Synthesis, structure and photochromism of zinc(II) complexes of alkylthioarylazoimidazoles

Article abstract of DOI:10.1016/j.poly.2013.09.014

The [Zn(SRaaiNR′)(H₂O)X₂] (SRaaiNR′ = 1-alkyl-2-{(o-thioalkyl)phenylazo}imidazole; X = Cl, Br, I) complexes have been characterized by spectroscopic studies. The X-ray structure of [Zn(SEtaaiNEt)(H₂O)Cl₂] (where SEtaaiNEt = 1-ethyl-2-{(o-thioalkyl)phenylazo}imidazole) shows bidentate imidazolyl-N, azo-N chelation of SEtaaiNEt and the other donors are H₂O and 2Cl ^-. UV light irradiation to a MeCN solution of the complexes shows E-to-Z (E and Z refer to trans and cis-configurations about -NN-, respectively) isomerisation of the coordinated azoimidazole. The rate of isomerisation follows the sequence: [Zn(SRaaiNR′)(H₂O)Cl₂] < [Zn(SRaaiNR′)(H₂O)Br₂] < [Zn(SRaaiNR′) (H₂O)I₂]. The quantum yields (_E→Z) and the activation energies (E_a) of the isomerisation of the complexes are lower than those of the free ligands. The observation has been explained considering the molecular association that increases the mass and rotor volume of the complexes.

Full text of DOI:10.1016/j.poly.2013.09.014

Polyhedron 67 (2014) 321–328
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, structure and photochromism of zinc(II) complexes
of alkylthioarylazoimidazoles
Shefali Saha (Halder) a, Partha Mitra b, Chittaranjan Sinha a,
⇑
a Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700 032, India
b Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata 700 032, India
a r t i c l e i n f o
a b s t r a c t
The [Zn(SRaaiNR0)(H2O)X2] (SRaaiNR0 = 1-alkyl-2-{(o-thioalkyl)phenylazo}imidazole; X = Cl, Br, I) com-
plexes have been characterized by spectroscopic studies. The X-ray structure of [Zn(SEtaaiNEt)(H2O)Cl2]
(where SEtaaiNEt = 1-ethyl-2-{(o-thioalkyl)phenylazo}imidazole) shows bidentate imidazolyl-N, azo-N
chelation of SEtaaiNEt and the other donors are H2O and 2Clꢀ. UV light irradiation to a MeCN solution
of the complexes shows E-to-Z (E and Z refer to trans and cis-configurations about –N@N–, respectively)
isomerisation of the coordinated azoimidazole. The rate of isomerisation follows the sequence: [Zn(SRa-
aiNR0)(H2O)Cl2] < [Zn(SRaaiNR0)(H2O)Br2] < [Zn(SRaaiNR0)(H2O)I2]. The quantum yields (/E?Z) and the
activation energies (Ea) of the isomerisation of the complexes are lower than those of the free ligands.
The observation has been explained considering the molecular association that increases the mass and
rotor volume of the complexes.
Article history:
Received 18 July 2013
Accepted 2 September 2013
Available online 25 September 2013
Keywords:
Zn(II)-thioalkylphenyl-azoimidazole
Distorted Td-symmetry
Photochromism
Activation energy
Effect of halide
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
of the effect of the substituents on the optical properties. We have
synthesized 1-alkyl-2-{(o-thioalkyl)phenylazo}imidazole (SRa-
Zinc, the fourth most widely used metal after iron, aluminum
and copper [1], is anticorrosive and is used to protect bridges, build-
ings etc. from loss through rusting. It is an essential trace element
and more than 150 metalloenzymes are either directly or indirectly
dependent on zinc(II) [2,3] and fight cancer [4]. DNA bases and other
nucleotides and nucleosides strongly bind Zn(II). Imidazole and its
derivatives form complexes of the type [Zn(imzH)6]Cl2 [5]. Imidazo-
lato bridged Zn(II) complexes enhance the activity of Cu(II) in metal-
lo-proteins [6,7]. Studies on Zn(II)-imidazole/imidazole containing
ligands are of interest. This has prompted us to synthesize 1-alkyl-
2-(arylazo)imidazoles (RaaiR0) and investigate their coordination
aiNR0) and their properties were examined [26,27]. In this work
Zn(II)-SRaaiNR0 complexes were structurally characterized and the
light stimulated property of the coordinated SRaaiNR0 has been
examined. The effect of Zn–X (X = Cl, Br, I) on the photoisomerisa-
tion rate and quantum yields have been discussed.
2. Experimental
2.1. Material
Anhydrous ZnCl2, ZnBr2 and ZnI2 were obtained from Merck.
1-Alkyl-2-{(o-thioalkyl) phenylazo}imidazole (SRaaiNR0) were
synthesized by a reported procedure [27]. All other chemicals
and solvents were of reagent grade and used as received.
chemistry [8–13]. The molecules are p-acidic and the active func-
tion is an azoimine group (–N@N–C@N–). The coordination chemis-
try of Zn(II) with RaaiR0 has recently been started by us [14–16]. One
of the interesting properties of RaaiR0 and many of their complexes
is photochromism [17–20], which is a reversible transformation be-
tween E (trans) and Z (cis) isomers upon light irradiation. The pho-
tochromic compounds are applicable in different areas, such as
liquid crystal alignment [21], optical data storage [22], non-linear
optics [23], photoswitching [24] and molecular-photonic devices
[25]. Very few azo compounds, including azobenzenes, arylazopyri-
dines and arylazoimidazoles [17–25], show photochromic activity.
This has stimulated us to functionalize RaaiR0 for an investigation
2.2. Physical measurements
Microanalytical data (C, H, N) were collected on 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 were collected on
a Bruker (AC) 300 MHz FTNMR spectrometer.
⇑
Corresponding author. Tel.: +91 9433621872; fax: +91 2414 6584.
E-mail address: c_r_sinha@yahoo.com (C. Sinha).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.09.014

322
S. Saha (Halder) et al. / Polyhedron 67 (2014) 321–328
2.3. Synthesis
(7.7). 1H NMR (300 MHz, CDCl3), d (ppm), (J(Hz)): 7.41 (bs, 4H),
7.24 (bs, 5H), 7.35 (d, 7.00 Hz, 8H), 7.44 (m, 9 and 10H), 8.10 (d,
8.00 Hz, 11H), 4.32 (s, 1-CH3), 2.60 (s, S-CH3). [Zn(SMeaaiEt)
(H2O)Cl2] (3b). Anal. Calc. for ZnC12H16N4OSCl2: C, 35.97; H, 3.99;
N, 13.99. Found: C, 35.91; H, 4.03; N, 13.92.% FT-IR (KBr disc,
2.3.1. Synthesis of [Zn(SMeaaiNMe)(H2O)Cl2] (3a)
To a methanol solution of 1-methyl-2-{(o-thiomethyl)pheny-
lazo}imidazole (SMeaaiNMe) (52 mg, 0.23 mmol), a ZnCl2 (30 mg,
0.22 mmol) solution (5 ml in MeOH) was added in drops and stir-
red for 2 h. The resultant reddish solution was then collected by fil-
tration. Slow evaporation of the solution gives orange-red crystals.
The yield was 52 mg (61%).
cmꢀ1):
in CH3CN (kmax (nm) (10ꢀ3
m
(N@N), 1429;
m
(C@N), 1624. UV–Vis spectroscopic data
ꢀ
(dm3 molꢀ1 cmꢀ1))): 363 (14.76),
417 (9.9). 1H NMR (300 MHz, CDCl3), d (ppm), (J(Hz)): 7.42 (bs,
4H), 7.23 (bs, 5H), 7.32 (d, 7.00 Hz, 8H), 7.43 (m, 9 and 10H),
7.98 (d, 8.00 Hz, 11H), 4.48 (q, 6.60 Hz, 1-CH2), 1.65 (t, 7.30 Hz,
(1-CH2)-CH3)), 2.67 (s, S-CH3). [Zn(SEtaaiMe)(H2O)Cl2] (3c). Anal.
Calc. for ZnC12H16N4OSCl2: C, 35.97; H, 3.99; N, 13.99. Found: C,
Other complexes were prepared under identical conditions
from MeOH solution and the yields were 60–70%.
Microanalytical data of the complexes are as follows: [Zn(SMea-
aiMe)(H2O)Cl2] (3a). Anal. Calc. for ZnC11H14N4OSCl2: C, 34.17; H,
3.62; N, 14.50. Found: C, 34.21; H, 3.60; N, 14.43%. FT-IR (KBr disc,
35.94; H, 4.02; N, 13.97%. FT-IR (KBr disc, cmꢀ1):
m(N@N), 1428;
m
(C@N), 1623. UV–Vis spectroscopic data in CH3CN (kmax (nm)
cmꢀ1):
m
(N@N), 1424;
m
(C@N), 1622. UV–Vis spectroscopic data in
(10ꢀ3
ꢀ
(dm3 molꢀ1 cmꢀ1))): 364 (12.3), 417 (10). 1H NMR
CH3CN (kmax (nm) (10ꢀ3
ꢀ
(dm3 molꢀ1 cmꢀ1))): 362 (11.5), 416
(300 MHz, CDCl3), d (ppm), (J(Hz)): 7.40 (bs, 4H), 7.23 (bs, 5H),
7.34 (d, 7.40 Hz, 8H), 7.46 (m, 9 and 10H), 7.94 (d, 7.90 Hz, 11H),
4.28 (s, 1-CH3), 3.01 (q, 7.20 Hz, S-CH2), 1.31 (t, 7.35 Hz, (S-CH2)-
CH3). [Zn(SEtaaiNEt)(H2O)Cl2] (3d). Anal. Calc. for ZnC13H18N4OSCl2:
C, 37.65; H, 4.34; N, 13.52. Found: C, 37.72; H, 4.38; N, 15.49%.
Table 1
Summarized crystallographic data for [Zn(SMeaaiNMe)(H2O)Cl2] (3a).
[Zn(SMeaaiNMe)(H2O)Cl2] (3a)
FT-IR (KBr disc, cmꢀ1):
troscopic data in CH3CN (kmax (nm) (10ꢀ3
m
(N@N), 1430;
m
(C@N), 1623. UV–Vis spec-
Empirical formula
Formula weight
T (K)
Crystal system
Space group
C11H14N4OSCl2Zn
ꢀ
(dm3 molꢀ1 cmꢀ1))):
386.59
293(2)
triclinic
363 (13.87), 415 (10.8). 1H NMR (300 MHz, CDCl3), d (ppm),
(J(Hz)): 7.41 (bs, 4H), 7.22 (bs, 5H), 7.35 (d, 7.00 Hz, 8H), 7.45
(m, 9 and 10H), 7.97 (d, 8.00 Hz, 11H), 4.59 (q, 6.70 Hz, 1-CH2),
1.60 (t, 7.20 Hz, (1-CH2)-CH3), 3.04 (q, 7.20 Hz, S-CH2), 1.39 (t,
7.36 Hz, (S-CH2)-CH3).
ꢀ
P1
Unit cell dimensions
a (Å)
b (Å)
c (Å)
7.675(3)
8.805(3)
13.245(5)
[Zn(SMeaaiNMe)(H2O)Br2] (4a). Anal. Calc. for C11H14N4OSBr2Zn:
C, 27.78; H, 2.95; N, 11.79. Found: C, 27.72; H, 2.92; N, 11.84%.
a
(°)
74.700(7)
FT-IR (KBr disc, cmꢀ1):
m
(N@N), 1423;
m
(C@N), 1612. UV–Vis spec-
b (°)
75.615(7)
troscopic data in CH3CN (kmax (nm) (10ꢀ3
ꢀ
(dm3 molꢀ1 cmꢀ1))):
c
(°)
70.012(7)
798.8(5)
2
V (Å3)
Z
363 (10.84), 417 (6.9). 1H NMR (300 MHz, CDCl3), d (ppm),
(J(Hz)): 7.40 (bs, 4H), 7.23 (bs, 5H), 7.36 (d, 7.40 Hz, 11H), 7.55
(m, 9 and 10H), 7.84 (d, 8.00 Hz), 4. 72 (s, 1-CH3), 2.55 (s, S-CH3).
[Zn(SMeaaiNEt)(H2O)Br2] (4b). Anal. Calc. for C12H16N4OSBr2Zn: C,
29.44; H, 3.27; N, 11.45. Found: C, 29.37; H, 3.32; N, 11.52%.
k (Å)
0.71073
2.001
1.697
l
(Mo K
a
) (mmꢀ1
)
Dcalc (Mg mꢀ3
Index range
h range (°)
)
ꢀ9 6 h 6 9, ꢀ10 6 k 6 10, ꢀ15 6 l 6 15
1.62–25.00
191
FT-IR (KBr disc, cmꢀ1):
troscopic data in CH3CN (kmax (nm) (10ꢀ3
m
(N@N), 1424;
m
(C@N), 1610. UV–Vis spec-
Refine parameters
ꢀ
(dm3 molꢀ1 cmꢀ1))):
Total reflection
2806
365 (14.81), 417 (9.78). 1H NMR (300 MHz, CDCl3), d (ppm),
(J(Hz)): 7.41 (bs, 4H), 7.20 (bs, 5H), 7.33 (d, 7.00 Hz, 8H), 7.52
(m, 9 and 10H), 7.88 (d, 8.00 Hz, 11H), 4.47 (q, 7.00 Hz, 1-CH2),
1.69 (t, 7.30 Hz, (1-CH2)-CH3)), 2.64 (s, S-CH3). [Zn(SEtaaiNMe)(H2-
O)Br2] (4c). Anal. Calc. for C12H16N4OSBr2Zn: C, 29.44; H, 3.27; N,
11.45. Found: C, 29.50; H, 3.23; N, 11.53%. FT-IR (KBr disc, cmꢀ1):
Unique data [I > 2
r(I)]
2492
a
R1 [I > 2
wR2
r(I)]
0.0307
0.0804
1.131
0.255
ꢀ0.517
b
Goodness-of-fit
Dmax (e Åꢀ3
Dmin (e Åꢀ3
)
)
a
R =
wR = [
R
|F0 ꢀ Fc|/
R F0.
b
2
4 1/2
R
w(F0 ꢀ Fc2)/
R
w
F0
]
are general but w are different, w = 1/
m
(N@N), 1425;
m(C@N), 1611. UV–Vis spectroscopic data in CH3CN
[r
2(F2) + (0.0360P)2 + 0.2100P] where P = (Fo2 + 2Fc2)/3.
(kmax (nm) (10ꢀ3 ꢀ (dm3 molꢀ1 cmꢀ1))): 364 (13.12), 418 (9.88). 1H
(i) Imidazole, pH 7; (ii) NaH in THF and R/I
[Zn(SRaaiNR/)(H2O)X2] (X = Cl (3), Br (4), I (5)) [R = R/ = Me (a); R = Me, R/ = Et (b);
R = Et, R/ = Me (c); R = R/ = Et (d)]
Scheme 1. The ligands and the complexes.

S. Saha (Halder) et al. / Polyhedron 67 (2014) 321–328
323
NMR (300 MHz, CDCl3), d (ppm), (J(Hz)): 7.42 (bs, 4H), 7.20 (bs,
5H), 7.35 (d, 7.00 Hz, 8H), 7.56 (m, 9 and 10H), 7.83 (d, 8.00 Hz,
11H), 4.30 (s, 1-CH3), 3.00 (q, 7.40 Hz, S-CH2), 1.30 (6.20 Hz,
(S-CH2)-CH3). [Zn(SEtaaiNEt)(H2O)Br2] (4d). Anal. Calc. for
C
13H18N4OSBr2Zn: C, 31.01; H, 3.58; N, 11.13. Found: C, 31.00; H,
3.60; N, 11.18%. FT-IR (KBr disc, cmꢀ1):
(N@N), 1426; (C@N),
1614. UV–Vis spectroscopic data in CH3CN (kmax (nm) (10ꢀ3
m
m
ꢀ
(dm3 molꢀ1 cmꢀ1))): 363 (13.52), 419 (10.2). 1H NMR (300 MHz,
CDCl3), d (ppm), (J(Hz)): 7.41 (bs, 4H), 7.22 (bs, 5H), 7.32 (d,
7.00 Hz), 7.53 (m, 9 and 10-), 7.86 (d, 8.00 Hz, 11H), 4.52 (q,
7.25 Hz, 1-CH2), 1.62 (t, 7.20 Hz, (1-CH2)-CH3), 3.04 (q, 7.30 Hz,
S-CH2), 1.35 (t, 6.20 Hz, (S-CH2)-CH3). [Zn(SMeaaiNMe)(H2O)I2]
(5a). Anal. Calc. for C11H14N4OSI2Zn: C, 23.19; H, 2.46; N, 9.84.
Found: C, 23.23; H, 2.50; N, 9.80%. FT-IR (KBr disc, cmꢀ1):
m
(N@N), 1424; m(C@N), 1601. UV–Vis spectroscopic data in CH3CN
(kmax (nm) (10ꢀ3 ꢀ (dm3 molꢀ1 cmꢀ1))): 364 (9.65), 415 (6.23). 1H
NMR (300 MHz, CDCl3), d (ppm), (J(Hz)): 7.41 (bs, 4H), 7.28 (bs,
5H), 7.31 (d, 7.00 Hz, 8H), 7.55 (m, 9 and 10H), 7.80 (d, 8.00 Hz,
11H), 4.14 (s, 1-CH3), 2.52 (s, S-CH3). [Zn(SMeaaiNEt)(H2O)I2] (5b).
Anal. Calc. for C12H16N4OSI2Zn: C, 24.70; H, 2.74; N, 9.60. Found:
Fig. 1. Molecular structure of [Zn(SMeaaiNMe)(H2O)Cl2] (3a).
Table 2
Selected bond distances and bond angles of [Zn(SMeaaiMe)(H2O)Cl2] (3a).
C, 24.74; H, 2.71; N, 9.66%. FT-IR (KBr disc, cmꢀ1):
m
(N@N), 1428;
Bond lengths (Å)
Bond angles (°)
m
(C@N), 1605. UV–Vis spectroscopic data in CH3CN (kmax (nm)
Zn(1)–N(1)
Zn(1)–N(2)
Zn(1)–O(13)
Zn(1)–Cl(2)
Zn(1)–Cl(3)
N(2)–N(6)
N(2)–C(6)
2.007(2)
2.608(6)
2.130(2)
2.2442(10)
2.2445(11)
1.269(3)
1.399(3)
N(1)–Zn(1)–O(13)
N(1)–Zn(1)–N(2)
N(1)–Zn(1)–Cl(2)
N(1)–Zn(1)–Cl(3)
O(13)–Zn(1)–Cl(2)
O(13)–Zn(1)–Cl(3)
Cl(2)–Zn(1)–Cl(3)
N(2)–Zn(1)–Cl(2)
N(2)–Zn(1)–Cl(3)
N(2)–Zn(1)–O(13)
92.36(9)
69.88(3)
119.99(7)
118.32(7)
98.75(8)
96.05(8)
118.74(3)
94.56(2)
88.57(4)
169.31(4)
(10ꢀ3 2 (dm3 molꢀ1 cmꢀ1))): 365 (14.81), 419 (9.64). 1H NMR
(300 MHz, CDCl3), d (ppm), (J(Hz)): 7.40 (bs, 4H), 7.23 (bs, 5H),
7.34 (d, 7.00 Hz, 8H), 7.56 (m, 9 and 10H), 7.82 (d, 8.00 Hz, 11H),
4.51 (q, 7.00 Hz, 1-CH2), 1.63 (t, 7.30 Hz, (1-CH2)-CH3)), 2.59
(s, S-CH3). [Zn(SEtaaiNMe)(H2O)I2] (5c). Anal. Calc. C12H16N4OSI2Zn:
C, 24.70; H, 2.74; N, 9.60. Found: C, 24.67; H, 2.79; N, 9.63%. FT-IR
(KBr disc, cmꢀ1):
m(N@N), 1426; m(C@N), 1604. UV–Vis spectro-
scopic data in CH3CN (kmax (nm) (10ꢀ3 2 (dm3 molꢀ1 cmꢀ1))):
363 (13.55), 418 (9.71). 1H NMR (300 MHz, CDCl3), d (ppm),
Fig. 2. 1D chain constituted by O–Hꢁ ꢁ ꢁCl bonds with neighboring molecules.

324
S. Saha (Halder) et al. / Polyhedron 67 (2014) 321–328
Fig. 3. Formation of a 2D supramolecular plane via Cg(3)ꢁ ꢁ ꢁCg(3) interactions (
Cg(3):
N
N
N
N
N
Zn
Zn
N
N
N
N
N
N
R
R
R
R
N
N
N
N
N
S
S
S
S
R
R
R
R
E- or trans-
Z- or cis-
Free SRaaiNR/
Coordinated SRaaiNR/
Scheme 2. Photochromism of free SRaaiNR0 and the coordinated ligand in [Zn(SRaaiNR0)(H2O)Cl2].
(J(Hz)): 7.43 (bs, 4H), 7.27 (bs, 5H), 7.32 (d, 7.40 Hz, 8H), 7.55 (m, 9
and 10H), 7.81 (d, 8.00 Hz, 11H), 4.17 (s, 1-CH3), 3.00 (q, 7.30 Hz,
S-CH2), 1.29 (t, 7.00 Hz, (S-CH2)-CH3). [Zn(SEtaaiNEt)(H2O)I2] (5d).
Anal. Calc. for C13H18N4OSBr2Zn: C, 26.13; H, 3.01; N, 9.38. Found:
cal absorption corrections based on w-scans were applied. The
structures were solved by the direct method using SHELXS-97 [28]
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 calcula-
tions were carried out using the SHELXL-97 [29], PLATON-99 [30] and
ORTEP-32 [31] programs.
C, 26.10; H, 3.05; N, 9.42%. FT-IR (KBr disc, cmꢀ1):
m
(N@N), 1427;
m
(C@N), 1605. UV–Vis spectroscopic data in CH3CN (kmax (nm)
(10ꢀ3 2 (dm3 molꢀ1 cmꢀ1))): 364 (13.68), 417 (9.8). 1H NMR
(300 MHz, CDCl3), d (ppm), (J(Hz)): 7.42 (bs, 4H), 7.22 (bs, 5H),
7.33 (d, 7.00 Hz, 8H), 7.56 (m, 9 and 10H), 7.80 (d, 8.00 Hz, 11H),
4.44 (q, 7.30 Hz, 1-CH2), 1.61 (t, 7.30 Hz, (1-CH2)-CH3), 3.05 (q,
7.40 Hz, S-CH2), 1.30 (t, 5.30 Hz, (S-CH2)-CH3) (bs, broad singlet;
s, singlet; d, doublet; t, triplet; q, quintet; m, multiplet).
2.5. Photometric measurements
Absorption spectra were taken with a PerkinElmer Lambda 25
UV–Vis spectrophotometer in a 1 ꢂ 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. Upon UV light irradiation at kmax of
the pp⁄ absorption to a solution of the complex, the intense pp⁄
peak decreases, which is accompanied by a slight increase at the
tail portion of the spectrum around 450 nm (np⁄) until a photosta-
tionary state (PSS-I) is reached. Subsequent irradiation at the new-
ly appeared longer wavelength peak (np⁄) reverses the course of
the reaction and the original spectrum is recovered up to a point,
2.4. X-ray diffraction study
Single crystals suitable for X-ray diffraction data collection were
grown from slow evaporation of the complexes in methanol. The
crystal data are given in Table 1. A suitable single crystal of
[Zn(SMeaaiMe)(H2O)Cl2] (3a) (0.26 ꢂ 0.24 ꢂ 0.22 mm) was
mounted on a Bruker SMART APEX CCD diffractometer and data
were collected by the use of
x scans. Unit cell parameters were
determined from least-squares refinement. Data were corrected
for Lorentz polarization effects and for linear decay. Semi-empiri-
which is another photostationary state (PSS-II) under
n
p⁄

S. Saha (Halder) et al. / Polyhedron 67 (2014) 321–328
325
increasing
decerasing
trans
trans
0.8
0.6
0.4
0.2
0.0
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.75
340
340
cis
cis
339
0.50
0.25
0.00
339
0.50
0.25
250 300 350 400 450 500 550 600
Wavelength (nm)
250 300 350 400 450 500 550 600
Wavelength(nm)
0.00
270
360
450
540
Wavelength (nm)
Fig. 6. Z ? E isomerisation of [Zn(SMeaaiNEt)(H2O)Cl2] in MeCN at 303 K.
250 300 350 400 450 500 550 600
Wavelength(a.u)
ꢀ
ꢁ
m
¼ ð/I0=VÞ 1 ꢀ 10ꢀAbs
Fig. 4. Photochromism of [Zn(SMeaaiNEt)(H2O)Cl2] (3b), E ? Z isomerisation in
CH3CN at 25 °C.
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
p–
p⁄ excitation [32]) under the same irradiation
decreasing
conditions.
trans
0.8
The thermal rates of Z-to-E isomerisation were obtained by
monitoring absorption changes intermittently for a cis-rich solu-
tion kept in the dark at constant temperatures (T) in the range
298–321 K. The activation energy (Ea) and the frequency factor
(A) were obtained from the Arrhenius plot:
337
337
0.6
cis
0.6
0.4
0.2
0.4
0.0
300
400
500
600
Wave length(nm)
ln k ¼ ln A ꢀ Ea=RT
0.2
0.0
where k is the measured rate constant, R is the gas constant and T is
temperature. The values of the activation free energy (
D
G⁄) and
activation entropy (
D
S⁄) were obtained through the relationships,
200
300
400
500
600
wave length(nm)
D
Gꢃ ¼ Ea ꢀ RT ꢀ T
D
Sꢃ=and=
D
Sꢃ ¼ ½ln A ꢀ 1 ꢀ lnðkBT=hÞ=R
Fig. 5. Photochromism of SMeaaiNEt (2b) in CH3CN at 25 °C.
where kB and h are Boltzmann’s and Plank’s constants
3. Results and discussion
irradiation. An isosbestic point at 335–340 nm is reached during
this process. The absorption spectra of the cis isomers were ob-
tained by extrapolation of the absorption spectra of a cis-rich mix-
ture for which the composition is known from 1H NMR integration.
Quantum yields (/) were obtained by measuring the initial E-to-Z
3.1. The formulation of the complexes
1-Alkyl-2-{(o-thioalkyl)phenylazo}imidazoles [SRaaiNR0 (R =
R0 = Me (2a); R = Me, R0 = Et (2b); R = Et, R0 = Me (2c); R = R0 = Et
(2d)] have been reacted with anhydrous ZnX2 (X = Cl, Br, I) in meth-
anol, and orange-red crystalline compounds [Zn(SRaaiNR0)(H2O)X2]
isomerization rates (m) in a well-stirred solution within the above
instrument using the equation:
Table 3
ꢃ
Excitation wavelength (k ), rate of E ? Z conversion and quantum yield (UE?Z).
pp
ꢃ
Compound
k
(nm)
Isosbastic
points (nm)
Rate of E ? Z
UE?Z
pp
conversion ꢂ108 (sꢀ1
)
SMeaaiNMe (2a)#
357
358
357
356
362
363
362
361
363
362
364
363
364
365
363
364
337
337
336
335
339
339
338
337
338
337
337
339
337
338
336
337
4.908
3.108
4.67
2.948
2.88
2.33
2.45
1.89
3.22
2.68
2.77
2.03
3.65
3.05
3.18
2.21
0.317
0.232
0.290
0.1974
0.161
0.1305
0.142
0.0913
0.179
0.148
0.155
0.105
0.231
0.159
0.164
0.121
SMeaaiNEt (2b)#
SEtaaiNMe (2c)#
SEtaaiNEt (2d)#
[Zn(SMeaaiNMe)(H2O)Cl2] (3a)
[Zn(SEtaaiNMe)(H2O)Cl2] (3b)
[Zn(SMeaaiNEt)(H2O)Cl2] (3c)
[Zn(SEtaaiNEt)(H2O)Cl2] (3d)
[Zn(SMeaaiNMe)(H2O)Br2] (4a)
[Zn(SEtaaiNMe)(H2O)Br2] (4b)
[Zn(SMeaaiNEt)(H2O)Br2] (4c)
[Zn(SEtaaiNEt)(H2O)Br2] (4d)
[Zn(SMeaaiNMe)(H2O)I2] (5a)
[Zn(SEtaaiNMe)(H2O)I2] (5b)
[Zn(SMeaaiNEt)(H2O)I2] (5c)
[Zn(SEtaaiNEt)(H2O)I2] (5d)
#
Ref. [26].

326
S. Saha (Halder) et al. / Polyhedron 67 (2014) 321–328
Table 4
Rate and activation parameters for E ? Z thermal isomerisation of [Zn(SRaaiNR0)(H2O)Cl2] (3–5).
Compd
Temp (K)
Rate of thermal E ? Z
Ea (kJ molꢀ1
)
D
H⁄ (kJ molꢀ1
)
D
S⁄ (J molꢀ1Kꢀ1
)
D
G⁄
conversion ꢂ104 (sꢀ1
)
(kJ molꢀ1
)
§
2a
298
303
308
313
4.329
5.295
6.191
7.091
25.42
22.88
20.45
19.38
16.34
14.60
13.37
13.017
11.09
15.73
14.89
14.95
11.60
16.83
15.58
15.82
13.18
ꢀ232.41
ꢀ240.02
ꢀ244.04
ꢀ253.96
ꢀ259.14
ꢀ262.90
ꢀ263.97
ꢀ270.00
ꢀ255.22
ꢀ257.76
ꢀ257.40
ꢀ268.19
ꢀ251.50
ꢀ255.39
ꢀ254.51
ꢀ262.81
92.13
93.29
94.45
95.62
§
2b
298
303
308
313
4.614
5.598
6.293
7.269
22.99
21.92
18.88
17.18
15.95
15.60
13.67
18.31
17.47
17.53
14.18
19.41
18.16
18.40
15.76
91.97
93.17
94.37
95.57
§
2c
298
303
308
313
4.425
5.123
5.985
6.729
92.11
93.33
94.55
95.77
§
2d
298
303
308
313
4.625
5.123
5.985
6.589
92.02
93.29
94.56
95.83
3a
3b
3c
3d
4a
4b
4c
4d
5a
5b
5c
5d
303
308
313
318
5.67
78.53
79.83
81.13
82.42
6.063
6.857
7.812
303
308
313
318
5.875
6.263
6.975
7.905
79.67
80.99
82.30
83.61
303
308
313
318
5.921
6.363
7.101
7.9
79.99
81.31
82.63
83.95
303
308
313
318
6.18
6.51
7.19
7.95
81.82
83.17
84.52
85.87
303
308
313
318
5.811
6.251
7.222
8.11
77.34
78.62
79.9
81.17
303
308
313
318
5.991
6.401
7.332
8.24
78.12
79.40
80.69
81.98
303
308
313
318
6.111
6.483
7.484
8.395
78.01
79.29
80.58
81.87
303
308
313
318
6.288
6.612
7.348
8.159
81.27
82.62
83.96
85.30
303
308
313
318
5.901
6.309
7.441
8.369
76.22
77.48
78.74
79.99
303
308
313
318
6.051
6.474
7.5
77.40
78.68
79.95
81.23
8.411
303
308
313
318
6.11
77.13
78.41
79.68
80.95
6.508
7.591
8.518
303
308
313
318
6.409
6.823
7.618
8.581
79.64
80.96
82.27
83.59
§
Received from Ref. [26].

S. Saha (Halder) et al. / Polyhedron 67 (2014) 321–328
327
have been isolated. Microanalytical data has confirmed the compo-
sition of the complexes. Thermal studies of the complexes show one
step elimination of H2O within temperature range 105–140 °C,
which supports the coordination of H2O to Zn(II). The complexes
are stable up to 245 °C.
molecule in a row interacts via hydrogen bonding with two adjacent
molecules [(Zn)Cl(2)ꢁ ꢁ ꢁH(O)Hꢁ ꢁ ꢁCl(2A)(Zn). . .] of a second row and
vice versa. Two hydrogen bonds, Zn–O(H)–Hꢁ ꢁ ꢁCl(2/3), make eight
member supracycles and form hydrogen bonding with adjacent
units, leading to the formation of 1D chain. The pendant thiophenyl
The infrared spectra of the complexes show moderately intense
ring (C(6)–C(7)–C(8)–C(9)–C(10)–C(11)) in each row shows a
interaction with the same ring of an adjacent layer (3.639(2) Å and
p p
ꢁ ꢁ ꢁ
stretching vibrations at 1609–1625 and 1419–1430 cmꢀ1, which
are assigned to
m
(C@N) and
m(N@N) by comparing with published
symmetry 1 ꢀ x, 1 ꢀ y, 2 ꢀ z) to constitute a 2D network (Fig. 3.).
work [26,27]. The 1H-NMR spectra of the complexes are recorded
in CDCl3 and the signals are assigned unambiguously via spin–spin
interactions, the effect of substitutions therein and on comparing
with free ligand data [26,27]. The atom numbering pattern is
shown in the Scheme 1. Imidazolyl 4 and 5H show a broad singlet
at 7.4 and 7.2 ppm, respectively for the complexes [Zn(SRa-
aiNR0)(H2O)X2] (3–5) and these are downfield shifted by 0.1–
0.2 ppm compared to the free ligand data. The broadening may
be due to rapid proton exchange between these imidazolyl protons
and/or exchange with solvent protons. The phenyl protons (8 to
11H) of –S–(R)–C6H4 remain almost unperturbed. This spectral
information supports the single crystal structure (Fig. 1), where
SMeaaiNMe acts as a bidentate imidazolyl-N and azo-N chelating
agent. The N(1)–Me peak appears as a 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 sin-
glet signal at 2.6 ppm for S–Me of SMeaaiNR0 (3a,b–6a,b). SEtaai-
NEt (3d–6d) shows two quartets at 4.5 (7 Hz) and 3.00 (7 Hz)
ppm for –CH2– protons of N–CH2–(CH3) and S–CH2–(CH3), respec-
tively and two triplets at 1.6 (7 Hz) and 1.3 (6 Hz) ppm for
–(N–CH2)–CH3 and –(S–CH2)–CH3, respectively.
3.3. UV–Vis spectra and photochromism
The solution electronic spectra of the compounds were re-
corded in CH3CN in the range 200–600 nm. There are two bands
in the UV–Vis region at 360–380 and 410–420 nm. On comparing
with the spectra of the free ligands [27] and that of Zn(II) com-
plexes, we may conclude that these bands come from intramolec-
ular charge-transfer transitions (n ? p⁄ p⁄).
, p ?
The E-to-Z (trans-to-cis) isomerisation of the complexes
(Scheme 2) has been investigated by irradiation of UV light in ace-
tonitrile solution (Fig. 4) and compared with free ligand spectral
changes (Fig. 5). The absorption spectra of the E-configuration of
the coordinated SRaaiNR0 ligand in complexes 3–5 in CH3CN show
a gradual decrease in absorbance at 350–355 nm and an increase in
absorbance at 245–255 nm, and an isosbestic point is assigned at
335–340 nm. The complexes show little degradation upon re-
peated irradiation of at least up to 15 cycles in each case, which
has been verified by measuring the absorbance before and after
irradiation at the
p ?
p⁄ band. The quantum yields were measured
for the E-to-Z (UE?Z) photoisomerisation of these compounds in
CH3CN on irradiation with UV light (Table 3). The UE?Z values
are significantly dependent on the nature of the substituents, ha-
lide type and molecular weight [18,19,26,34–36]. In the presence
of 1-Me and 1-Et groups, the quantum yield is reduced in the com-
plexes compared to the free ligand.
3.2. Molecular structure of Zn(SMeaaiNMe)(H2O)Cl2] (3a)
The crystal structure of [Zn(SMeaaiNMe)(H2O)Cl2] (3a) is given
in Fig. 1 and bond parameters are listed in Table 2. The ligand,
SMeaaiNMe has three potential donor centres – N(imidazolyl),
N(azo) and S(Me), while it shows, in this example, bidentate
Upon UV light irradiation, the E structure of SRaaiNR0 changes to
the Z structure about the azo (–N@N–) function and the Z molar ra-
tio reached >80% (Scheme 2). This technique may assist the rotation
of the azo-aryl group (–N@N–Ar), which isomerizes from the E to Z
structure. The rates of photoisomerisation increase with decreasing
electronegativity of X in the complexes (Table 3); the rate follows
[Zn(SRaaiNR0)Cl2] (3) < [Zn(SRaaiNR0)Br2] (4) < [Zn(SRaaiNR0)I2] (5),
although the molar mass follows the order 3 < 4 < 5. The higher
electronegativity of Cl in 3 may help to enhance the association
strength with neighboring molecules and may effectively increase
the rotor mass to more than that of 4 or 5. The molecular association
has been supported in the crystalline state by hydrogen bonds and
N(imidazolyl) and N(azo) chelation. The
unit has a Zn–N(azo)
distance of 2.608(6) Å, which is longer than previously reported
data, e.g. 2.546(3) Å for [Zn(MeaaiMe)(H2O)Cl2] (MeaaiMe =
1-methyl-2-(p-tolylazo)imidazole) [18], but shorter than the sum
of the van der Waals radii of Zn(II) (1.39 Å) and N(sp2) (1.55 Å).
The chelate angle is 69.88(3)° and this is comparable to the re-
ported data in a series of chelated arylazoimidazole complexes
[14,15,18,19,26]. The angles in the coordination sphere are
Cl(2)–Zn(1)–Cl(3), 118.74(3)°; N(1)–Zn(1)–Cl(2), 119.99(7)°;
N(1)–Zn(1)–Cl(3), 118.32(7)° and
s = 0.70, which imply a five coor-
dinated distorted square pyramidal geometry about Zn(II) [33]. The
distorted square plane is constituted by Zn(1), N(1), N(2), Cl(3) and
O(13); Cl(2) is assumed to be in the apical position of the square
pyramid. Two aromatic rings attached to –N@N–, imidazolyl and
thiophenyl make a dihedral angle of 5.19(10)°. The N@N bond
length is 1.269(3) Å and this is slightly elongated compared to that
of the free ligand value (1.250(1) Å) [17]. The two Zn–Cl bonds are
almost equal in length, Zn–Cl(2), 2.2442(10) and Zn–Cl(3),
2.2445(11) Å, and they are longer than related reported distances
(2.223(1) and 2.256(1) Å) [15].
The coordinated H2O–Zn unit shows two hydrogen bonds,
Zn–O(HA)–HBꢁ ꢁ ꢁCl(2) and Zn–O(HB)–HAꢁ ꢁ ꢁCl(3), with two adjacent
molecules and this is the reason for an infinite 1D chain (Fig. 2); and
pꢁ ꢁ ꢁp interactions between thiophenyl rings of adjacent molecules
p p interactions (vide supra).
ꢁ ꢁ ꢁ
The thermal Z-to-E isomerisation of the complexes was
followed by UV–Vis spectroscopy (Fig. 6) in MeCN at various
13.5
13.4
a
13.3
13.2
13.1
b
13.0
12.9
constitute a 2D structure (Fig. 3). One of the two O–H bonds of
H2O interacts interacts with Cl (Zn-–Cl bonds) of adjacent
molecules, (Zn)Cl(2/2A)ꢁ ꢁ ꢁH(O)Hꢁ ꢁ ꢁCl(3/3A)(Zn) (Cl(2/2A)ꢁ ꢁ ꢁH–O,
d(Oꢁ ꢁ ꢁCl(2/2A)), 3.18(6); d(Hꢁ ꢁ ꢁCl(2/2A)), 2.36(7) Å and \O–Hꢁ ꢁ ꢁCl,
170.9(3)°, to construct a 1D chain (symmetry 2 ꢀ x, 2 ꢀ y, 1 ꢀ z for
Cl(2) and 1 ꢀ x, 2 ꢀ y, 1 ꢀ z for Cl(3)). The coordinated H2O of one
0.00315 0.00320 0.00325 0.00330 0.00335
1/T (k-1)
Fig. 7. Arrhenius plots of Z-to-E thermal isomerisation of (a) SMeaaiNMe (2a) (j,
––) and (b) [Zn(SMeaaiNMe)(H2O)Cl2] (N, – – –) (3a) at various temperatures (303–
318 K).

328
S. Saha (Halder) et al. / Polyhedron 67 (2014) 321–328
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were obtained (Table 4) from Arrhenius plots (Fig. 7). In the com-
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S⁄) are more negative in the complexes
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square pyramidal complexes. UV light irradiation to a solution of
the complexes causes photoisomerisation, E (trans) M Z (cis) about
–N@N– of SRaaiNR0. The decrease in electronegativity of X in-
creases the rate of E-to-Z photoisomerisation. The rates and quan-
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volume of the photochrome – complexes of a higher rotor mass
and volume commonly show slower rates of isomerisation. The
Z-to-E isomerisation is a thermal process and the activation ener-
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Acknowledgments
Financial support from Department of Science & Technology,
New Delhi is thankfully acknowledged.
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Appendix Supplementary. material
CCDC 948112; contains the supplementary crystallographic
data for Zn(SMeaaiNMe)(H2O)Cl2] (3a). These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
ing.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.
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