Structural studies and photochromism of mercury(II)-iodo complexes of (arylazo)imidazoles

Article abstract of DOI:10.1021/ic061221u

Neat reaction between HgI₂ and 1-methyl-2-(phenylazo)imidazole (Pai-Me) under microwave irradiation has isolated a novel compound whose structure shows intercalated HgI₂ in the layers of Pai-Me. They exist independently in interpenetrated arrays. In a solution phase study, the same reaction has synthesized an iodo-bridged azoimidazole-Hg(II) complex, [Hg(RaaiR′)(μ-I)(I)]₂ (RaaiR′ = 1-alkyl-2-(arylazo) imidazole). The structures have been characterized by X-ray diffraction studies. Chloro-bridged Hg(II) complexes of azoimidazoles, [Hg(RaaiR′)(μ-Cl) (Cl)]₂, are also known. These complexes upon irradiation with UV light show trans-to-cis isomerization. The reverse transformation, cis-to-trans isomerization, is very slow with visible light irradiation. Quantum yields (φ_t→c) of trans-to-cis isomerization are calculated, and the free ligand shows higher φ than their Hg(II) complexes. The cis-to-trans isomerization is a thermally induced process. The activation energy (E _a) of cis-to-trans isomerization is calculated by controlled temperature reaction. The E_a's of free ligands are much higher than that of halo-bridged Hg(II)-azoimidazole complexes. Chloro-bridged Hg(II) complexes show lower E_a's than those of iodo-bridged complexes. DFT calculation has been adopted to rationalize the experimental results.

Full text of DOI:10.1021/ic061221u

Inorg. Chem. 2007, 46, 670−680
Structural Studies and Photochromism of Mercury(II)
of (Arylazo)imidazoles
−Iodo Complexes
K. K. Sarker,† B. G. Chand,† K. Suwa,‡ J. Cheng,§ T.-H. Lu,§ J. Otsuki,‡ and C. Sinha*,†
Department of Chemistry, Inorganic Chemistry Section, JadaVpur UniVersity, Kolkata-7000 032,
India, Department of Materials and Applied Chemistry, College of Science and Technology, Nihon
UniVersity, 1-8-4 Kanda Surugadai, Chiyoda-ku, Tokyo 101-8308, Japan, and Department of
Physics, National Tsing Hua UniVersity, Hsinchu 300, Taiwan
Received July 3, 2006
Neat reaction between HgI2 and 1-methyl-2-(phenylazo)imidazole (Pai-Me) under microwave irradiation has isolated
a novel compound whose structure shows intercalated HgI2 in the layers of Pai-Me. They exist independently in
interpenetrated arrays. In a solution phase study, the same reaction has synthesized an iodo-bridged azoimidazole
Hg(II) complex, [Hg(RaaiR )( -I)(I)]2 (RaaiR′ ) 1-alkyl-2-(arylazo)imidazole). The structures have been characterized
by X-ray diffraction studies. Chloro-bridged Hg(II) complexes of azoimidazoles, [Hg(RaaiR )( -Cl)(Cl)]2, are also
−
′
µ
′
µ
known. These complexes upon irradiation with UV light show trans-to-cis isomerization. The reverse transformation,
cis-to-trans isomerization, is very slow with visible light irradiation. Quantum yields (φtfc) of trans-to-cis isomerization
are calculated, and the free ligand shows higher
thermally induced process. The activation energy (Ea) of cis-to-trans isomerization is calculated by controlled
temperature reaction. The Ea’s of free ligands are much higher than that of halo-bridged Hg(II) azoimidazole
φ than their Hg(II) complexes. The cis-to-trans isomerization is a
−
complexes. Chloro-bridged Hg(II) complexes show lower Ea’s than those of iodo-bridged complexes. DFT calculation
has been adopted to rationalize the experimental results.
1. Introduction
(solvent, temperature, catalyst, etc.) and nature of reagents
have profusely influenced the composition, stereochemistry,
and molecular structure of the complexes. Exploration of the
material properties of organic molecules and their metal
complexes are needed to complete the understanding of the
compounds. Confirmative structure demonstration is a defi-
nite proof of bonding theories. Measurements of the metric
parameters of complexes provides concrete information about
coordination. It is assumed that a recognized bond has a
length less than that of the sum of the van der Waals radii
of participating atoms.5 In this work we perform a reaction
under two different reaction conditions: neat reaction;
solution-phase reaction. A neat reaction under microwave
treatment of the mixture of HgI2 and 1-alkyl-2-(arylazo)-
Despite extensive study, coordination chemistry has yet
to achieve complete understanding of the mechanism of
interaction between the metal ions and ligands. Attempts have
been made to explain this problem from both theoretical and
experimental techniques.1,2 The fundamental principle of the
formation of a complex is the interaction between the central
metal ion and ligands. Depending on the force of interaction,
the strength of bond is defined. The directional bonds are
covalent. Noncovalent bonding, viz., hydrogen bonds, C-H-
π, and πsπ interactions, provide rigidity to the main
covalently bonded complex.3,4 The reaction conditions
* To whom correspondence should be addressed. E-mail: c_r_sinha@
yahoo.com. Fax: 91-033-2413-7121.
† Jadavpur University.
(3) (a) Desiraju, G. R. Crystal Engineering: The Design of Organic Solids;
Elsevier: Amsterdam, 1989. (b) Desiraju, G. R. In The Crystals as a
Supramolecular Entity; Lehn, J. M., Ed.; John Wiley & Sons:
Chichester, U.K., 1996; Vol. 2.
(4) (a) Burrows, A. D.; Harrington, R. W.; Mahon, M. F.; Price, C. E. J.
Chem. Soc., Dalton Trans. 2000 3845. (b) Desiraju, G. R. Angew.
Chem., Int. Ed. Engl. 1995, 34, 2311. (c) Lehn, J. M. Supramolecular
Chemistry; VCH: Weinheim, Germany, 1995.
‡ Nihon University.
§ National Tsing Hua University.
(1) (a) Wilkinson, G., Gillard, R. D., McCleverty, J., Eds. ComprehensiVe
Coordination Chemistry; Pergamon Press: Oxford, U.K., 1987; Vols.
1-7. (b) Steel, C.; Vogtel, F. In PerspectiVes in Coordination
Chemistry; Williams, A. F., Merbach, A. E., Eds.; VCH: Weinheim,
Germany, 1992.
(2) Cais, M. Progress in Coordination Chemistry; American Elsevier: New
York, 1968.
(5) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry, 5th ed.;
John Wiley and Sons: New York, 1988.
670 Inorganic Chemistry, Vol. 46, No. 3, 2007
10.1021/ic061221u CCC: $37.00
© 2007 American Chemical Society
Published on Web 01/10/2007

Mercury(II)-Iodo Complexes of (Arylazo)imidazoles
imidazoles (RaaiR′) has synthesized a precoordination state
of the complexes. The X-ray structure determination of one
of the compounds shows that HgI2 (metal compound) is
intercalated in layers of ligand (1-methyl-2-(phenylazo)-
imidazole, Pai-Me) and independently exists in interpen-
etrated arrays without showing covalent interaction. On the
changing of reaction conditions, neat reaction to solution
reaction, iodo-bridged coordination complexes [Hg(RaaiR′)-
(µ-I)(I)]2 have been isolated. The complexes are characterized
by spectroscopic (IR, UV-vis, NMR) data and by single-
crystal X-ray diffraction studies. HgI2 is chosen because of
its efficient photoconductivty and has been used in radio-
graphic and fluoroscopic medical imaging.
R ) Me, R′ ) Me (Tai-Me, 1b); R ) H, R′ ) Et (Pai-Et,
2a); R ) Me, R′ ) Et (Tai-Et, 2b); R ) H, R′ ) CH2Ph
(Pai-CH2Ph, 3a); R ) Me, R′ ) CH2Ph (Tai-CH2Ph, 3b)).
Neat reaction between ligand (RaaiR′) and HgI2 under
Toward the study of properties of the complexes, we have
been interested to examine the photochromism of (arylazo)-
imidazoles and the effect of metal coordination thereof.
Photochromism is a reversible photoinduced transformation
between two molecular states whose absorption spectra differ
significantly.6 Incorporation of photochromic molecules into
organic or hybrid organic-inorganic materials leads to
development of very effective devices. Azo-conjugated metal
complexes exhibit unique properties upon light irradiation
in the area of photon-mode high-density information storage
photoswitching devices.7 A combination of metal ions and
π-conjugated systems with d-π electronic interaction might
be used to realize novel optical, electronic, and magnetic
properties.6-8 A preliminary report9 of the photochromism
of 1-methyl-2-(phenylazo)imidazole (Pai-Me) has inspired
us to examine this property on different other azoimidazoles
and their metal complexes. In this work we examine
photochromism of Pai-Et (1-ethyl-2-(phenylazo)imidazole),
Tai-Me (1-methyl-2-(p-tolyllazo)imidazole), Tai-Et (1-ethyl-
2-(p-tolyllazo)imidazole), and their chloro- and iodo-bridged
Hg(II) complexes. The structural description of chloro-
bridged complexes has been described previously.10 In this
report HgI2 intercalated into the layers of Pai-Me and iodo-
bridged Hg(II)-azoimidazoles have been characterized by
single-crystal X-ray diffraction studies.
microwave irradiation has synthesized a compound of
composition [HgI2][RaaiR′]2 (4-6) (a precoordination com-
pound). The solution-phase reaction between HgX2 (X )
Cl, I) and RaaiR′ in MeOH-2-methoxyethanol (2:1, v/v)
has synthesized halo-bridged coordination complex of com-
position [Hg(RaaiR′)(µ-X)(X)]2 (X ) Cl, 7-9, and X ) I,
10-12). Under neat reaction condition of ligand and HgCl2
by microwave irradiation, we have isolated only chloro-
bridged product [Hg(RaaiR′)(µ-Cl)(Cl)]2 (7-9). The solution-
phase reaction has also isolated chloro-bridged product. The
structures of chloro-bridged compounds are previously
described.10 The compounds are nonconducting, and their
compositions have been supported by microanalytical data.
The structures have been established in representative cases
by single-crystal X-ray diffraction studies.
2.2. Molecular Structures. The crystals of (1-methyl-2-
(phenylazo)imidazole)mercury(II) iodide (4a) were grown
by slow evaporation of the THF extract of the solid-phase
microwave-irradiated reaction. Diiodomercury(II)-1-methyl-
2-(phenylazo)imidazole (10a) and diiodomercury(II)-1-
benzyl-2-(p-tolylazo)imidazole (12b) were grown by slow
evaporation of the reaction mixture in MeOH-2-methoxy-
ethanol at ambient condition for 1 week.
2. Results and Discussion
2.2.a. Structure of [HgI2][PaiMe]2 (4a). The crystal
structure of the complex is shown in Figure 1, and the bond
parameters are listed in Table 1. The structure does not fit
into our general conception of coordination complexes.1,5
Linear HgI2 are intercalated into the layers of Pai-Me. The
ligand exists as a weakly interacted π-dimer (Figure 1b):
Ph(A)-Imz(B), 4.336(7) Å (symmetry: 1/2 - x, -1/2 + y,
2.1. Synthesis of Complexes. Two different reaction
conditions have been employed for the synthesis of halom-
ercury(II) compounds of 1-alkyl-2-(arylazo)imidazole
(RaaiR′: R ) H, R′ ) Me (Pai-Me, 1a); R ) H, R′ ) Me
(Pai-Me, 1a); R ) Me, R′ ) Me (Tai-Me, 1b); R ) H, R′
) Et (Pai-Et, 2a); R ) Me, R′ ) Et (Tai-Et, 2b); R ) H, R′
) CH2Ph (Pai-CH2Ph, 3a); R ) Me, R′ ) CH2Ph (Tai-CH2-
Ph, 3b);
1
1/2 - z); Ph(B)-Imz(A), 4.248(8) Å (symmetry: /2 - x, 1/2
+ y, 1/2 - z) (where A and B represent two series of Pai-Me
ligands in the dimer). Ph and Imz rings are connected by
-NdN-, and they make dihedral angle of 7.9(3)°. The azo
distance, -NdN-, of 1.245(6) Å is ∼0.02 Å shorter than
the previously reported free ligand value (1.261(2) Å).6 HgI2
exists independently in the packing (Figure 1). However,
weak interactions exist between Ph and Imz protons with
Hg and I centers separately: C(1)-H(1)-Hg, 3.556(8) Å
(symmetry: x, -y, 1/2 + z); C(6)-H(6)-Hg, 3.385(3) Å (-x,
(6) Rau, H. Photochromism: Molecules and Systems; Durr, H., Bouas-
Laurent, H., Eds.; Elsevier: Amsterdam, 1992; Chapter 4, pp 165-
192. Brown, G. H. Photochromism: Techniques of Chemistry; Wiley-
Interscience: New York, 1971; Vol. III.
(7) Ire, M. Chem. ReV. 2000, 100, 1683. Ikeda, T.; Tsutsumi, O. Science
1995, 268, 1873. Kawata, S.; Kawata, Y. Chem. ReV. 2000, 100, 1777.
(8) Nishihara, H. Bull. Chem. Soc. Jpn. 2004, 77, 407. Yutaka, T.; Mori,
I.; Kurihara, M.; Mizutani, J.; Tamai, N.; Kawai, T.; Irie, M.; Nishihara,
H. Inorg. Chem. 2002, 41, 7143.
(9) Otsuki, J.; Suwa, K.; Narutaki, K.; Sinha, C.; Yoshikawa, I.; Araki,
K. J. Phys. Chem. A 2005, 109, 8064.
(10) Chand, B. G.; Ray, U. S.; Santra, P. K.; Mostafa, G.; Lu, T.-H.; Sinha,
C. Polyhedron 2003, 22, 1205.
1
1
y, /2 - z); C(10)-H(10b)-I, 3.294(3) Å (-x, y, /2 - z).
The presence of a C-H-π interaction also enhances the
Inorganic Chemistry, Vol. 46, No. 3, 2007 671

Sarker et al.
Figure 1. Unit cell of precoordination compound showing (a) interpenetrated layers of HgI2 and Pai-Me and (b) π-π interaction of 4a.
Table 1. Selected Bond Distances (Å) and Angles (deg) for
124.661(16)°, and I(1)-Hg-I(2)*, 103.787(15)°, for 10a (/
refers to symmetry -x, -y, -z) and I(1)-Hg-I(2), 137.98-
(3)°, and I(1)-Hg-I(2)*, 99.79(2)°, for 12b (/ refers
symmetry 1 - x, 2 - y, 1 - z) support for distorted
geometry. Steric crowding of 1-CH2Ph of 12b may be the
reason for larger deviation than 10a, which bears the 1-Me
group. The acute bite angle 62.66(3)° in 10a is extended by
Pai-Me or that of 64.9(3)° is extended by Tai-CH2Ph in 12b
on coordination to Hg(II). The small chelate angle may be
one of the reasons for geometrical distortion.10,11 The ligand
(Pai-Me or Tai-CH2Ph) is almost planar (maximum deviation
<0.08 Å except for the 1-Me or 1-CH2Ph group). The Hg-
N(imidazole) (2.290(4) Å, 10a; 2.302(9) Å, 12b) is shorter
than Hg-N(azo) (2.842(2) Å, 10a; 2.739(7) Å, 12b) which
reflects stronger interaction between Hg(II) and N(imidazole).
The Hg-N distances are longer than chloro-bridged Hg(II)-
azoimidazole complexes (Hg-N(imidazole), 2.204(5), and
Hg-N(azo), 2.757(5) Å).10,11 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 significant bonding interaction between these com-
ponents. Strong coordination of imidazole-N to Hg(II) has
significant biochemical implications and explains the great
toxicity of Hg(II).12 Weak and flexible bonds are very
effective to induce some functional property in the mol-
[HgI2][HaaiMe]2 (4a)
bond distances
bond angles
N(3)-N(4)
N(3)-C(3)
N(4)-C(4)
Hg-I(2)
1.245(6)
I-Hg-I
179.99(2)
118.4(4)
112.9(4)
112.1(4)
122.3(4)
117.0(4)
1.415(6)
1.443(5)
2.8972(5)
N(3)-C(3)-N(1)
N(4)-N(3)-C(3)
N(3)-N(4)-C(4)
N(4)-C(4)-C(9)
N(4)-C(4)-C(5)
strength between the dimers: C(7)-H(7)-Cg(1), 3.333(9)
1
1
Å (1/2 - x, /2 + y, /2 - z) (Cg(1): C4-C5-C6-C7-
C8-C9).
2.2.b. Structures of [Hg(Pai-Me)(µ-I)(I)]2 (10a) and
[Hg(Tai-CH2Ph)(µ-I)(I)]2 (12b). The molecular structures
of [Hg(Pai-Me)(µ-I)(I)]2 (10a) and [Hg(Tai-CH2Ph)(µ-I)(I)]2
(12b) are shown in Figure 2a,b, respectively. The bond
parameters are listed in Table 2. Each discrete molecular
unit consists of a dinuclear iodo-bridged Hg2I2 fragment. Pai-
Me or Tai-CH2Ph acts as an N,N′-donor end-capping agent,
and a nonbridged I atom lies in a semiaxial position. The
bridged Hg2I2 forms a unsymmetric tetraatomic rhombohe-
dral plane (Hg-I(2), 2.7611(6) Å, and Hg-I(2)*, 3.1114-
(5) Å, in 10a (/ symmetry: -x, -y, -z); Hg-I(1), 2.6943(8)
Å, and Hg-I(1)**, 3.3046(8) Å, in 12b (// symmetry: 1
- x, 2 - y, 1 - z)). The atomic arrangements Hg, N(4),
C(1), N(1), N(3) (in 10a) or Hg, N(1), C(3), N(3), N(4) (in
12b) constitute chelate plane(s) with a maximum deviation
<0.04 Å. The pendent aryl ring makes a small dihedral (13.5-
(3)° for 10a and 8.6(2)° for 12b) with the respective chelated
azoimidazole ring. The bond angular values I(1)-Hg-I(2),
(11) Chand, B. G.; Ray, U. S.; Cheng, J.; Lu, T.-H.; Sinha, C. Inorg. Chim.
Acta 2005, 358, 1927.
(12) Fergusson, J. E. The heaVy elements: chemistry, enVironment impact
and health effects; Pergamon: Oxford, U.K., 1990.
672 Inorganic Chemistry, Vol. 46, No. 3, 2007

Mercury(II)-Iodo Complexes of (Arylazo)imidazoles
tion and hence the canting phenomenon and remnant
magnetism.14 Because of the long Hg(II)-N(azo) distance,
the molecule may exhibit photophysical activation via
cleavage of this bond followed by rotation to introduce
photoisomerization. In fact, we have examined photo-
chromism of these molecules (vide infra). The NdN distance
is 1.262(6) Å in 10a and 1.272(9) Å in 12b. The Hg-
I(bridged) (Hg-I(2), 2.7611(6) Å, in 10a and Hg-I(1),
2.6943(8) Å, in 12b) distance is longer than the Hg-
I(nonbridged) (Hg-I(1), 2.6441(5) Å, in 10a and Hg-I(2),
2.6406(8) Å, in 12b) bond length. The bond parameters are
in corroboration with previously reported data.10,11 Hg-I(1)
in 10a and Hg-I(2) in 12b are in semiaxial positions in their
respective planes. The chelate ring around Hg is twisted in
a manner so that the Hg-N(azo) inclines toward the axial
Hg-I bond [N(3)-Hg-I(1), 100.63(3)°, and N(4)-Hg-I(1),
123.9(1)°, in 10a and N(4)-Hg-I(2), 98.86(15)°, and N(1)-
Hg-I(2), 107.23(18)°, in 12b].
A π-π interaction (imidazole-phenyl, 4.15(1) Å; sym-
metry, -x, -y, 1 - z) in 10a between imidazole and phenyl
rings with the partners of neighboring molecule has generated
a supramolecular arrangement (Figure 3). The presence of
1
C-H-π (2.772 Å; symmetry 3/2 - x, -1/2 + y, /2 - z;
between imidazole C-H and the pendent p-tolyl ring of
neighboring molecule) and π-π interactions (4.103 Å;
symmetry 1 - x, -y, -z; between imidazole and p-tolyl
rings of neighboring molecules) in 12b have generated a 1-D
supramolecule.
Figure 2. Molecular structures of iodo-bridged dimers: (a) [Hg(Pai-Me)-
(µ-I)(I)]2 (10a); (b) [Hg(Tai-CH2Ph)(µ-I)(I)]2 (12b).
2.3. Spectral Studies. IR spectra of [Hg(RaaiR′)(µ-X)-
(X)]2 (7-12) exhibit ν(CdN) and ν(NdN) at 1580-1600
and 1375-1380 cm-1, respectively, and are shifted to lower
frequency by 15-20 cm-1 with reference to the free ligand
values.10,15 Precoordination compounds [HgI2][RaaiR′]2 (4-
6) exhibit hardly any shift of NdN and CdN frequencies
compared to the free ligand values.15 This, in fact, helps us
to comment on the no-coordination situation of N-donor
centers of the ligand with Hg(II). This is also supported from
the X-ray crystal structure study (Figure 1).
Table 2. Selected Bond Distances (Å) and Angles (deg) for
[Hg(Pai-Me)(µ-I)(I)]2 (10a) and [Hg(Tai-CH2Ph)(µ-I)(I)]2 (12b)
bond distances
bond angles
[Hg(Pai-Me)(µ-I)(I)]2 (10a)
Hg-I(1)
2.6441(5)
2.7611(6)
3.1114(5)
2.290(4)
2.842(2)
1.382(7)
1.434(7)
1.262(6)
I(1)-Hg-I(2)
I(1)-Hg-I(2)a
N(4)-Hg-I(1)
N(4)-Hg-I(2)
N(4)-Hg-I(2)a
I(2)-Hg-I(2)a
Hg-I(2)-Hga
N(3)-Hg-I(1)
N(3)-Hg-I(2)
N(3)-Hg-N(4)
124.661(16)
103.787(15)
123.89(13)
107.67(12)
89.86(12)
94.324(13)
85.676(13)
100.63(4)
85.01(2)
Hg-I(2)
Hg-I(2)a
Hg-N(4)
Hg-N(3)
N(1)-C(1)
N(3)-C(4)
N(1)-N(3)
The solution electronic spectra of 7-12 were recorded in
MeCN in 200-900 nm. There are four bands in the UV-
vis region, and three of them are intense (ꢀ ∼ 104 mol-1
dm3 cm-1) at 255-270, 350-365, and 380-390 nm. On
comparing with free ligand spectra15 and that of the chloro-
bridged Hg(II) complex,10 we may conclude that they are
coming from intramolecular charge-transfer transitions (n-
π*, π-π*), which is also supported by frontier molecular
orbital calculations using X-ray diffraction parameters (vide
supra). The fourth band appears at 435-445 nm in the case
of 7-12 and is weak in intensity (ꢀ ∼ 103 mol-1 dm3 cm-1).
This may refer to Hg(II) f π*(azoimine) charge-transfer
transitions.10,11
62.66(3)
[Hg(Tai-CH2Ph)(µ-I)(I)]2 (12b)
Hg-I(1)
2.6943(8)
2.6406(8)
2.302(9)
2.739(7)
3.3046(8)
1.426(11)
1.385(9)
1.272(9)
I(1)-Hg-I(2)
I(1)-Hg-I(2)b
I(1)-Hg-I(1)b
N(1)-Hg-I(1)
N(1)-Hg-I(2)
N(1)-Hg-N(4)
I(2)-Hg-N(4)
I(1)-Hg-N(4)
N(1)-Hg-I(1)b
N(4)-Hg-I(1)
Hg-I(1)-Hgb
137.98(3)
99.79(2)
89.69(2)
114.15(18)
107.23(18)
64.9(3)
98.86(15)
92.24(15)
86.8(2)
Hg-I(2)
Hg-N(1)
Hg-N(4)
Hg-I(1)b
N(3)-C(3)
N(4)-C(4)
N(3)-N(4)
149.70(19)
90.31(2)
a Symmetry: -x, -y, -z. b Symmetry: 1 - x, 2 - y, 1 - z.
1
The H NMR spectra of the complexes are recorded in
ecules.13 The weak bonding interaction between the Mn(II)
and N(azo) center in [Mn(N3)2(TaiEt)]n (TaiEt ) 1-ethyl-
2-(p-tolylazo)imidazole) is responsible for structural distor-
CDCl3, and the signals are assigned (Table 3) unambiguously
(14) Ray, U. S.; Ghosh, B. K; Monfort, M; Ribas, J.; Mostafa, G.; Lu,
T.-H.; Sinha, C. Eur. J. Inorg. Chem. 2004, 250.
(15) Misra, T. K. Ph.D. Thesis. Transition Metal Chemistry of 2-(Arylazo)-
imidazoles: Synthesis, Characterisation and Electrochemical Studies;
Burdwan University: Burdwan, India, 1999.
(13) Kepert, C. J. Chem. Commun. 2006, 695. Nathan C.; Gianneschi, M.;
Masar, S., III; Mirkin, A. Acc. Chem. Res. 2005, 38, 825.
Inorganic Chemistry, Vol. 46, No. 3, 2007 673

Sarker et al.
Figure 3. (a) Intermolecular π-π interactions in 10a (red, Ring2-Ring1#2; blue, Ring2sRing1#1). (b) C-H-π (a) and πsp (b) interactions in the
dimeric unit of [Hg(Tai-CH2Ph)(µ-I)(I)]2 (12b).
Table 3. 1H NMR Spectral Data for Hg(II) Complexes (4-6 and 10-12) in CDCl3 at 300 K
δ (ppm ) (J (Hz))
4-Ha
5-Ha
7,11-Hb
8,10-Hb
9-R
1-CH3
1-CH2
(1-CH2)CH3
Ph-H
e
compd
4a
4b
5a
5b
6a
6b
10a
10b
11a
11b
12a
12b
7.13
7.07
7.22
7.19
7.24
7.22
7.30
7.37
7.36
7.37
7.45
7.44
7.00
6.95
7.11
7.06
7.09
7.14
7.24
7.26
7.25
7.21
7.31
7.26
7.55 (7.2)
7.53 (7.2)
7.60 (7.8)
7.48 (7.2)
7.69 (7.2)
7.65 (7.2)
8.09 (7.0)
8.00 (7.0)
8.05 (7.0)
8.02 (7.0)
8.00 (7.0)
7.93 (7.0)
7.50c
7.50d
2.40f
7.03d
2.45f
7.49d
2.40f
7.57d
2.48f
7.48d
2.40c
7.55d
2.40f
4.03
4.04
7.33 (7.0)
7.43c
4.53g (12.0)
4.52g (12.0)
5.53e
1.59h (7.0)
1.53h (7.0)
7.30 (7.0)
7.49c
7.30-7.40
7.30-7.45
7.32 (7.2)
5.52e
7.57c
4.14
4.13
7.48 (7.0)
7.50c
4.63g (10.0)
4.58g (12.0)
5.73e
1.58h (7.5)
1.55 (7.5)
7.32 (7.0)
7.55c
7.30-7.40
7.30-7.40
7.39 (7.0)
5.77e
a Broad singlet. b Doublet. c Multiplet. d δ(9-H). e Singlet. f δ(9-Me). g Quartet.
by spin-spin interaction, the effect of substitution therein,
and on comparing previously reported cases.10,11 The atom-
numbering pattern is shown in the structure. Data reveal that
the signals in the spectra of 10-12 in general are shifted
downfield compared to those of the the spectra of the free
ligand.15 The signal pattern and chemical shift data in the
spectra of 4-6 are similar to those of the free ligand. This
supports that the ligand is coordinated to Hg(II) in 10-12
and not in the case of 4-6. The 7,11-H’s are shifted to higher
δ by ∼0.5 ppm when we move from 4-6 to 10-12. This
observation supports the existence of strong interaction
between ligands and Hg(II) in 10-12 compared to 4-6. An
674 Inorganic Chemistry, Vol. 46, No. 3, 2007

Mercury(II)-Iodo Complexes of (Arylazo)imidazoles
important feature of the spectra of 10-12 with their chloro-
analogue [Hg(RaaiR′)(µ-Cl)(Cl)]210 is the shifting of imida-
zole protons, 4-H and 5-H, to higher δ values (by >0.2 ppm),
while aryl protons (7-H-11-H) remain almost at the same
signal positions. This is because iodo makes mercury(II)
softer16 in the Hg-I (10-12) system than chloro in [Hg-
(RaaiR′)(µ-Cl)(Cl)]2 (7-9). Thus, Hg(II) prefers to bind
imidazole-N more efficiently in the present case than that
of [Hg(RaaiR′)(µ-Cl)(Cl)]2. Aryl signals are shifted to the
lower field side on Me-substitution to the aryl ring. This is
due to the electron-donating effect of the Me- group. N(1)-
R′ shows the usual signal pattern as earlier.10
2.4. Photochromism of Ligands and Complexes. The
trans-to-cis isomerization of Tai-Me (1b), Pai-Et (2a), and
Tai-Et (2b), chloro-bridged [Hg(Pai-Me)(µ-Cl)(Cl)]2 (7a),
[Hg(Tai-Me)(µ-Cl)(Cl)]2 (7b), [Hg(Pai-Et)(µ-Cl)(Cl)]2 (8a),
and[Hg(Tai-Et)(µ-Cl)(Cl)]2 (8b), and iodo-bridged-Hg(II)
[Hg(Pai-Me)(µ-I)(I)]2 (10a), [Hg(Tai-Me)(µ-I)(I)]2 (10b),
[Hg(Pai-Et)(µ-I)(I)]2 (11a), and [Hg(Tai-Et)(µ-I)(I)]2 (11b)
have been investigated. The photochromism of Pai-Me is
reported earlier.9 The complexes [Hg(Pai-CH2Ph)(µ-X)(X)]2
(X ) Cl, 9a, and X ) I, 12a) and [Hg(Tai-CH2Ph)(µ-X)-
(X)]2 (X ) Cl, 9b, and X ) I, 12b) are very sluggish to
change their absorption spectra upon UV light irradiation
and have not been studied further. The photoisomerization
of ligand in the complexes is dependent on the nature of the
metal ion and its oxidation state.8 To investigate possible
modulation of photochromes by metal ions, we carried out
some experiments with metal ions added to a methanol
solution of Pai-Me.9 The addition of Cu(II), Zn(II), and Ag-
(I) to Pai-Me solution has shifted the absorption spectra to
longer wavelengths. The illumination at the π-π* peak (355
nm) isomerizes trans-Pai-Me to cis-Pai-Me, and subsequent
illumination at 454 nm reversed the course of reaction. The
chelate complexes of these metal ions are structurally
known.17-19 Herein, we are investigating photoisomerization
of Hg-RaaiR′ complexes in the presence of chloride (Cl-)
and iodide (I-). Iodide is chosen because of its light-sensitive
oxidative reactions20 intending its participation in the ex-
perimental process. It is observed that upon irradiation with
UV light trans-to-cis photoisomerization proceeded and the
cis molar ratio reached >95%. The absorption spectra of
the trans ligands (Pai-Et, Tai-Me, Tai-Et) in toluene changed
with isosbestic points upon excitation (Figures 4 and 5) into
the cis isomer and also in the case of the respective
Figure 4. Spectral changes of Tai-Me in toluene upon repeated irradiation
at 365 nm at a 3 min interval at 25 °C. The inset figure shows spectra of
the cis and trans isomers of Tai-Me.
Figure 5. Spectral changes of [Hg(Tai-Me)(µ-I)I]2 in MeCN upon repeated
irradiation at 367 nm at a 3 min interval at 25 °C. The inset figure shows
spectra of the cis and trans isomers of the complex.
measure the percentage composition of the irradiated solu-
tion, which supports the composition obtained from absorp-
1
tion spectra. The H NMR signals of the aromatic ring
protons are significantly shifted to the upfield portion after
light irradiation. The ligands and complexes show little sign
of degradation upon repeated irradiation at least up to 15
cycles in each case. We have also examined the UV-light
irradiation behavior of green tcc-Ru(Pai-Me)2Cl2 and blue
ctc-Ru(Pai-Me)2Cl220 in MeCN. Ruthenium(II) complexes are
silent to photoisomerization. There may be several reasons
on the basis of structure and energy ordering of the
participating functions. Pai-Me is chelated to Ru(II), and the
X-ray structure determination shows that the Ru-N(azo)
bond length is shorter than Ru-N(imidazole) distances. This
has also prompted charge flow dπ(Ru) f π* (azo) which
synergistically enhances Ru-N(azo) bond strength. This
structural rigidity may resist photoisomerization. Besides, the
energy transfer from π-π* to the MLCT state may cause
very fast bleaching.22 The structures of Hg(II)-RaaiR′ halo
complexes (vide supra) (Figure 2) show very long Hg(II)-
N(azo) bond distances10 compared to Hg-N(imidazole)
distances. Upon UV-light irradiation the Hg-N(azo) bond
1
complexes. The H NMR technique has been adopted to
(16) Softening, I.; Wang, X.; Knobler, C. B.; Zebeng, Z; Hawthorne, M.
F. J. Am. Chem. Soc. 1994, 116, 7142. Pauling, L. The Nature of the
Chemical Bond, 3rd ed.; Cornell University Press: Ithaca, NY, 1960.
(17) Ray, U. S.; Banerjee, D.; Mostafa, G.; Lu, T.-H.; Sinha, C. New J.
Chem. 2004, 28, 1432.
(18) Chand, B. G.; Ray, U. S.; Mostafa, G.; Lu, T.-H.; Sinha, C. J. Coord.
Chem. 2004, 57, 627. Chand, B. G.; Ray, U. S.; Cheng, J.; Lu, T.-H.;
Sinha, C. Polyhedron 2003, 22, 1213.
(19) Dinda, J.; Jasimuddin, Sk.; Mostafa, G.; Hung, C. H.; Sinha, C.
Polyhedron 2004, 23, 793.
(20) O’Regan, B.; Moser, J.; Anderson, M. J. Phys. Chem. 1990, 94, 8720.
Fitzmaurice, D. J.; Esche, M.; Frei, H.; Moser, J. J. Phys. Chem. 1993,
97, 3806.
(21) Misra, T. K.; Das, D.; Sinha, C.; Ghosh P. K.; Pal, C. K. Inorg. Chem.
1998, 37, 1672.
Inorganic Chemistry, Vol. 46, No. 3, 2007 675

Sarker et al.
Table 4. Excitation Wavelength (λπ,π*), Rate of Trans (t) f Cis (c) Conversion and Quantum Yield (φtfc
)
rate of tfc
compd
Pai-Me (1a)
Tai-Me (2a)
Pai-Et (1b)
λπ,π* (nm)
isosbestic point(s) (nm)
conversn × 108 (s-1
)
φtfc
363
365
364
365
363
365
367
369
367
367
368
371
333, 431
331, 443
333, 434
328, 444
333, 447
348, 438
342, 449
343, 458
336, 452
329, 442
341, 449
345, 462
5.06
3.70
4.83
4.59
3.21
3.43
3.19
3.49
4.20
3.22
3.28
3.63
0.25 ( 0.03
0.218 ( 0.007
0.241 ( 0.012
0.206 ( 0.006
0.181 ( 0.020
0.157 ( 0.010
0.162 ( 0.020
0.144 ( 0.010
0.211 ( 0.011
0.177 ( 0.009
0.202 ( 0.016
0.197 ( 0.010
Tai-Et (2b)
[Hg(Pai-Me)(µ-Cl)Cl]2 (7a)
[Hg(Tai-Me)(µ-Cl)Cl]2 (7b)
[Hg(Pai-Et)(µ-Cl)Cl]2 (8a)
[Hg(Tai-Et)(µ-Cl)Cl]2 (8b)
[Hg(Pai-Me)(µ-I)I]2 (10a)
[Hg(Tai-Me)(µ-I)I]2 (10b)
[Hg(Pai-Et)(µ-I)I]2 (11a)
[Hg(Tai-Et)(µ-I)I]2 (11b)
may cleave easily to make a free rotatory azo-aryl group
which isomerizes to the cis form. The quantum yields were
measured for the trans-to-cis (φtfc) photoisomerization of
these compounds in toluene (free ligand)/MeCN (complexes)
on irradiation of UV wavelength (Table 4). The φtfc values
are significantly dependent on the nature of substituents and
are less so than that of Pai-Me (φtfc ) 0.25 ( 0.03). The
Me substituent at the azophenyl group (Pai-R′ (1) to Tai-R′
(2)) and Et substituent at the N(1)-position (1-Me (a) to 1-Et
(b)) both reduce φtfc values. The effect is significant for
the Tai-R′ ligand. Thus, the Me substituent at the -NdN-
phenyl group reduces the rate of isomerization (Table 5). In
the complexes [Hg(Raai-R′)(µ-X)(X)]2 (X ) Cl for 7 and 8
and X ) I for 10 and 11) the φtfc values are significantly
less than that of free ligand data. Two reasons may be
considered for this decrement: (i) the presence of the
coordinated HgX2 motif heavily increases the molecular
weight of the complex unit which may severely interfere with
the motion of the -NdN-Ar moiety; (ii) the photobleaching
efficiency of a heavy atom like Hg(II) and the iodo group23
may snatch out energy from the π-π* excited state and may
cause very fast deactivation other than the photochromic
route. One should note that the rotor volume has significant
influence on the isomerization rate and quantum yields.8,22
The complexation increases the rotor volume leading to a
less efficient isomerization.
Thermal cis-to-trans isomerizations of the ligands (1b, 2a,
2b) and complexes (7, 8, 10, 11) were followed by UV-vis
spectroscopy in toluene (ligands)/MeCN (complexes) at
varied temperatures, 298-313 K. The Arrhenius plots in the
range 298-313 K gave a linear graph from which the
activation energies were obtained (Table 6, Figure 6). The
Ea’s of free ligands are closer to that of Pai-Me but lower
than that of azobenzene. In the complexes 7, 8, 10, and 11,
the Ea’s are severely reduced, which means faster cis-to-
trans thermal isomerization of the complexes. The entropies
of activation (∆S*) are more negative in the complexes than
that of the free ligand. This is also in support of an increase
in rotor volume in the complexes. The chloro complexes (7,
8) have slightly lower activation energy (lowered by 3-8
kJ mol-1, Table 4) than that of iodo-bridged complexes (10,
11), which are also in agreement with lower quantum yields
in the former (Table 5).
2.5. DFT Calculation and Correlation with Spectral
Properties. Density functional theory was employed to
obtain insight into the electronic structure of the mercury
complexes of (arylazo)imidazoles. The calculations for 10a
were conducted on using the coordinates of the crystal
structure obtained in this work and were compared with the
results obtained for the ligand, 1a. Although the DFT
calculations on 1a have been reported previously,9 we have
calculated them again using the same functionals and basis
functions as for 10a to compare the results. The results
obtained here for 1a using B3LYP with LanL2DZ for Hg
and MIDI! for other elements are qualitatively consistent with
those obtained previously using the B88 and LYP functionals
and the DZVP basis set.
The major results of the calculations are graphically
illustrated in Figure 7. For 1a, the LUMO is π*, which is
delocalized over the entire conjugated system. The HOMO
and HOMO-1 are close in energy. A π orbital is dominant
in the HOMO, but the n orbitals on the azo nitrogens also
contribute. This mixing occurs probably because of the slight
nonplanarity of the crystal structure of 1a, on which the
calculations were based. Meanwhile, the azo n orbitals are
major contributors to the HOMO-1. The LUMO in 10a is
on the Pai-Me ligand, and the symmetry of the orbital is the
same as the LUMO of 1a. On the other hand, the occupied
frontier orbitals are dominated by the π orbitals of iodines
with some contributions from the d orbitals of mercury and
the azo n orbitals from the ligands. The ligand π orbital of
a symmetry similar to that of the HOMO of 1a is found
below these iodine-centered orbitals (MO no. 206). Further
below in energy, an orbital with a major contribution from
the azo n orbitals is found (MO no. 204). By complexation
with mercury ion, the unoccupied and occupied frontier
orbitals in that part of the ligand are stabilized to a similar
extent. Also the symmetries of the orbitals are largely
retained upon complexation. These results explain the
experimental results that there is little shift in λmax in the
absorption spectra and that the photochromic activity is well
retained in the complex.
(22) Yutaka, T.; Kurihara, M.;. Nishihara, H. Mol. Cryst. Liq. Cryst. 2000,
343, 193. Yutaka, T.; Mori, L.; Kurihara, M.; Mizutani, J.; Kubo, K.;
Furusho, S.; Matsumura, K.; Tamai, N.; Nishihara, H. Inorg. Chem.
2001, 40, 4986.
(23) Zhao, X.; Chan, W.; Wong, M.; Xiao, D.; Li, Z. Am. Lab. 2003, 13.
Hutton, A. T.; Irving, H. M. N. H. J. Chem. Soc., Dalton Trans. 1982,
2299. Zentai, G.; Partain, L.; Pavlyuchkova, R.; Proano, C.; Virshup,
G.; Melekshov, L.; Zuck, A.; Breen, B. N.; Dagan, O.; Vilensky, A.;
Schieber, M.; Gilboa, H.; Bennet, P.; Shah, K.; Dimitriev, Y.; Thomas,
J.; Yaffe, M.; Hunter, D. Proc. SPIE 2003, 5030 (June).
676 Inorganic Chemistry, Vol. 46, No. 3, 2007

Mercury(II)-Iodo Complexes of (Arylazo)imidazoles
Table 5. Rate and Activation Parameters for Cis (c) f Trans (t) Thermal Isomerization
rate of thermal c f t
compd
temp (K)
conversn × 105 (s-1
)
Ea (kJ mol-1
)
∆H* (kJ mol-1
)
∆S* (J mol-1 K-1
-77.1
)
∆G* c (kJ mol-1
)
Pai-Me (1a)a,d
298
303
313
323
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
2.2
4.0
79.0
77.05
100.00
8.8
27.5
7.3
12.0
26.0
37.0
3.3
5.8
9.7
18.0
6.4
9.8
Tai-Me (1b)a
87.57
86.87
87.63
32.31
27.82
28.58
27.42
40.89
30.97
35.53
36.00
85.03
84.33
87.08
29.77
25.28
26.04
24.88
38.36
28.43
32.99
34.12
-38.84
-47.83
96.60
98.58
97.06
93.73
92.73
91.85
91.48
93.85
92.24
91.69
90.81
Pai-Et (2a)a
Tai-Et (2b)a
-40.20
18.7
34.0
22.3
29.9
35.1
42.3
34.3
41.9
49.7
58.9
48.3
61.0
73.1
84.2
56.7
69.6
82.1
96.7
21.6
30.0
37.1
48.4
41.4
53.3
61.7
79.3
50.6
59.0
79.9
98.3
76.7
92.6
124.0
143.2
[Hg(Pai-Me)(µ-Cl)Cl]2 (7a)
[Hg(Tai-Me)(µ-Cl)Cl]2 (7b)
[Hg(Pai-Et)(µ-Cl)Cl]2 (8a)
[Hg(Tai-Et)(µ-Cl)Cl]2 (8b)
[Hg(Pai-Me)(µ-I)I]2b (10a)
[Hg(Tai-Me)(µ-I)I]2b (10b)
[Hg(Pai-Et)(µ-I)I]2b (11a)
[Hg(Tai-Et)(µ-I)I]2b (11b)
-214.65
-226.36
-220.84
-223.49
-186.20
-214.1
-197.53
-190.21
a In toluene. b In acetonitrile. c At 298 K. d Reference 9.
in air. Orange-red crystals were separated. They were collected by
filtration and dried over CaCl2 in vacuo. The yield was 0.19 g
(42%). Other complexes were prepared under identical conditions,
and yield varied in the range 45-50%. Anal. Calcd for C20H20N8I2-
Hg (4a): C, 29.03; H, 2.42; N, 13.55. Found: C, 29.15; H, 2.49;
N, 13.40. Calcd for C22H24 N8I2Hg (4b): C, 30.89; H, 2.81; N,
13.11. Found: C, 30.97; H, 2.90; N, 13.05. Calcd for C22H24N8I2-
Hg (5a): C, 30.89; H, 2.81; N, 13.11. Found: C, 30.93; H, 2.72;
N, 13.15. Calcd for C24H28N8I2Hg (5b): C, 32.63; H, 3.17; N, 12.69.
Found: C, 32.70; H, 3.10; N, 12.60. Calcd for C32H28N8I2Hg (6a):
C, 39.24; H, 2.86; N, 11.45. Found: C, 39.17; H, 2.90; N, 11.50.
Calcd for C34H32N8I2Hg (6b): C, 40.53; H, 3.18; N, 11.13. Found:
C, 40.45; H, 3.12; N, 11.05.
3. Experimental Section
3.1. Materials. HgI2 was obtained from Loba Chemicals,
Bombay, India. 1-Alkyl-2-(arylazo)imidazoles were synthesized by
reported procedure.21 All other chemicals and solvents were reagent
grade as received.
3.2. Physical Measurements. Microanalytical data (C, H, N)
were collected on a Perkin-Elmer 2400 CHNS/O elemental analyzer.
Spectroscopic data were obtained using the following instruments:
UV-vis spectra from a Perkin-Elmer Lambda 25 spectrophotom-
eter; IR spectra (KBr disk, 4000-200 cm-1) from a Perkin-Elmer
RX-1 FTIR spectrophotometer; luminescence studies from a Perkin-
1
Elmer LS-55 spectrofluorometer; H NMR spectra from a Bruker
(AC) 300 MHz FTNMR spectrometer.
3.3.b. Synthesis of [Hg(HaaiMe)(µ-I)(I)]2 (10a). 1-Methyl
2-(phenylazo)imidazole (0.1 g, 0.54 mmol) in MeOH (10 mL) was
added dropwise to a 2-methoxyethanol solution (5 mL) of HgI2
(0.22 g, 0.48 mmol), and the solution was refluxed for 1 h. It was
then cooled, filtered, and left undisturbed for 1 week. Bright orange-
red crystals were obtained by slow evaporation. The crystals were
3.3.a. Synthesis of [HgI2][HaaiMe]2 (4a). A finely powdered
mixture of 1-methyl-2-(phenylazo)imidazole (Pai-Me, 1a) (0.5 g,
2.69 mmol) and HgI2 (0.25 g, 0.55 mmol) was taken in a Teflon
reactor (100 mL capacity) and placed in microwave oven of 450
W for 5 min × 3 with a 5 min interval at each step. The reactor
was then cooled, and the solution was filtered and evaporated slowly
Inorganic Chemistry, Vol. 46, No. 3, 2007 677

Sarker et al.
Table 6. Summarized Crystallographic Data for [HgI2][Pai-Me]2 (4a), [Hg(Pai-Me)(µ-I)(I)]2 (10a), and [Hg(Tai-CH2Ph)(µ-I)(I)]2 (12b)
param
4a
C20H20N8I2Hg
826.83
10a
C10H10N4I2Hg
640.61
12b
C17H16N4I2Hg
730.73
empirical formula
fw
temp (K)
cryst system
space group
cryst size (mm3)
a (Å)
b (Å)
c (Å)
296(2)
monoclinic
C2/c
0.20 × 0.10 × 0.10
27.539(3)
6.6508(6)
17.2364(17)
90.00
124.911(2)
90.00
2588.8(4)
4
8.353
1.8-28.31
-35 e h e 23; -8 e k e 8;
-19 e l e 22
2.121
294(2)
triclinic
P1h
294(2)
monoclinic
P21/n
0.20 × 0.05 × 0.05
11.6496(13)
14.3615(16)
12.8105(14)
90.00
109.002(2)
90.00
0.20 × 0.12 × 0.10
7.5841(7)
10.408(1)
10.506(1)
113.784(1)
93.111(2)
99.851(2)
740.7(1)
2
R (deg)
â (deg)
γ (deg)
V (Å)3
2026.5(4)
4
10.648
Z
µ(Mo KR) (mm-1
θ range
)
14.544
2.14-28.31
-9 < h < 9; -13 < k < 13;
-13 < l < 13
2.873
2.05-28.34
hkl range
-13 < h < 15; -19 < k < 17;
-17 < l < 17
D
calc (mg m-3
)
2.395
refine params
tot. reflcns
141
8420
3121
0.0410
0.1362
0.933
154
7949
3519
0.0302
0.0743
0.989
187
13 566
4921
0.0398
0.0529
0.750
unique reflcns
R1a [I > 2σ(I)]
b
wR2
goodness of fit
2
2
2
a R ) Σ||Fo| - |Fc||/Σ|Fo|. b wR2 ) [Σw(Fo - Fc )2/Σw(Fo )2]1/2; w ) 1/[σ2(Fo)2 + (0.1000P)2 + (0.0000P)] for 4a, w ) 1/[σ2(Fo)2 + (0.0420P)2 +
(0.0000P)] for 10a, and w ) 1/[σ2(Fo)2 + (0.0010P)2 + (0.0000P)] for 12b, where P ) ((Fo + 2Fc )/3.
2
2
(I). All these structures were solved by direct methods and followed
by successive Fourier and difference Fourier syntheses. Full-matrix
least-squares refinements on F2 were carried out using SHELXL-
97 with anisotropic displacement 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 SHELXS 97,25 SHELXL97,26 PLATON 99,27 and
ORTEP-328 programs.
3.5. Photometric Measurements. Absorption spectra were taken
with a Perkin-Elmer Lambda 25 UV/vis spectrophotometer in a 1
× 1 cm quartz optical cell maintained at 25 °C with a Peltier
thermostat. The light source of a Perkin-Elmer LS 55 spectrofluo-
rometer 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 isomers 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 (φ) were obtained by measuring initial trans-to-
cis isomerization rates (ν) in a well-stirred solution within the above
instrument using the equation
Figure 6. Arrhenius plots of rate constants of the cis-to-trans thermal
isomerization of (a) Tai-Me and (b) [Hg(Tai-Me)(µ-I)(I)]2 at different
temperatures.
collected by filtration, washed with cold methanol, and dried over
CaCl2 in vacuo. The yield was 0.23 g (74%). Other complexes were
prepared under identical conditions, and yield varied in the range
65-75%. Anal. Calcd for C10H10N4I2Hg (10a): C, 18.73; H, 1.56;
N, 8.74. Found: C, 18.65; H, 1.49; N, 8.80. Calcd for C11H12N4I2-
Hg (10b): C, 20.17; H, 1.83; N, 8.55. Found: C, 20.08; H, 1.90;
N, 8.65. Calcd for C11H12N4I2Hg (11a): C, 20.17; H, 1.83; N, 8.55.
Found: C, 20.12; H, 1.76; N, 8.68. Calcd for C12H14N4I2Hg (11b):
C, 21.54; H, 2.09; N, 8.38. Found: C, 21.70; H, 2.00; N, 8.45.
Calcd for C16H14N4I2Hg (12a): C, 26.79; H, 1.95; N, 7.81. Found:
C, 26.66; H, 1.90; N, 7.90. Calcd for C17H16N4I2Hg (12b): C, 27.92;
H, 2.19; N, 7.67. Found: C, 28.05; H, 2.12; N, 7.78.
3.4. X-ray Diffraction Study. The crystallographic data are
shown in Table 6. Suitable single crystals of complexes 4a (0.20
× 0.10 × 0.10 mm3), 10a (0.20 × 0.12 × 0.10 mm3), and 12b
(0.20 × 0.05 × 0.05 mm3) were mounted on a Siemens CCD
diffractometer equipped with graphite-monochromated Mo KR (λ
) 0.710 73 Å) radiation. The unit cell parameters and crystal-
orientation matrices were determined for two complexes by least-
squares refinements of all reflections. The intensity data were
corrected for Lorentz and polarization effects, and an empirical
absorption correction were also employed using the SAINT
program.24 Data were collected by applying the condition I > 2σ-
ν ) (φI0/V)(1 - 10-Abs
)
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 azobenzene
(φ ) 0.11 for π-π* excitation29) under the same irradiation
condition.
(24) SMART and SAINT; Bruker AXS Inc.: Madison, WI, 1998.
(25) Sheldrick, G. M. SHELXS 97, Program for the Solution of Crystal
Structure; University of Gottingen: Gottingen, Germany, 1997.
(26) Sheldrick, G. M. SHELXL 97, Program for the Solution of Crystal
Structure; University of Gottingen: Gottingen, Germany, 1997.
(27) Spek, A. L. PLATON, Molecular Geometry Program; University of
Utrecht: Utrecht, The Netherlands, 1999.
(28) Farrugia, L. J. ORTEP-3 for windows. J. Appl. Crystallogr. 1997,
30, 565.
(29) Zimmerman, G.; Chow, L.; Paik, U. J. Am. Chem. Soc. 1958, 80,
3528.
678 Inorganic Chemistry, Vol. 46, No. 3, 2007

Mercury(II)-Iodo Complexes of (Arylazo)imidazoles
Figure 7. Frontier molecular orbitals of 1a and 10a. The ordinate indicates orbital energies in hartrees. The numbers in parentheses indicate molecular
orbital numbers. Orbitals of similar symmetries in parts of the ligand are connected.
The thermal cis-to-trans isomerization rates were obtained by
monitoring absorption changes intermittently for a cis-rich solution
kept in the dark at constant temperatures in the range from 298 to
313 K. The activation energy (Ea) and the frequency factor (A)
were obtained from the Arrhenius plot,
GaussView visualization program.31 The hybrid functional by
Becke, B3LYP,32 was used, which include a mixture of Hartree-
Fock exchange with DFT exchange-correlation. For C, H, N, and
I, we used MIDI! basis functions,33 as this basis set is applicable
to all these elements and includes polarization functions. For Hg,
we used the LanL2DZ basis set including Los Alamos effective
core potentials.34-36
ln k ) ln A - Ea/RT
4. Conclusion
where k is the measured rate constant, R is the gas constant, and T
is temperature. The values of activation free energy (∆G*) and
activation entropy (∆S*) were obtained through the relationships
The effect of reaction phase on the synthesis of compounds
is examined in this work. Neat reaction between HgI2 and
1-alkyl-2-(arylazo)imidazole (RaaiR′) under microwave ir-
radiation has synthesized [HgI2][RaaiR′]2 in which layers of
reactants separately positioned in the crystal structure. In
solution phase we have isolated iodo-bridged azoimine-
chelated coordination complexes of the composition [Hg-
(RaaiR′)(µ-I)(I)]2. Single-crystal X-ray diffraction study has
determined the structure of the complexes. Both ligands and
iodo-bridged Hg(II) complexes show photochromism upon
UV light irradiation. Quantum yields of trans-to-cis isomer-
ization are determined in toluene for ligands and in MeCN
for complexes. The cis-to-trans isomerization is a thermally
driven process. The activation energies (Ea’s) of cis-to-trans
isomerization have been calculated. The slow rate of isomer-
ization in complexes may be due to higher rotor volume than
that of free ligands.
∆G* ) Ea - RT - T∆S*
∆S* ) [ln A - 1 - ln(kBT/h)]/R
where kB and h are Boltzmann’s and Plank’s constants, respectively.
3.5. DFT Calculations. The density functional theory (DFT)
calculations on the crystal structures of 1a9 and 10a were carried
out using the Gaussian 03w program package30 with the aid of the
(30) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G.
A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.;
Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R.
E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J.
W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.;
Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.;
Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari,
K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.;
Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A.
Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford. CT, 2004.
(31) Dennington, R., II; Keith, T.; Millam, J.; Eppinnett, K.; Hovell, W.
L.; Gilliland, R. GaussView, version 3.09; Semichem, Inc.: Shawnee
Mission, KS, 2003.
(32) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(33) Easton, R. E.; Giesen, D. J.; Welch, A.; Cramer, C. J.; Truhlar, D. G.
Theor. Chim. Acta 1996, 93, 281.
(34) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270.
(35) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284.
(36) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299.
Inorganic Chemistry, Vol. 46, No. 3, 2007 679

Sarker et al.
also been deposited with the Cambridge Crystallographic Data
Center (CCDC No.: 293801, [HgI2][HaaiMe]2 (4a); 293802, [Hg-
(HaaiMe)(µ-I)(I)]2 (10a); 293803, [Hg(MeaaiCH2Ph)(µ-I)(I)]2 (12b)).
Copies of this information may be obtained free of charge from
the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K.
(e-mail,deposit@ccdc.cam.ac.uk,orwww,htpp://www.ccdc.cam.ac.uk).
Acknowledgment. Financial support from the Council
of Scientific and Industrial Research (CSIR) and Department
of Science & Technology (DST), New Delhi, is gratefully
acknowledged. K.K.S. thanks the CSIR for a fellowship.
Supporting Information Available: Crystallographic data in
CIF format. This material is available free of charge via the Internet
at http://pubs.acs.org. Crystallographic data for the structures have
IC061221U
680 Inorganic Chemistry, Vol. 46, No. 3, 2007