The synthesis, structure and photochromism of mercury(II)-iodide complexes of 1-CnH2n+1-2-(arylazo)imidazoles (n = 4, 6, 8)

Article abstract of DOI:10.1016/j.poly.2011.10.002

The reaction of HgI₂ with 1-C_nH_{2 n+1}-2-(arylazo)imidazole (Raai-C_nH _2n+1 where n = 4, 6, 8) has isolated iodide bridging dimeric complexes, [Hg(RaaiC_nH_{2n +1})(μ-I)(I)]₂. The structures of the ligand and the complexes have been established by spectral (UV-Vis, IR, ¹H NMR) data. One of these complexes [Hg(1-hexyl-2-(p-tolylazo)imidazole)(μ-I)(I)] ₂ has been structurally confirmed by single crystal X-ray diffraction study. The ligand, Raai-C_nH_2n+1 exists at ambient condition in trans geometry about azo (-NN-) group; the UV light irradiation in MeOH solution shows E-to-Z isomerisation. The reverse transformation, Z-to-E, is very slow with visible light irradiation while isomerises rapidly on heating. The coordinated ligand, Raai-C_nH _2n+1 in the complexes exhibit similar behaviour in DMF solution. Quantum yields (φ_{E→ Z}) of E-to-Z isomerisation are higher for free ligands than that of their metal complexes. The Z-to-E isomerisation is a thermally induced process. The activation energy (E_a) is calculated by controlled temperature experiment.

Full text of DOI:10.1016/j.poly.2011.10.002

Polyhedron 31 (2012) 506–514
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Polyhedron
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The synthesis, structure and photochromism of mercury(II)-iodide complexes
of 1-CnH2n+1-2-(arylazo)imidazoles (n = 4, 6, 8)
Debashis Mallick 1, Avijit Nandi, Shilpi Datta, Kamal Krishna Sarker 2, Tapan Kumar Mondal,
⇑
Chittaranjan Sinha
Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of HgI2 with 1-CnH2n+1-2-(arylazo)imidazole (Raai-CnH2n+1 where n = 4, 6, 8) has isolated
Received 13 August 2011
Accepted 4 October 2011
Available online 12 October 2011
iodide bridging dimeric complexes, [Hg(RaaiCnH2n+1)(
plexes have been established by spectral (UV–Vis, IR, 1H NMR) data. One of these complexes [Hg(1-hexyl-
2-(p-tolylazo)imidazole)( -I)(I)]2 has been structurally confirmed by single crystal X-ray diffraction
l-I)(I)]2. The structures of the ligand and the com-
l
study. The ligand, Raai-CnH2n+1 exists at ambient condition in trans geometry about azo (–N@N–) group;
the UV light irradiation in MeOH solution shows E-to-Z isomerisation. The reverse transformation, Z-to-E,
is very slow with visible light irradiation while isomerises rapidly on heating. The coordinated ligand,
Raai-CnH2n+1 in the complexes exhibit similar behaviour in DMF solution. Quantum yields (/E?Z) of E-
to-Z isomerisation are higher for free ligands than that of their metal complexes. The Z-to-E isomerisation
is a thermally induced process. The activation energy (Ea) is calculated by controlled temperature
experiment.
Keywords:
Arylazoimidazole
Mercury-iodide complexes
Spectral study
Photochromism
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
are found in the literature [19–25]. The photochromism of
1-alkyl-2-(arylazo)imidazole [20,21] and their Cu(I) [22], Cd(II)
Organic–inorganic hybrid materials are of current research
interest [1–7]. The properties of the materials can be varied by
changing the ligand types, presence of substituents, different car-
bocyclic and heterocyclic rings, and also by using different metal
ions. Towards the study of properties of the complexes we have
been interested to examine the photochromism of azoheterocycles
and the effect on metal coordination thereof. Photochromism is a
reversible photo-induced transformation between two molecular
states whose absorption spectra differ significantly [8–11].
Azo-conjugated metal complexes exhibit unique properties upon
light irradiation in the area of photon-mode high-density informa-
tion storage photoswitching devices [12–14]. Arylazoimidazoles
constitute an interesting class of heterocyclic azo compounds,
since imidazole is a ubiquitous and essential group in chemistry
and biology, especially as a metal coordinating site. The azohetero-
cycles have been extensively used as ligands for metal ions by us
[15–17] and others [18,19]. However, very few reports concerning
the photochromic property (Scheme 1) of arylazoimidazole dyes
[23], Hg(II) [24] and Pd(II) [25] complexes are so far reported in
the literature. An important advantage of imidazole is its chemical
and biological ubiquity and alkylation of N(1) and/or N(3) centres
with number of carbon centres as per one’s requirement. Double
alkylation has synthesised imidazolium salts those are potent ionic
liquids [26] and at present are using as green solvents in chemical
reactions. Presence of long chain alkyl group is useful in the disper-
sion of organic–inorganic hybrid material in organic media, forma-
tion of thin film, soft matter, etc. In this work we use, first time,
long chain alkyl group consisting of four (n-butyl), six (n-hexyl)
and eight (n-octyl) carbons in the alkylation of 2-(arylazo)imida-
zoles to synthesise 1-CnH2n+1-2-(arylazo)imidazole (Raai-CnH2n+1
where n = 4, 6, 8). The ligands so-obtained are used to synthesise
hitherto unknown mercury(II) complexes. Both ligands and com-
plexes are characterised by spectroscopic studies and in one case
by single crystal X-ray diffraction study. The photochromic activity
of the ligands and complexes are examined.
2. Results and discussion
⇑
Corresponding author. Fax: +91 33 2414 6584.
E-mail address: c_r_sinha@yahoo.com (C. Sinha).
Address: Department of Chemistry, Mrinalini Datta Mahavidyapith, Birati,
2.1. Synthesis and formulation of compounds
1
2-(Arylazo)imidazoles were synthesised by the diazotisation of
arylamine in NaNO2/HCl and followed by coupling with imidazole
in aqueous sodium carbonate solution (pH ꢀ7). The [Hg(Haai-
Kolkata 700051, India.
2
Present address: Department of Chemistry, Mahadevananda Mahavidyalaya,
Barrackpore, Monirampore, Kolkata 700120, India.
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.10.002

D. Mallick et al. / Polyhedron 31 (2012) 506–514
507
2.2. Molecular structure of [Hg(Meaai-C6H13)(
l-I)(I)]2 (5b)
N
N
N
R
N
N
The crystals of [Hg(Meaai-C6H13)( -I)(I)]2 (5b) are grown by
l
R
N
R
slow evaporation of the reaction mixture in MeOH–DMF at ambi-
ent condition for a week. The molecular structure is shown in
Fig. 1. The bond parameters are listed in Table 1. Each discrete
molecular unit consists of dinuclear iodide bridging Hg2I2 frag-
ment. Meaai-C6H13 acts as N,N0-donor end capping agent where
N and N0 refer to N(imidazole) and N(azo) donor centres and a
non-bridged-I atom lies in a semi-axial position. The Hg2I2 bridge
has an asymmetric tetra-atomic puckered rhombohedral geome-
try: Hg(1)–I(2), 2.6818(6) Å; Hg(1)–I(2a), 3.421(6) Å (symmetry
code (a) = ꢀx, y, 1/2 ꢀ z). Atoms Hg(1), N(4), N(3), C(8), N(1) consti-
tute the chelate plane with a maximum deviation <0.04 Å. The pen-
dant aryl ring makes a small dihedral of 4.7° with the chelated
azoimidazole ring. The bond angles I(1)–Hg(1)–I(2), 135.36(3)°
and I(1)–Hg–I(2a), 97.85(3)° also support this distorted geometry.
The structural demand of long chain 1-C6H13– group may be the
reason for larger deviation of both bridging and non-bridging met-
ric parameters compared to previously reported data [24]. Interest-
ingly two 1-C6H13– groups in Hg2I2 dimmer stereochemically
disposed to make 34.33° torsion angle. The small chelate angle
(64.52(3)°) may be one of the reasons for geometrical distortion.
The Hg–N(imidazole) (Hg(1)–N(1/1a), 2.306(8) Å) is shorter than
Hg–N(azo) (Hg(1)–N(4/4a), 2.717(2) Å, which reflects stronger
interaction between Hg(II) and N(imidazole). Although Hg–
N(azo) bond length is very long but it is less than the sum of van
N
N
R
trans- (E)
cis- (Z)
Scheme 1. Isomerisation of 1-alkyl-2-(arylazo)imidazole.
C4H9)(
C6H13)(
C8H17)(
l
l
l
-I)(I)]2 (4a), [Hg(Meaai-C4H9)(
-I)(I)]2 (5a), [Hg(Meaai-C6H13)(
-I)(I)]2 (6a), [Hg(Meaai-C8H17)(
l
l
-I)(I)]2 (4b) [Hg(Haai-
-I)(I)]2 (5b), [Hg(Haai-
-I)(I)]2 (6b) alkylation
l
was carried out by adding alkyl halide in dry THF solution to the
corresponding 2-(arylazo)imidazole in presence of NaH (Scheme
2). The n-butyl (C4H9–), n-hexyl (C6H13–) and n-octyl (C8H17–)
are appended at N(1) of imidazole unit of 2-(arylazo)imidazole to
synthesise present series of molecules, Raai-CnH2n+1 (n = 4, 6, 8).
The ligands are unsymmetrical N,N0-bidentate chelator where N
and N0 refer to N(imidazole) and N(azo) donor centres, respec-
tively. The reaction between Raai-R0 and HgI2 in 1:1 mole ratio in
MeOH and ethyleneglycol monomethyl ether (EGME) mixture
has isolated compounds of chemical formula [Hg(Raai-R0)(
l-I)I]2.
µ
µ
µ
[Hg(Ha
µ
µ
µ
Scheme 2. Preparation of ligands and Hg(II)-iodide complexes.

508
D. Mallick et al. / Polyhedron 31 (2012) 506–514
Fig. 1. The crystal structure of [Hg(Meaai-C6H13)(l-I)(I)]2 (5b).
Table 1
C6H13 show
p–p interactions with aromatic groups of neighbour-
Selected bond distances (Å) and angles (°) for [Hg(Meaai-C6H13)(
l
-I)(I)]2 (5b).
ing molecule leading to the formation of 1D chain (Fig. 2). The
interaction units are p-tolyl (Cg(4) of molecule A)–imidazole
(Cg(3) of molecule B), 3.764 Å (symmetry, 1/2 ꢀ x, 1/2 ꢀ y, ꢀz)
and p-tolyl (Cg(4) of molecule A)–p-tolyl (Cg(4) of molecule B),
3.865 Å (symmetry, ꢀx, y, 1/2 ꢀ z). A weak hydrogen bond is also
observed at C(11)–H(11B)ꢁ ꢁ ꢁN(3) where distance of H(11B)ꢁ ꢁ ꢁN(3)
and C(11)ꢁ ꢁ ꢁN(3) are 2.50 and 2.92 Å, respectively, and angle is
106° (Fig. 2). Although iodine is not sufficiently electronegative
to form hydrogen bonded structure but coordinated semi-axial io-
dine, Hg(1)–I(1), has ability to form hydrogen bond with stereo-
chemically close hydrogen of –C(13)H2– of neighbouring
dimmer: C(13)–H(13b)ꢁ ꢁ ꢁI(1), H(13b)ꢁ ꢁ ꢁI(1), 3.60 Å, C(13)ꢁ ꢁ ꢁI(1),
4.084 Å and \C(13)–H(13b)ꢁ ꢁ ꢁI(1), 159.51° which makes dim-
mer–dimmer interaction to constitute 1D chain. The disposal of
dimmer lies in a manner such that 1-C6H13 groups of two dimmers
are interpenetrated to constitute a tetrameric motif that serves as
crystal building motif (Fig. 3).
Bond distances (Å)
Bond angles (°)
Hg(1)–I(1)
Hg(1)–I(2)
Hg(1)–I(2a)a
Hg(1)–N(1/1a)
Hg(1)–N(4)
N(1)–C(8)
2.6476(9)
2.6818(9)
3.421(6)
2.306(8)
2.717(2)
1.352(12)
1.363(13)
1.263(11)
I(1)–Hg(1)–I(2)
I(1)–Hg(1)–I(2a)a
N(1)–Hg(1)–I(1)
N(4)–Hg(1)–I(2)
N(4)–Hg(1)–I(2)b
I(2a)a–Hg(1)–I(2)b
Hg(1)–I(2a)Hg(1a)
N(1)–Hg(1)–N(4)
N(4)–Hg(1)–I(1)
N(1)–Hg(1)–I(2)
135.36(3)
97.85(3)
114.9(2)
93.98(3)
148.74(12)
92.23(10)
83.86(1)
64.52(3)
98.84(4)
109.3(2)
N(2)–C(8)
N(4)–N(3)
a
Symmetry: ꢀx, ꢀy, ꢀz.
b
Symmetry: 1 ꢀ x, 2 ꢀ y, 1 ꢀ z.
der Waals radii of Hg(II) (1.55 Å) and N(sp2) (1.53 Å). This implies
significant bonding interaction between these components. The
bond lengths of Hg–N and Hg–I are slightly longer than that of pre-
viously reported results [24]. The stronger coordination of imidaz-
ole-N to Hg(II) compared to azo-N has significant biochemical
implication and explains strong toxicity of Hg(II) [27]. Because of
long Hg(II)–N(azo) distance, the molecule becomes useful for
photo-activation via cleavage of this bond followed by rotation to
introduce photoisomerisation (vide infra). The N@N distance is
1.263(11) Å and is comparable to previously reported result
(1.262(6) Å) [20,21]. The Hg–I(bridge) (Hg(1)–I(2), 2.6818(9) Å) is
longer than Hg–I(non-bridge) (Hg(1)–I(1), 2.6476(9) Å). The Hg–
I(1/1a) lies at semi-axial position to their respective plane. The che-
late ring around Hg is twisted in a manner so that the Hg–N(azo)
inclines to axial Hg–I bond, N(4)–Hg(1)–I(1), 99.03(4)°. Two aro-
matic rings (p-tolyl and imidazole) of each coordinated Meaai-
2.3. Spectral studies
The bands in the FT-IR spectra of the ligands (1–3) and the com-
plexes (4–6) are assigned based on comparing with reported work
[22,27]. The point of interest is the band due to the azo (–N@N–)
and imine (–C@N–) groups in compounds. The complexes, 4–6,
show moderately intense stretching at 1580–1695 and 1370–
1385 cmꢀ1 for
m(C@N) and m(N@N), respectively, and free ligand,
1–3, values appear at higher frequency, 1620–1625 and 1400–
1410 cmꢀ1, respectively. In the complexes, the frequencies are
shifted to lower value than free ligand which support the coordina-
tion of azo-N and imine-N to Hg(II).

D. Mallick et al. / Polyhedron 31 (2012) 506–514
509
Fig. 2. 1D chain structure of [Hg(Meaai-C6H13)(
l-I)(I)]2 (5b) formed by C–Hꢁ ꢁ ꢁN/I hydrogen bonding and p–p interactions.
Fig. 3. View of interdigitated tetramer that serves as motif to pack crystal in 2D geometry.

510
D. Mallick et al. / Polyhedron 31 (2012) 506–514
in DMSO-d6 (Table 2) because of solubility problem in former sol-
vent. The atom numbering pattern is shown in Scheme 1. The
alkylation of imidazole is supported by the disappearance of
d(N–H) at ꢂ10.30 ppm and the appearance of N(1)-alkyl signal at
0.85–4.40 ppm for 1, 2 and, 3; –N–CH2–(CH2)n–CH3 shows a triplet
for –N–CH2– at 4.39–4.46 ppm, a triplet at 0.85–89 ppm for –CH3
group and a multiplet for –(CH2)n– at 1.30–1.90 ppm. Imidazole
4- and 5-H appears as broad singlet at 7.26–7.33 and 7.13–
7.18 ppm, respectively. Broadening may be due to rapid proton ex-
change between these imidazole protons The aryl protons (7-H to
11-H) are upfield shifted on going from phenylazo (a) to p-tolylazo
(b) which may be due to +I effect of –Me group. Data (Table 2) re-
veal that the signals in the spectra of the complexes are shifted to
downfield side relative to free ligand values at 6.5–8.5 ppm. The
signals at 1.00–5.00 ppm hardly exhibit any change. Important
features of the spectra are the shifting of imidazole protons 4-H
and 5-H to lower d-values, in general, relative to perturbation of
aryl protons (7-H–11-H). Imidazole protons suffer downfield shift-
ing by 0.2–0.5 ppm compared to the free ligand signal position.
This supports the strong preference of binding of imidazole-N to
Hg(II). Aryl signals shift to the lower field side on Me-substitution
to the aryl ring. This is due to electron donating effect of the Me-
group. This conclusion is also supported by single crystal X-ray
structure of one of the complexes (Fig. 1).
Fig. 4. UV–Vis spectra of (a) Meaai-C6H13 (2b) in MeOH and (b) [Hg(Meaai-
C6H13)(l-I)(I)]2 (5b) in DMF solution.
The absorption spectra were recorded in MeOH solution for the
ligands and in DMF solution (because of sparing solubility of the
complexes in MeOH) for the complexes, in the wavelength range
200–700 nm (Fig. 4). The spectra of the ligands show absorption
band at 340–380 nm with a molar absorption coefficient in the or-
der of 103 Mꢀ1 cm2 and a weak band at 450–455 nm. The intense
2.4. Photochromism
Upon UV-light irradiation at fixed time interval at kmax to a
MeOH solution of the ligands shows the changes of absorption
spectrum that is corresponding to the structural change of the li-
gand from trans-Raai-CnH2n+1 (E-isomer) to cis-Raai-CnH2n+1 (Z-iso-
mer) (Fig. 5). The intense peak at kmax decreases, which is
accompanied by a slight increase at the tail portion of the spectrum
until a stationary state is reached. Subsequent irradiation at the
newly appeared longer wavelength peak reverses the course of
the reaction and the original spectrum is recovered up to a point,
which is another photostationary state under irradiation at the
longer wavelength peak. The quantum yields of the E-to-Z photo-
isomerisation are determined using those of azobenzene [24,28]
as a standard and the results are tabulated in Table 3.
The E-to-Z isomerisation of Hg(II) complexes of Raai-CnH2n+1
(n = 4, 6, 8) in presence of Iꢀ are carried out in DMF solution be-
cause of sparing solubility in MeOH. It is observed that upon irra-
diation with UV light E-to-Z change proceeds and the Z molar ratio
is reached to ꢂ75%. The absorption spectra of the coordinated Raai-
CnH2n+1 in E-form have changed with isosbestic points upon excita-
tion (Fig. 6) into the Z-isomer. The ligands and the complexes show
little sign of degradation upon repeated irradiation at least up to 15
cycles in each case. The quantum yields were measured for the
absorption band is assigned to p–
p⁄ transitions, while the tail cor-
responds to n-p⁄ transition. The p⁄ band exhibits bathochromic
p–
shifts by ꢂ30 nm compared with azobenzene [28], while the n-p⁄
band shows little shift. As a consequence, the energy separation
between the p–
p⁄ and n-p⁄ transitions in arylazoimidazoles is nar-
rower than that of azobenzene.
The characteristics common to the complexes are a structured
absorption band around 360–390 nm with a molar absorption
coefficient on the order of 103 Mꢀ1 cm2 and a weak band at 450–
460 nm (
e
ꢂ 103 Mꢀ1 cm2). From the analogy with the absorption
spectra of ligands (1–3) it is likely that the large absorption band
around 360–380 nm corresponds to p–
p⁄ transitions, while the tail
corresponds to n-p⁄ transition. The transitions are shifted to longer
wavelength by av. 8 nm. This may due to the overlapping of MLCT
transition from Hg(II) ? p⁄(azoimine) (Fig. 4). The p⁄ absorption
p–
peaks (kmax) for derivatives of (2-phenylazo)imidazole are within a
range of 365–385 nm, which is between the
p
–
p⁄ absorption bands
of azobenzene (313 nm) and 4-N,N-dimethylaminoazobenzene
(390 nm) [29,30].
The 1H NMR spectra of ligands (1–3) are recorded in CDCl3 and
those of complexes [Hg(Raai-CnH2n+1)(l-I)(I)]2 (4–6) are recorded
Table 2
1H NMR spectral data in DMSO-d6 at room temperature.
4-Hs
5-Hs
7,11-Hd
8,10-Hd
9-Rs
12-CH2
LCH3
t
t
*
bs
LCH3—ðCH2Þ – CH2ꢀ
Compounds
Haai-C4H9 (1a)
Meaai-C4H9 (1b)
Haai-C6H13 (2a)
Meaai-C6H13 (2b)
Haai-C8H17 (3a)
Haai-C8H17 (3b)
7.29
7.26
7.32
7.31
7.33
7.32
7.75
7.73
7.78
7.75
7.79
7.77
7.16
7.13
7.17
7.15
7.18
7.15
7.33
7.30
7.35
7.32
7.36
7.34
7.86
7.79
7.40
7.35
7.41
7.29
7.42
7.30
7.55
7.40
7.58
7.42
7.61
7.60
7.40
2.40
7.41
2.42
7.42
2.43
7.55
2.38
7.58
2.40
7.60
2.42
4.40
4.39
0.87
0.85
1.85–1.31
1.84–1.30
1.86–1.30
1.87–1.31
1.89–1.33
1.88–1.32
1.82–1.23
1.80–1.22
1.82–1.23
1.82–1.23
1.83–1.24
1.85–1.29
7.97 (8.2)
7.89 (8.6)
7.99 (8.0)
7.95 (7.5)
7.95 (8.2)
7.82 (7.6)
7.98 (8.2)
7.86 (7.6)
7.99 (8.5)
7.90 (8.7)
4.45 (6.9)
4.41 (7.0)
4.46 (7.1)
4.43 (6.4)
4.40 (6.45)
4.39 (7.0)
4.42 (6.45)
4.45 (7.0)
4.43 (7.4)
4.46 (7.5)
0.88 (5.8)
0.86 (6.1)
0.89 (6.5)
0.87 (6.2)
0.81 (6.0)
0.79 (6.4)
0.82 (6.0)
0.80 (6.4)
0.84 (6.1)
0.85 (7.0)
[Hg(Haai-C4H9)(
l
-I)(I)]2 (4a)
-I)I]2 (4b)
-I)(I)]2 (5a)
-I)(I)]2 (5b)
-I)(I)]2 (6a)
-I)(I)]2 (6b)
[Hg(Meaai-C4H9)(
l
[Hg(Haai-C6H13)(
l
[Hg(Meaai-C6H13)(
l
[Hg(Haai-C8H17)(
l
Hg(Meaai-C8H17)(
l
s, singlet; d, doublet; t, triplet; bs, broad singlet; L, last –CH3 of respective N-alkyl group.
*
All Hs of middle ‘ACH2A’ of CnH2n+1
.

D. Mallick et al. / Polyhedron 31 (2012) 506–514
511
1.2
1.0
0.8
0.6
0.4
0.2
0.0
means faster Z-to-E thermal isomerisation of the complexes. The
entropy of activation (
S⁄) are highly negative in the complexes
than that of free ligands. This is also in support of increase in rotor
volume of the complexes.
D
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Trans-isomer
cis-isomer
Decreasing
3. Experimental
323 nm
433 nm
3.1. Materials
Isobestic point
HgI2 was prepared by adding KI solution to Hg(NO3)2 solution,
filtered and washed with profuse amount of water and dried in a
CaCl2 desiccator. 2-(Arylazo)imidazoles were synthesised by re-
ported procedure [29]. All other chemicals and solvents were re-
agent grade as received.
300 350 400 450 500 550 600 650
Wavelength(nm)
323 nm
3.2. Physical measurements
433 nm
Increasing
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,
4000–400 cmꢀ1) from a Perkin-Elmer RX-1 FT-IR spectrophotome-
ter; photo excitation has been carried out using a Perkin-Elmer LS-
55 spectrofluorimeter and 1H NMR spectra were recorded from a
Bruker (AC) 300 MHz FTNMR spectrometer.
Isobestic point
300 350
400
450
500
550
600
650
Wavelength(nm)
Fig. 5. Spectral changes of Meaai-C6H13 (2b) in MeOH upon repeated irradiation at
364 nm at 3 min interval at 25 °C.
E-to-Z (/E?Z) photoisomerisation of these ligands in MeOH and
that of the complexes in DMF on irradiation of UV wavelength (Ta-
ble 3). The /E?Z values are significantly depend on nature of sub-
stituents; the Me substituent at azoaryl group and the
substituents (CnH2n+1: n = 4, butyl; n = 6, hexyl; n = 8, octyl) at
N(1)-position both reduce /E?Z values. The photoisomerisation
rate and quantum yields of coordinated ligand are decreased com-
pared to free ligands and in general, increase in mass of the mole-
cule reduces the rate and quantum yield of isomerisation.
3.3. Preparation of compounds
3.3.1. Synthesis of [N(1)-hexyl-2-(p-tolylazo)imidazole], Meaai-C6H13
(2b)
To dry THF solution (75 ml) of 2-(p-tolylazo)imidazole (2 g,
10.8 mmol), NaH (50% paraffin) (0.56 g) was added in small portion
and stirred at cold condition on ice bath for 0.5 h. 1-Bromohexane
(3.57 g, 11.69 mmol) was added slowly through pressure equalis-
ing system under stirring condition for a period of 1 h and then
warm for another 1 h on steam bath. The solution was evaporated
to dryness, extracted with CH2Cl2, washed with NaOH solution
(10%, 3ꢃ 10 ml) and finally by distilled water (3ꢃ 20 ml). The
CH2Cl2 extract was chromatographed over silica gel column (45ꢃ
1 ml) prepared in benzene. The elution was performed by ben-
zene–acetonitrile (10:1 v/v) mixture. On slow evaporation in air
orange crystalline product was obtained and then dried over
P4O10 under vacuum.
The lowering of /E?Z in the complexes may be due to the pres-
ence of coordinated HgI2 that increases molecular weight and se-
verely interferes the motion of the –N@N–Ar moiety and photo
bleaching efficiency of halide [31] may snatch out energy from
p–
p⁄ excited state. These may cause very fast deactivation other
than photochromic route. Both rotor mass and volume are in-
creased upon coordination of ligand to HgI2. These two factors have
significant influence on the isomerisation rate and quantum
yields.
The microanalytical data of the complexes are as follows: Haai-
C4H9 (1a), Anal. Calc. for C13H16N4: C, 68.39; H, 7.06; N, 24.54.
Found: C, 68.35; H, 7.08; N, 24.50%. FT-IR (KBr disc, cmꢀ1),
The Z-to-E isomerisation of Raai-CnH2n+1 (1–3) and their Hg(II)
complexes (4–6) are followed by spectra measurements in MeOH
and DMF, respectively, at varied temperatures, 298–313 K. The
Eyring plots in the range 298–313 K gave a linear graph from
m
(N@N), 1404;
m
(C@N), 1621 cmꢀ1. UV–Vis spectroscopic data in
MeOH (kmax (nm) (10ꢀ3
e
(dm3 molꢀ1 cmꢀ1): 363 (13.1), 376
which the activation parameters
D D
S⁄ and H⁄ are calculated (Table
(12.4), 448 (2.3). Meaai-C4H9 (1b), Anal. Calc. for C14H18N4: C,
69.39; H, 7.49; N, 23.12. Found: C, 69.34; H, 7.47; N, 23.14%. FT-IR
4 and Fig. 7). In the complexes, the Eas are severely reduced which
Table 3
Results of photochromism, rate of conversion and quantum yields upon UV light irradiation.
pꢄ
Compounds
k
(nm)
Isobestic point (nm)
Rate of E ? Z conversion ꢃ 109 (sꢀ1
)
/
E?Z
conversion ꢃ 109
p;
1a
1b
2a
2b
3a
3b
4a
4b
5a
5b
6a
6b
363
362
363
364
364
365
367
366
367
368
368
369
328, 437
327, 430
325, 435
323, 433
330, 427
324, 435
335, 439
331, 440
336, 438
332, 436
333, 438
332, 448
36.01
35.71
34.92
34.09
33.76
32.14
32.69
32.01
31.09
30.11
28.12
27.03
1.90 0.005
1.87 0.002
1.81 0.001
1.79 0.003
1.69 0.004
1.65 0.002
1.75 0.002
1.71 0.002
1.65 0.001
1.61 0.003
1.57 0.003
1.55 0.001

512
D. Mallick et al. / Polyhedron 31 (2012) 506–514
Table 4
Rate and activation parameters for Z(c) ? E(t) thermal isomerisation.
Compounds
T (K)
Rate of thermal Z ?
Ea (kJ molꢀ1
)
D
H⁄ (kJ molꢀ1
)
D
S⁄ (J molꢀ1 Kꢀ1
)
D )
G⁄c (kJ molꢀ1
E conversion ꢃ 105 (sꢀ1
)
1a
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
3.62
5.10
10.5
19.0
6.01
88.20
85.66
85.93
89.19
89.57
90.85
91.12
27.12
25.97
25.82
25.01
23.80
22.90
ꢀ43.27
ꢀ37.96
98.9
1b
2a
2b
3a
3b
4a
4b
5a
5b
6a
6b
88.47
91.73
92.11
93.38
93.66
29.65
28.51
28.36
27.55
25.62
25.43
97.53
98.36
98.38
98.13
97.07
89.5
9.02
18.04
32.01
4.20
6.20
13.2
23.5
6.09
11.21
19.01
37.03
4.40
ꢀ30.09
ꢀ25.12
ꢀ23.83
7.60
13.20
27.30
7.11
ꢀ19.45
10.21
21.01
42.03
24.00
27.33
36.50
41.20
52.55
58.36
69.07
91.87
34.55
39.36
45.07
60.87
55.55
64.36
72.07
96.87
39.57
49.61
57.64
65.21
59.15
67.36
74.52
98.97
ꢀ204.24
ꢀ220.90
ꢀ224.82
ꢀ223.53
ꢀ232.39
ꢀ230.12
93.46
94.51
93.3
94.01
93.12
(KBr disc, cmꢀ1),
m
(N@N), 1402;
m
(C@N), 1622 cmꢀ1. UV–Vis spec-
(dm3 molꢀ1 cmꢀ1): 362
3.3.2. Synthesis of [Hg(Meaai-C6H13)(
l
-I)(I)]2 (5b)
troscopic data in MeOH (kmax (nm) (10ꢀ3
e
To methanol (25 ml) solution of Meaai-C6H13 (0.025 g,
0.093 mmol), HgI2 (0.040 g, 0.087 mmol) in ethyleneglycol mono-
methylether (EGME, 10 ml) was added and refluxed for 2 h. Or-
ange-yellow precipitate appeared. The precipitate was collected
by filtration, washed with cold MeOH and dried over CaCl2 in vac-
uum. The yield was 0.046 g (72%). Other complexes were prepared
under identical conditions and the yield varied in the range 65–
75%.
(12.9), 379 (11.9), 447 (1.9). Haai-C6H13 (2a), Anal. Calc. for
15H20N4: C, 70.28; H, 7.81; N, 21.86. Found: C, 70.32; H, 7.81; N,
C
22.00%. FT-IR (KBr disc, cmꢀ1),
m
(N@N), 1408;
m
(C@N), 1625 cmꢀ1
.
UV–Vis spectroscopic data in MeOH (kmax (nm) (10ꢀ3
e
(dm3 molꢀ1 cmꢀ1): 363 (13.5), 380 (11.4), 451 (2.1). Meaai-C6H13
(2b), Anal. Calc. for C16H22N4: C, 71.08; H, 8.20; N, 20.72. Found:
C, 71.20; H, 8.30; N, 20.78%. FT-IR (KBr disc, cmꢀ1),
m(N@N), 1403;
m
(C@N), 1627 cmꢀ1. UV–Vis spectroscopic data in MeOH (kmax
The microanalytical data of the complexes are as follows:
(nm) (10ꢀ3
e
(dm3 molꢀ1 cmꢀ1): 364 (13.9), 379 (12.9), 447 (1.9).
[Hg(Haai-C4H9)(
22.85; H, 2.34; N, 8.21. Found: C, 22.89; H, 2.37; N, 8.19%. FT-IR
(KBr disc, cmꢀ1), (C@N), 1583 cmꢀ1. UV–Vis spec-
(N@N), 1372;
troscopic data in DMF (kmax (nm) (10ꢀ3 (dm3 molꢀ1 cmꢀ1): 285
(8.0), 366 (11.21), 381 (9.7), 455 (2.4). [Hg(Meaai-C4H9)( -I)(I)]2
l-I)(I)]2 (4a), Anal. Calc. for C13H16N4HgI2: C,
Haai-C8H17 (3a), Anal. Calc. for C17H24N4: C, 71.80; H, 8.51; N,
19.70. Found: C, 71.76; H, 8.45; N, 19.77%. FT-IR (KBr disc, cmꢀ1),
m
m
m
(N@N), 1405;
m
(C@N), 1620 cmꢀ1. UV–Vis spectroscopic data in
e
MeOH (km (nm) (10ꢀ3
e
(dm3 molꢀ1 cmꢀ1): 360 (12.7), 380 (11.5),
l
451 (2.2). Meaai-C8H17 (3b), Anal. Calc. for C18H26N4: C, 72.44; H,
(4b), Anal. Calc. for C14H18N4HgI2: C, 24.11; H, 2.60; N, 8.04. Found:
8.78; N, 18.77. Found: C, 72.47; H, 8.73; N, 18.72%. FT-IR (KBr disc,
C, 24.16; H, 2.55;ꢀN1 , 8.00%. FT-IR (KBr disc, cmꢀ1),
m(N@N), 1374;
cmꢀ1),
m
(N@N), 1407;
m
(C@N), 1615 cmꢀ1. UV–Vis spectroscopic
m
(C@N), 1591 cm . UV–Vis spectroscopic data in DMF (km (nm)
data in MeOH (kmax (nm) (10ꢀ3
e
(dm3 molꢀ1 cmꢀ1): 365 (13.3),
(10ꢀ3
e
(dm3 molꢀ1 cmꢀ1): 280 (8.2), 367 (11.1), 383 (9.7), 460
-I)(I)]2 (5a), Anal. Calc. for C15H20N4HgI2:
380 (12.03), 449 (1.9).
(1.9). [Hg(Haai-C6H13)(
l

D. Mallick et al. / Polyhedron 31 (2012) 506–514
513
Table 5
0.55
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
Decreasing
Summarised crystallographic data for [Hg(Meaai-C6H13)(l-I)(I)]2 (5b).
0.55
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
Compound
5b
Trans-isomer
Cis-isomer
Empirical formula
Formula weight
T (K)
Crystal system
Space group
Crystal size (mm)3
a (Å)
C
32H44N8I4Hg2
1449.53
293(2)
monoclinic
C2/c
0.34 ꢃ 0.30 ꢃ 0.29
14.6328(4)
19.0566(6)
15.0012(5)
90.00
91.152(2)
90.00
4182.3(2)
4
332 nm
b (Å)
c (Å)
300 350 400 450 500 550 600 650 700 750 800
a
(°)
Wavelength(nm)
b (°)
c
(°)
V (Å3)
Z
Increasing
l
(Mo K
a
) (mmꢀ1
)
10.317
1.75–30.68
ꢀ20 < h < 20; ꢀ20 < k < 27; ꢀ21 < l < 20
h range
439nm
hkl range
Dcalc (mg mꢀ3
)
2.1302
208
32102
6360
0.0423
0.1143
0.994
300 350 400 450 500 550 600 650 700 750 800
Wavelength(nm)
Refine parameters
Total reflections
Unique reflections
a
Fig. 6. Spectral changes of [Hg(Meaai-C6H13)(
irradiation at 368 nm at 5 min interval at 25 °C.
l
-I)(I)]2 (5b) in DMF upon repeated
R1 [I > 2
r
(I)]
b
wR2
Goodness-of-fit (GOF) on F2
w = 1/[
r
2(Fo)2 + (0.0482P)2 + (4.8416P)] for 5b; where P ¼ ððFo2 þ 2F2c Þ=3.
P
P
C, 25.35; H, 2.84; N, 7.88. Found: C, 25.42; H, 2.81; N, 7.80%. FT-IR
(KBr disc, cmꢀ1), (C@N), 1580 cmꢀ1. UV–Vis spec-
(N@N), 1373;
troscopic data in DMF (km (nm) (10ꢀ3 (dm3 molꢀ1 cmꢀ1): 290
(7.8), 367 (11.4), 383 (9.9), 458 (2.4). [Hg(Meaai-C6H13)( -I)(I)]2
(5b), Anal. Calc. for C16H22N4HgI2: C, 26.49; H, 3.04; N, 7.73. Found:
C, 26.57; H, 3.00; N, 7.6%. FT-IR (KBr disc, cmꢀ1),
(N@N), 1375;
a
R = ||Fo| ꢀ |Fc||/ |Fo|.
P
P
2
2 1=2
wR2 ¼ ½ ðF2o ꢀ Fc2Þ = wðF2oÞ ꢅ
.
b
m
m
e
l
3.4. X-ray diffraction study
m
m
(C@N), 1597 cmꢀ1. UV–Vis spectroscopic data in DMF (km (nm)
The crystallographic data are shown in Table 5. Suitable single
crystal of 5b was mounted on a Siemens CCD diffractometer
equipped with graphite monochromated Mo K
radiation. The unit cell parameters and crystal-orientation matri-
ces were determined for two complexes by least squares refine-
ments of all reflections. The intensity data were corrected for
Lorentz and polarisation effects and an empirical absorption cor-
rection were also employed using the SAINT program. Data were col-
(10ꢀ3
e
(dm3 molꢀ1 cmꢀ1): 288 (7.9), 368 (11.30), 381 (9.8), 454
-I)(I)]2 (6a), Anal. Calc. for C17H24N4HgI2:
C, 27.62; H, 3.25; N, 7.58. Found: C, 27.73; H, 3.29; N, 7.63%. FT-
IR (KBr disc, cmꢀ1), (C@N), 1599 cmꢀ1. UV–Vis
(N@N), 1374;
spectroscopic data in DMF (km (nm) (10ꢀ3 (dm3 molꢀ1 cmꢀ1):
287 (7.82), 368 (11.2), 387 (9.9), 457 (2.1). [Hg(Meaai-C8H17)(
(1.9). [Hg(Haai-C8H17)(
l
a (k = 0.71073 Å)
m
m
e
l
-
I)(I)]2 (6b), Anal. Calc. for C18H26N4HgI2: C, 28.70; H, 3.49; N,
7.44. Found: C, 28.69; H, 3.49; N, 7.41%. FT-IR (KBr disc, cmꢀ1),
lected applying the condition I > 2r(I). All these structures were
m
(N@N), 1379; m
(C@N), 1590 cmꢀ1. UV–Vis spectroscopic data in
solved by direct methods and followed by successive Fourier and
difference Fourier syntheses. Full matrix least squares refinements
on F2 were carried out using SADBAS [32] with anisotropic displace-
ment parameters for all non-hydrogen atoms. Hydrogen atoms
were constrained to ride on the respective carbon or nitrogen
atoms with an isotropic displacement parameters equal to 1.2
times the equivalent isotropic displacement of their parent atom
in all cases. Complex neutral atom scattering factors were used
throughout for all cases. All calculations were carried out using
SHELXS97 [33], SHELXL97 [33], PLATON99 [34], ORTEP-3 [35] program.
DMF (km (nm) (10ꢀ3
e
(dm3 molꢀ1 cmꢀ1): 288 (8.1), 369 (10.8),
386 (9.7), 455 (2.3).
3.5. Photometric measurements
Absorption spectra were taken with a Perkin-Elmer Lambda 25
UV–Vis Spectrophotometer in a 1 ꢃ 1 cm quartz optical cell main-
tained at 25 °C with a Peltier thermostat. The light source of a Per-
kin-Elmer 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
initial trans-to-cis isomerisation rates (
m
) in a well-stirred solution
= (/I0/
within the above instrument using the equation,
m
V)(1 ꢀ 10ꢀAbs) 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
Fig. 7. The Eyring plots of rate constants of Z-to-E isomerisation of (a) Meaai-C6H13
(2b) and (b) [Hg(Meaai-C6H13)( -I)(I)]2 (5b) at 298–313 K.
l

514
D. Mallick et al. / Polyhedron 31 (2012) 506–514
at the irradiation wavelength. The value of I0 was obtained by using
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.
azobenzene (/ = 0.11 for p–
p⁄ excitation [28]) under the same irra-
diation conditions.
The thermal cis-to-trans isomerisation rates were obtained by
monitoring absorption changes intermittently for a cis-rich solu-
tion kept in the dark at constant temperatures (T) in the range from
298 to 313 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
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Acknowledgements
Financial support from Department of Science & Technology,
New Delhi is thankfully acknowledged. One of us (A. Nandi) thanks
the University Grants Commission, New Delhi for fellowship.
Appendix A. Supplementary data
CCDC 835026 contains the supplementary crystallographic data
for [Hg(Meaai-C6H13)(l-I)(I)]2 (5b). These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or