46 (2012) 25–32
31
spectra that corresponds to a structural change of the ligand from
13.4
13.2
13.0
12.8
12.6
trans-SRaaiNR0 (E-isomer) to cis-SRaaiNR0 (Z-isomer) (Scheme 2,
Fig. 3). The intense peak in the UV zone decreases with a slight in-
crease in the tail portion of the spectra in the visible region, until a
stationary state is reached. The irradiation at the newly appearing
longer wavelength peak (ꢃ420 nm) reverses the process very
slowly and the original spectra are recovered. The quantum yields
of the E-to-Z photoisomerization are given in Table 4. It is observed
that upon irradiation with UV light, the E-to-Z change proceeds and
the Z molar ratio reaches ꢃ85%. The absorption spectra of the coor-
dinated SRaaiNR0 ligands in the E-form change upon excitation
(Fig. 3) into the Z-isomer. The ligands and the complexes show lit-
tle sign of degradation upon repeated irradiation for up to at least
15 cycles in each case. The quantum yields were measured for the
E-to-Z (/E?Z) acetonitrile solution on irradiation with UV light. The
a
b
0.00315 0.00320 0.00325 0.00330 0.00335
(1/T)(K-1)
/
values are significantly dependent on nature of substituents.
E?Z
Fig. 6. The Eyring plots of rate constants of Z (cis) to E (trans) thermal isomerisation
of (a) SMeaaiNEt (2b) and (b) [Hg(SMeaaiNEt)I2] (5b) at 298, 303, 308 and 313 K.
The photoisomerisation rate and quantum yields of coordinated li-
gands are decreased compared to the free ligands and in general,
an increase in mass of the molecule reduces the rate and quantum
yield of isomerisation.
quantum yields of the trans-to-cis isomerisation. The Eas follow
the sequence (a) > (c) > (b) > (d).
The 1H NMR spectra of [Hg(SMeaaiNMe)Cl2] in CD3CN were re-
corded before and after UV irradiation at 360 nm and it is observed
that the aromatic ring protons are significantly shifted upfield after
the light irradiation. The 1H NMR spectra of the irradiated mole-
cules show the presence of two closely associated signals of differ-
ent intensity ratios, which are distinguishable in the aliphatic
region of the N–Me group (Fig. 4), whilst it was difficult to analyze
the aromatic region because of the complexity of overlapping
proton signals. The lower intense signal, which appears at higher
d values (shifted by 0.05–0.15 ppm), may be the contribution from
the cis-configuration of coordinated SMeaaiNMe. However, we
have not carried out an NMR experiment of irradiated solution of
all the complexes.
4. Conclusion
Hg(1-alkyl-2-{(o-thioalkyl)phenylazo}imidazole)X2 complexes
have been characterized by spectral data and in one case by a sin-
gle crystal X-ray diffraction study. The photoisomerisation of the
complexes have been examined by UV light irradiation in CH3CN
solution and the results are compared with free ligand data. The
decrease in electronegativity of X increases the rate of E ? Z photo-
isomerisation. The Z ? E isomerisation is a thermally driven pro-
cess. The activation energies (Eas) of Z ? E isomerisation have
been calculated and the values are lower than the free ligand data.
The slow rate of isomerisation in the complexes may be due to a
higher rotor volume than that of the free ligands.
On comparing with previously reported photoisomerisation
results of [Hg{1-alkyl-2-(arylazo)imidazole}(l-X)X]2 [27], we ob-
serve that the present complexes show a higher E ? Z rate and
better quantum yields, although the formula weight of the present
molecules are higher than the earlier examples. The dimeric nature
of the previous complexes may increase the rotor mass relative to
the monomeric nature of the present complexes and may be a
probable reason for the decrease in the rate and quantum yields.
The rate and quantum yields (/t?c) are considerably dependent
on the position of the substituents. Substitution at the imidazolyl
ring (–NR) is recognizably different from –SR substitution
towards the regulation of photochromism. These parameters fol-
low the sequence [Hg(SMeaaiNMe)X2] (a) > [Hg(SEtaaiNMe)X2]
(c) > [Hg(SMeaaiNEt)X2] (b) > [Hg(SEtaaiNEt)X2] (d). The –NR
group has a higher mass influence than –SR, which may be due
to the coordination of imidazole-N to Hg(II) in which the ring –
NR is added, while the –SR group is free. Besides, the overall mass
influence on the rate and photochromism is common in all these
complexes. In general, an increase in mass of a molecule reduces
the rate of the trans-to-cis isomerisation.
Acknowledgment
Financial support from the Department of Science and Technol-
ogy, New Delhi is thankfully acknowledged.
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