Cadmium(II) complexes of (Arylazo)imidazoles: Synthesis, structure, photochromism, and density functional theory calculation

Article abstract of DOI:10.1021/ic7012073

Reaction between CdX₂ and 1-alkyl-2-(phenylazo)imidazole (RaaiR′) has isolated complexes of composition Cd(RaaiR′) ₂X₂ in MeOH or MeCN. Crystallization of Cd(RaaiR′)₂I₂ from N,N-dimethylformamide (DMF) has separated [Cd(RaaiR′)I₂·DMF], while Cd(RaaiR′) ₂X₂ (X = Cl and Br) remains unchanged in its composition upon crystallization under identical conditions. The structure has been established by spectral (UV-vis and ¹H NMR) data and confirmation in the latter case by a single-crystal X-ray diffraction study of [Cd(TaiMe)I ₂·DMF] [where TaiMe = 1-methyl-2-(p-tolylazo)imidazole]. UV-light irradiation in a MeCN solution of Cd(RaaiR′)₂I ₂ and [Cd(RaaiR′)I₂-DMF] shows trans-to-cis isomerization of coordinated azoimidazole. The reverse transformation, cis-to-trans, 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 values than their cadmium(II) iodo complexes. The cis-to-trans isomerization is a thermally induced process. The activation energy (E_a) of cis-to-trans isomerization is calculated by a controlled-temperature experiment. The effects of the anions (Cl^-, Br^-, I^-, and ClO₄^-) and the number of coordinated azoimidazoles (RaaiR′) [Cd(RaaiR′) or Cd(RaaiR′)₂] on the rate and quantum yields of photochromism are established in this work. A slow rate of photoisomerization of [Cd(RaaiR′)4](ClO₄)₂ compared to Cd(RaaiR′)I₂ or Cd(RaaiR′)₂X₂ may be associated with the increased mass and rotor volume of the complexes. The rate of isomerization is also dependent on the nature of X and follows the sequence Cd(RaaiR′)₂Cl₂ < Cd(RaaiR′) ₂Br₂ < Cd(RaaiR′)₂I₂. It may be related to the size and electronegativity of halide, which reduces the effective molar association in the order of I < Br < Cl and hence the rate. Gaussian 03 calculations of representative complexes and free ligands are used to explain the difference in the rates and quantum yields of photoisomerization.

Full text of DOI:10.1021/ic7012073

Inorg. Chem. 2007, 46, 8291−8301
Cadmium(II) Complexes of (Arylazo)imidazoles: Synthesis, Structure,
Photochromism, and Density Functional Theory Calculation
K. K. Sarker,† D. Sardar,† K. Suwa,‡ J. Otsuki,‡ and C. Sinha*,†
Department of Chemistry, Inorganic Chemistry Section, JadaVpur UniVersity, Kolkata 7000 032,
India, and Department of Materials and Applied Chemistry, College of Science and Technology,
Nihon UniVersity, 1-8-4 Kanda Surugadai, Chiyoda-ku, Tokyo 101-8308, Japan
Received June 20, 2007
Reaction between CdX2 and 1-alkyl-2-(phenylazo)imidazole (RaaiR
(RaaiR )2X2 in MeOH or MeCN. Crystallization of Cd(RaaiR )2I2 from N,N-dimethylformamide (DMF) has separated
[Cd(RaaiR )I2 DMF], while Cd(RaaiR )2X2 (X Cl and Br) remains unchanged in its composition upon crystallization
under identical conditions. The structure has been established by spectral (UV
vis and 1H NMR) data and confirmation
in the latter case by a single-crystal X-ray diffraction study of [Cd(TaiMe)I2 DMF] [where TaiMe 1-methyl-2-(p-
tolylazo)imidazole]. UV-light irradiation in a MeCN solution of Cd(RaaiR )2I2 and [Cd(RaaiR )I2 DMF] shows trans-
′) has isolated complexes of composition Cd-
′
′
′
‚
′
)
−
‚
)
′
′ ‚
to-cis isomerization of coordinated azoimidazole. The reverse transformation, cis-to-trans, is very slow with visible
light irradiation. Quantum yields (φtfc) of trans-to-cis isomerization are calculated, and the free ligand shows higher
φ
values than their cadmium(II) iodo complexes. The cis-to-trans isomerization is a thermally induced process. The
activation energy (Ea) of cis-to-trans isomerization is calculated by a controlled-temperature experiment. The effects
of the anions (Cl-, Br-, I-, and ClO4-) and the number of coordinated azoimidazoles (RaaiR
) [Cd(RaaiR ) or
Cd(RaaiR )2] on the rate and quantum yields of photochromism are established in this work. A slow rate of
photoisomerization of [Cd(RaaiR )4](ClO4)2 compared to Cd(RaaiR )I2 or Cd(RaaiR )2X2 may be associated with the
increased mass and rotor volume of the complexes. The rate of isomerization is also dependent on the nature of
X and follows the sequence Cd(RaaiR )2Cl2 < Cd(RaaiR )2Br2 < Cd(RaaiR )2I2. It may be related to the size and
′
′
′
′
′
′
′
′
′
electronegativity of halide, which reduces the effective molar association in the order of I < Br < Cl and hence the
rate. Gaussian 03 calculations of representative complexes and free ligands are used to explain the difference in
the rates and quantum yields of photoisomerization.
1. Introduction
photochromic molecules into organic or hybrid organic-
inorganic materials leads to the 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.2
(Arylazo)imidazoles constitute an interesting class of
heterocyclic azo compounds as a potential switching group
in biological applications and in coordination chemistry
because imidazole is a ubiquitous and essential group in
biology, especially as a metal-coordinating site. This family
Exploration of the material properties of the organic
molecules and their metal complexes is of current research
interest. The properties can be varied by changing the ligand
types, the presence of substituents, and the different car-
bocyclic and heterocyclic rings and also by using different
metal ions. Toward the study of the properties of the
complexes, we have been interested in examining 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.1 Incorporation of
(1) Rau, H. In Photochromism, Molecules and Systems; Du¨rr, H., Bounas-
Laurent, H., Eds.; Elsevier: Amsterdam, The Netherlands, 1990; pp
165-192. Nishihara, H. Bull. Chem. Soc. Jpn. 2004, 77, 407. Tamai,
N.; Miyasaka, H. Chem. ReV. 2000, 100, 1857. Yagai, S.; Karatsu,
T.; Kitamura, A. Chem.sEur. J. 2005, 11, 4054.
(2) Ire, M. Chem. ReV. 2000, 100, 1683. Ikeda, T.; Tsutsumi, O. Science
1995, 268, 1873. Kawata, S.; Kawata, Y. Chem. ReV. 2000, 100, 1777.
Akitasu, T.; Einaga, Y. Polyhedron 2007, 26, in press.
* To whom correspondence should be addressed. E-mail: c_r_sinha@
yahoo.com. Fax: 91-033-2413-7121.
† Jadavpur University.
‡ Nihon University.
10.1021/ic7012073 CCC: $37.00
Published on Web 09/07/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 20, 2007 8291

Sarker et al.
Scheme 1. Isomerization of (Phenylazo)imidazole
are nonconducting, and their compositions have been sup-
ported by microanalytical data. The reaction of Cd(ClO4)2‚
6H2O and RaaiR′ has separated compounds of composition
[Cd(RaaiR′)4](ClO4)2 (11 and 12; Scheme 2). The structures
of [Cd(PaiMe)2Cl2] (3a) and [Cd(PaiEt)4](ClO4)2 (12a) are
reported elsewhere.16 The complexes 5-10 are new and have
been established in one case by a single-crystal X-ray
diffraction study.
2.2. Spectral Studies. The bands in the Fourier transform
infrared (FTIR) spectra of the complexes [Cd(RaaiR′)2(X)2]
(3-8) and [Cd(RaaiR′)(I)2‚DMF] (9 and 10) were assigned
upon comparison with the free-ligand data and reported
complexes.16,17 Moderately intense stretchings at 1595-1600
and 1435-1445 cm-1 are due to ν(CdN) and ν(NdN),
respectively.
of compounds has been extensively used as ligands for metal
ions by us3-8 and others.9-11 However, very few reports con-
cerning the photochromic property (Scheme 1) of (arylazo)-
imidazole dyes are found in the literature.12-15 The photo-
chromism of 1-methyl-2-(phenylazo)imidazole (PaiMe)14 and
mercury(II) azoimidazole complexes15 has inspired us to
examine the photochromic property of cadmium(II) com-
plexes of 1-alkyl-2-(arylazo)imidazoles. The influence of
halides (Cl-, Br-, and I-), and ClO4- on the rates of isomeri-
zation and quantum yields will be compared in this report.
The absorption spectra were recorded in a MeCN solution
for the new complexes 5-10 in 200-900 nm. The charac-
teristics common to the complexes are as follows: (1) a
structured absorption band around 370-390 nm with a molar
absorption coefficient on the order of 104 M-1 cm-1 and (2)
a tail extending into 450 nm. From the analogy with the
absorption spectra of PaiMe,14 it is likely that the large
absorption band around 370-390 nm corresponds to ππ*
transitions, while the tail corresponds to a nπ* transition.
The assignment is also supported by theoretical calculations,
as described later. The ππ* absorption peaks (λmax) for
derivatives of (2-phenylazo)imidazole are within the range
of 361-375 nm, which is between the ππ* absorption bands
of azobenzene (313 nm) and 4-(N,N-dimethylamino)azoben-
zene (390 nm).18
2. Results and Discussion
2.1. Synthesis of Complexes. PaiMe (1a), 1-methyl-2-
(p-tolylazo)imidazole (TaiMe, 1b), 1-ethyl-2-(phenylazo)-
imidazole (PaiEt, 2a), and 1-ethyl-2-(p-tolylazo)imidazole
(TaiEt, 2b) are used in this work to prepare cadmium(II)
complexes. The reaction between CdX2 and the ligand in a
MeOH-MeCN (2:1, v/v) mixture has synthesized a coor-
dination complex of composition [Cd(RaaiR′)2(X)2] [X )
Cl (3 and 4), Br (5 and 6), I (7 and 8); RaaiR′ is used as a
general abbreviation for 1-alkyl-2-(arylazo)imidazole]. Crys-
tallization of [Cd(RaaiR′)2(I)2] in MeCN is unsuccessful, and
we have tried to crystallize it from a MeCN solution in the
presence of N,N-dimethylformamide (DMF; 1:0.5, v/v).
However, the compositions of the crystallized products are
different from the previous one and are [Cd(RaaiR′)(I)2‚
DMF] (9 and 10), while the crystallization of [Cd(RaaiR′)2-
(Cl)2] and [Cd(RaaiR′)2(Br)2] from DMF-MeCN does not
change the composition of the complexes. The compounds
1
The H NMR spectra of [Cd(RaaiR′)2(X)2] are recorded
in CD3CN, and the signals are assigned (Table 1) unambigu-
ously by spin-spin interaction, by the effect of substitution
therein, and on comparison with previously reported
data.15-17,19-21 The atom-numbering pattern is shown in the
structure (Scheme 2). Data reveal that the signals in the
spectra, in general, are shifted downfield compared with the
spectra of the free ligand.17 The 7,11-H are shifted to higher
δ by ∼0.5 ppm. This observation supports the existence of
strong interaction between ligands and cadmium(II) in the
complexes. Aryl signals are shifted to the lower field side
upon Me substitution to the aryl ring. This is due to the
electron-donating effect of the Me group. N1-R′ shows the
usual signal pattern, as earlier.17
(3) Chand, B.; Ray, U.; Mostafa, G.; Lu. T.-H.; Sinha, C. J. Coord. Chem.
2004, 57, 627.
(4) Ray, U.; Banerjee, D.; Mostafa, G.; Lu, T.-H.; Sinha, C. New J. Chem.
2004, 28, 1437.
(5) Dinda, J.; Jasimuddin, S.; Mostafa, G.; Hung, C.-H.; Sinha, C.
Polyhedron 2004, 23, 793.
(6) Jasimuddin, S.; Sinha, C. Transition Met. Chem. 2004, 29, 566.
(7) Chand, B.; Ray, U.; Mostafa, G.; Cheng, J.; Lu, T.-H.; Sinha, C. Inorg.
Chim. Acta 2005, 358, 1927.
(8) Dinda, J.; Senapoti, S.; Mondal, T.; Jana, A. D.; Chiang, M.; Lu T.-
H.; Sinha, C. Polyhedron 2006, 25, 1125. Mathur, T.; Ray, U. S.;
Baruri, B. N.; Sinha, C. J. Coord. Chem. 2005, 58, 399.
(9) Raj, S. S. S.; Fun, H.-K.; Chen, X.-F.; Zhu, X.-H.; You, X.-Z. Acta
Crystallogr. 1999, C55, 1644.
(10) Dash, A. C.; Acharya, A.; Sahoo, R. K. Indian J. Chem. 1998, 37A,
759.
(11) Ackermann, M. N.; Robinson, M. P.; Maher, I. A.; LeBlanc, E. B.;
Raz, R. V. J. Organomet. Chem. 2003, 682, 248.
(12) Endo, M.; Nakayama, K.; Kaida, Y.; Majima, T. Tetrahedron Lett.
2003, 44, 6903.
2.3. Molecular Structure of [Cd(TaiMe)I2(C3H7NO)].
The crystals obtained from the reaction between CdI2 and
TaiMe from a MeOH-MeCN mixture decompose upon
(16) Chand, B. G.; Ray, U. S.; Mostafa, G.; Lu, T.-H.; Falvello, L. R.;
Soler, T.; Tomas, M.; Sinha, C. Polyhedron 2003, 22, 3161.
(17) Misra, T. K. Transition Metal Chemistry of 2-arylazoimidazoles:
Synthesis, Characterisation and Electrochemical Studies. Ph.D. Thesis,
Burdwan University, Burdwan, India, 1999.
(18) Nishimura, N.; Sueyoshi, T.; Yamanaka, H.; Imai, E.; Yamamoto, S.;
Hasegawa, S. Bull. Chem. Soc. Jpn. 1976, 49, 1381.
(13) Fukuda, N.; Kim, J. Y.; Fukuda, T.; Ushijima, H.; Tamada, K. Jpn. J.
Appl. Phys. 2006, 45, 460.
(14) Otsuki, J.; Suwa, K.; Narutaki, K.; Sinha, C.; Yoshikawa, I.; Araki,
K. J. Phys. Chem. A 2005, 109, 8064.
(19) Misra, T. K.; Das, D.; Sinha, C.; Ghosh, P. K.; Pal, C. K. Inorg. Chem.
1998, 37, 1672.
(20) Sarker, K. K.; Chand, B. G.; Jana, A. D.; Mostafa, G.; Sinha, C. Inorg.
Chim. Acta 2005, 359, 695.
(15) Sarker, K. K.; Chand, B. G.; Suwa, K.; Cheng, J.; Lu, T.-H.; Otsuki,
J.; Sinha, C. Inorg. Chem. 2007, 46, 670. Otsuki, J.; Suwa, K.; Sarker,
K. K.; Sinha, C. J. Phys. Chem. A 2007, 111, 1403.
(21) (a) Chand, B. G.; Ray, U. S.; Cheng, J.; Lu, T.-H.; Sinha, C.
Polyhedron 2003, 23, 1213. (b) Chand, B. G.; Ray, U. S.; Santra, P.
K.; Mostafa, G.; Lu, T.-H.; Sinha, C. Polyhedron 2003, 22, 1205.
8292 Inorganic Chemistry, Vol. 46, No. 20, 2007

Cadmium(II) Complexes of (Arylazo)imidazoles
Scheme 2. Structures of Cd(RaaiR′)2(X)2 (3-8), [Cd(RaaiR′)(I)2‚DMF] (9 and 10), and [Cd(RaaiR′)4](ClO4)2 (11 and 12)a
a [Cd(PaiMe)2(Cl)2] (3a), [Cd(TaiMe)2(Cl)2] (3b), [Cd(PaiEt)2(Cl)2] (4a), [Cd(TaiEt)2(Cl)2] (4b), [Cd(PaiMe)2(Br)2] (5a), [Cd(TaiMe)2(Br)2] (5b),
[Cd(PaiEt)2(Br)2] (6a), [Cd(TaiEt)(Br)2] (6b), [Cd(PaiMe)2(I)2] (7a), [Cd(TaiMe)2(I)2] (7b), [Cd(PaiEt)2(I)2] (8a), [Cd(TaiEt)2(I)2] (8b), [Cd(PaiMe)(I)2.DMF]
(9a), [Cd(TaiMe)(I)2.DMF] (9b), [Cd(PaiEt)(I)2.DMF] (10a), [Cd(TaiEt)(I)2.DMF] (10b), [Cd(PaiMe)4](ClO4)2 (11a), [Cd(TaiMe)4](ClO4)2 (11b),
[Cd(PaiEt)4](ClO4)2 (12a), and [Cd(TaiEt)4](ClO4)2 (12b)
Table 1. 1H NMR Spectral Data of [Cd(RaaR′)2(X)2] [X ) Br (5 and 6), I (7 and 8)] Complexes 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
e
compd
Cd(PaiMe)2(Br)2 (5a)
Cd(TaiMe)2(Br)2 (5b)
Cd(PaiEt)2(Br)2 (6a)
Cd(TaiEt)2(Br)2 (6b)
Cd(PaiMe)2(I)2 (7a)
Cd(TaiMe)2(I)2 (7b)
Cd(PaiEt)2(I)2 (8a)
Cd(TaiEt)2(I)2 (8b)
7.44
7.47
7.46
7.41
7.48
7.42
7.33
7.33
7.34
7.36
7.35
7.31
7.38
7.37
7.28
7.29
8.09 (7.0)
8.00 (7.0)
8.05 (7.0)
8.02 (7.0)
7.93 (7.0)
8.02 (7.0)
7.65 (7.0)
8.00 (7.0)
7.57c
7.57d
2.48f
7.48d
2.40c
7.49d
2.46f
7.58d
2.42c
4.14
4.13
7.38 (7.0)
7.50c
4.60g (10.0)
4.52g (12.0)
1.55h (7.5)
1.56h (7.5)
7.32 (7.0)
7.49c
4.01
4.10
7.37 (7.0)
7.47c
4.24g (10.0)
4.44g (12.0)
1.32h (7.5)
1.30h (7.5)
7.35 (7.0)
a Broad singlet. b Doublet. c Multiplet. d δ(9-H). e Singlet. f δ(9-Me). g Quartet. h Triplet.
Table 2. Selected Bond Distances (Å) and Angles (deg) for
in apexes of the trigonal plane. The Cd-N4 [Cd-N(azo)]
bond is the longest [2.639(5) Å] one in the family of
M-N(azo) bond lengths so far known in the literature.15,16,19-23
The elongation may be due to axial coordination of N4 with
reference to the trigonal plane described by CdNI2 and from
the small chelate bite angle. Although the Cd-N(azo) bond
length is very long, it is less than the sum of the van der
Waals radii of CdII (1.58 Å) and N(sp2) (1.53 Å). This implies
significant bonding interaction between these components.
The strong coordination of imidazole N to CdII has significant
biochemical implications and explains the strong toxicity of
CdII.24 Weak and flexible bonds are very effective at inducing
some functional properties in the molecules. The weak
bonding interaction between MnII and the N(azo) center in
[Mn(N3)2(TaiEt)]n is responsible for structural distortion and,
hence, the canting phenomenon and remnant magnetism at
low temperature.22 Because of the long CdII-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 the photo-
chromism of these molecules (vide infra). Imidazole and
[Cd(TaiMe)I2(C3H7NO)]
bond distances (Å)
bond angles (deg)
Cd-I1
2.7140(12)
I1-Cd-I2
121.15(3)
115.72(15)
99.6(2)
66.23(17)
83.8(2)
149.9(2)
102.23(10)
91.97(10)
122.34(14)
95.3(2)
Cd-I2
2.7306(13)
2.250(5)
2.639(5)
2.380(7)
1.264(6)
0.986(12)
1.399(8)
1.421(7)
1.252(13)
I1-Cd-N1
I1-Cd-O1
N1-Cd-N4
N1-Cd-O1
N4-Cd-O1
I2-Cd-N4
I1-Cd-N4
I2-Cd-N1
I2-Cd-O1
Cd-N1
Cd-N4
Cd-O1
N3-N4
O1-C14
N3-C4
N4-C5
N5-C14
X-ray exposure. Good quality crystals of 1-methyl-2-(phe-
nylazo)imidazolecadmium(II) iodide were grown by slow
evaporation of a methanol-DMF (1:0.5, v/v) mixture. The
crystal structure of the complex is shown in Figure 1, and
the bond parameters are listed in Table 2. The structure
shows a distorted trigonal-bipyramidal (TBP) geometry
around CdII. The coordination includes the chelated azoimine
Cd-(NdC-NdN-) group [N(azo) is N4 and N(imine) is
N1], two Cd-I bonds, and one Cd-O(DMF) bond. The
atomic arrangements Cd1, I1, I2, and N1 constitute a trigonal
plane, and Cd deviates by ∼0.1 Å from the mean plane. The
chelate angle is 66.23(17)° and is comparable with results
reported in the series of chelated (arylazo)imidazole com-
plexes of d10 metal complexes.15,16,20,21 The small chelate
angle may be the cause for distortion from symmetric TBP
geometry. The N4 [N(azo)] and coordinated O (DMF) appear
(22) Ray, U. S.; Ghosh, B. K.; Monfort, M.; Ribas, J.; Mostafa, G.; Lu,
T.-H.; Sinha, C. Eur. J. Inorg. Chem. 2004, 250. Bhuina, P.; Ray, U.
S.; Mostafa, G.; Ribas, J.; Sinha, C. Inorg. Chim. Acta 2006, 359,
4660.
(23) Bhunia, P.; Baruri, B.; Ray, U. S.; Sinha, C.; Das, S.; Cheng, J.; Lu,
T.-H. Transition Met. Chem. 2006, 31, 310.
(24) Fergusson, J. E. The heaVy elements: chemistry, enVironment impact
and health effects; Pergamon: Oxford, U.K., 1990.
Inorganic Chemistry, Vol. 46, No. 20, 2007 8293

Sarker et al.
Figure 1. Crystal structure of 9b.
p-tolyl rings are joined by an azo group, and the dihedral
angle is 9.8(3)°. The NdN bond length is 1.264(6) Å and is
slightly longer than that of the free ligand [1.250(1) Å].14,23
The stability of chelated azoimine Cd-(NdC-NdN-) is
due to the metal-to-ligand π-back-bonding, and the azo group
is directly involved in this process.17,19 This reduces the
M-N(azo) bond length and subsequently increases the Nd
N bond length from that of the free ligand. The acute chelate
angle may develop a strain that is relieved partially by
structural distortion and bond-length elongation. The Cd-I
distances are comparable with published results.25
Figure 2. C-H(DMF)---π‚‚‚π-π‚-‚π---π-π---π‚‚‚CH(DMF)-π self-
assembly leading to the formation of a 2D supramolecular sheet in the (110)
plane.
A π-π interaction [4.069(4) Å; symmetry, 1 - x, 1 - y,
1 - z] between imidazole and p-tolyl rings with the partners
of a neighboring molecule has generated a supramolecular
motif. The observed weak interactions are C-H---π [3.033
Å, 132.97°, symmetry, 1 - x, 1 - y, 1 - z, where interaction
is taking place between C-H (N-CH3) and the p-tolyl ring;
2.882 Å, 148.33°, symmetry, 1 - x, 1 - y, -z, where
interaction is taking place between DMF C12-H12A and
the imidazole ring] and π---π: interactions between imida-
zole and p-tolyl rings of neighboring molecules (4.068 Å,
symmetry, 1 - x, 1 - y, 1 - z) and p-tolyl---p-tolyl (4.226
Å, symmetry, 2 - x, -y, 1 - z) have generated a 2D
supramolecular sheet (Figure 2).
2.4. Photochromism of [Cd(RaaiR′)2(X)2] and [Cd-
(RaaiR′)(I)2‚DMF] and Comparison with Other Cad-
mium(II) Azoimidazoles. Upon irradiation of light at λmax
to a MeCN solution of the complex, the absorption spectrum
changes (Figure 3) in a way similar to that observed for the
trans-to-cis isomerization of PaiMe14,15 and azobenzene.25 The
photochromic observation of a free ligand is shown in Figure
4. The intense peak at λmax decreases, which is accompanied
by a slight increase at the tail portion of the spectrum around
490 nm 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 trans-to-cis photoisomerization were
determined using those of azobenzene26 as a standard, and
Figure 3. Spectral changes of 7b in acetonitrile upon repeated irradiation
at 380 nm at 3 min intervals at 25 °C. The inset shows spectra of the cis
and trans isomers of the compound.
the results are tabulated in Table 3. Thermal cis-to-trans
isomerization rates of these (arylazo)imidazoles in MeCN
at 298-313 K are collected in Table 4.
The trans-to-cis isomerizations of CdII complexes of
PaiMe, TaiMe, PaiEt, and TaiEt in the presence of different
anions Cl-, Br-, I-, and ClO4- are carried out in this work.
Data are included in Tables 3 and 4 along with photochromic
data of PaiMe, TaiMe, PaiEt, and TaiEt. The photoisomer-
ization of the ligand in the complexes is dependent on the
nature of the metal ion, its oxidation state, and its structural
(25) Laskar, I. R.; Mostafa, G.; Maji, T. K.; Das, D.; Welch, A. J.;
Chaudhuri, N. R. J. Chem. Soc., Dalton Trans. 2002, 1066. Kropid-
lowska, A.; Chojnacki, J.; Becker, B. Acta Crystallogr., Sect. C: Cryst.
Struct. Commun. 2006, 62, m95.
(26) Zimmerman, G.; Chow, L.; Paik, U. J. Am. Chem. Soc. 1958, 80,
3528-3531.
8294 Inorganic Chemistry, Vol. 46, No. 20, 2007

Cadmium(II) Complexes of (Arylazo)imidazoles
Table 3. Excitation Wavelength (λππ*), Rate of Trans (t) f Cis (c)
Conversion, and Quantum Yield (φtfc) in MeCN
for this decrement: (i) the presence of a coordinated CdX2
motif heavily increases the molecular weight of the complex
unit, which may severely interfere with the motion of the
-NdN-Ar moiety, and (ii) the photobleaching efficiency
of the halo group28 may snatch out energy from the ππ*
excited state. These may cause very fast deactivation other
than the photochromic route. The rotor volume has a
significant influence on the isomerization rate and quantum
yields.27,29
rate of
t f c
isosbestic convn
λππ*
points
(nm)
× 108
compd
PaiMe (1a)
TaiMe (1b)
PaiEt (2a)
(nm)
(s-1
)
φtfc
363
365
364
365
381
382
375
383
383
381
377
388
375
380
373
390
385
393
382
391
333, 431 5.06
331, 443 3.70
333, 434 4.83
328, 444 4.59
0.25 ( 0.03
0.218 ( 0.007
0.241 ( 0.012
0.206 ( 0.006
TaiEt (2b)
Cd(PaiMe)2Cl2 (3a)
Cd(TaiMe)2Cl2 (3b)
Cd(PaiEt)2Cl2 (4a)
Cd(TaiEt)2Cl2 (4b)
Cd(PaiMe)2Br2 (5a)
Cd(TaiMe)2Br2 (5b)
Cd(PaiEt)2Br2 (6a)
Cd(TaiEt)2Br2 (6b)
Cd(PaiMe)2I2 (7a)
Cd(TaiMe)2I2 (7b)
Cd(PaiEt)2I2 (8a)
Cd(TaiEt)2I2 (8b)
Cd(PaiMe)I2‚DMF (9a)
Cd(TaiMe)I2‚DMF (9b)
Cd(PaiEt)I2.DMF (10a)
Cd(TaiEt)I2.DMF (10b)
338, 447 2.3810 0.121 ( 0.001
331, 447 1.8950 0.093 ( 0.002
338, 451 2.0135 0.106 ( 0.003
336, 459 1.7125 0.089 ( 0.004
335, 449 2.6972 0.134 ( 0.002
334, 451 2.1899 0.109 ( 0.003
338, 445 2.4721 0.116 ( 0.005
340, 463 1.7019 0.096 ( 0.006
334, 450 2.8560 0.142 ( 0.004
333, 451 2.5271 0.127 ( 0.002
332, 452 2.5045 0.121 ( 0.001
339, 463 1.9844 0.112 ( 0.001
337, 453 3.4330 0.167 ( 0.002
339, 465 3.0521 0.158 ( 0.003
338, 459 2.9625 0.143 ( 0.002
336, 463 2.3967 0.139 ( 0.003
338, 462 2.1159 0.103 ( 0.004
We have also examined the UV-light irradiation behavior
17
of green tcc-Ru(PaiMe)2Cl2 and blue ctc-Ru(PaiMe)2Cl2
in MeCN. RuII complexes are silent to photoisomerization.
There may be several reasons based on the structure and
energy ordering of the participating functions. PaiMe is
chelated to RuII, and X-ray structure determination shows
that the Ru-N(azo) bond length is shorter than Ru-
N(imidazole) distances. This has also prompted a charge flow
of dπ(Ru) f π*(azo), which synergistically enhances the
Ru-N(azo) bond strength. This structural rigidity may resist
photoisomerization. Besides, the energy transfer from ππ*
to the MLCT state may cause very fast bleaching.30
[Cd(PaiMe)4](ClO4)2 (11a) 382
[Cd(TaiMe)4](ClO4)2 (11b) 380
Thermal cis-to-trans isomerizations of the ligands and
complexes were followed by UV-vis spectroscopy in
toluene (ligands)/MeCN (complexes) at varied temperatures,
298-313 K. Eyring plots in the range 298-313 K gave a
linear graph from which the activation energies were obtained
(Table 4 and Figure 5). The Ea values of the free ligands are
closer to those of PaiMe but lower than those of azobenzene.
In the complexes, the Ea values are severely reduced, which
means faster thermal cis-to-trans isomerization of the com-
plexes. Hg-RaaiR′ halides also show higher Ea values than
Cd complexes, which is also supported by the rate data.15
The entropy of activation (∆S*) values are more negative
in the complexes than in the free ligand. This also supports
an increase in the rotor volume in the complexes.
331, 468 1.002
335, 458 1.8361 0.089 ( 0.002
334, 463 0.8052 0.043 ( 0.003
0.049 ( 0.001
[Cd(PaiEt)4](ClO4)2 (12a)
[Cd(TaiEt)4](ClO4)2 (12b)
381
380
state.27 The chelate complexes of these metal ions are
structurally known.15,18-20 It is observed that upon irradiation
with UV light trans-to-cis photoisomerization proceeded and
the cis molar ratio reaches >95%. The absorption spectra
of the trans ligands changed with isosbestic points upon
excitation (Figures 3 and 4) into the cis isomer and also in
1
the case of respective complexes. The H NMR technique
has been adopted to measure the percentage composition of
the irradiated solution, which supports the composition
1
obtained from absorption spectra. The H NMR signals of
the aromatic ring protons are significantly shifted to the
upper-field 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. The structures
of Cd(TaiMe)I2‚DMF (vide supra) (Figure 1) show long
CdII-N(azo) distances compared to the Cd-N(imidazole)
distances. Upon UV-light irradiation, the excitation may
cleave the Cd-N(azo) bond easily to make a free rotatory
arylazo group, which isomerizes to the cis form. The
2.5. Electronic Structure Calculation and Optical
Spectra. Density functioanl theory (DFT) calculations are
currently used to establish the electronic structure and spectral
transitions of organic and metalloorganic compounds. In this
work, DFT and time-dependent DFT (TD-DFT) calculations
have been performed on Cd(PaiMe)2Cl2 (3a), Cd(TaiMe)-
I2‚DMF (9b), and [Cd(PaiEt)4](ClO4)2 (12a) in the gas phase.
The highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) are abbreviated as
H and L, respectively, and hence other sets of MOs. There
are two sets of nearly degenerate orbitals on respective
ligands in Cd(PaiMe)2Cl2. Nearly degenerate H (-5.39 eV)
and H-1 (-5.44 eV) orbitals consist mainly of the 3p
quantum yields were measured for the trans-to-cis (φtfc
)
photoisomerization of these compounds in MeCN upon
irradiation of the UV wavelength (Table 3). The φtfc values
are significantly dependent on the nature of the substituents
and counterions and the number of RaaiR′. The Me sub-
stituent at the phenylazo group (PaiR′ to TaiR′) and the Et
substituent at the N1 position (1-Me to 1-Et) both reduce
the φtfc values. In general, an increase in the mass of the
molecule reduces the rate of isomerization, trans to cis. In
the complexes, the φtfc values are significantly less than
those of the free-ligand data. Two reasons may be considered
(28) 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.
(29) Kepert, C. J. Chem. Commun. 2006, 695. Nathan, C.; Gianneschi, M.;
Masar, S., III; Mirkin, A. Acc. Chem. Res. 2005, 38, 825.
(30) 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.
(27) 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.
Inorganic Chemistry, Vol. 46, No. 20, 2007 8295

Sarker et al.
Table 4. Rate and Activation Parameters for Cis (c) f Trans (t) Thermal Isomerization
rate of thermal
temp
(K)
c f t convn
Ea,
∆H*,
∆S*,
∆G*,c
compd
PaiMea,d (1a)
× 104 (s-1
0.22
)
kJ mol-1
kJ mol-1
J mol-1 K-1
kJ mol-1
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
298
303
313
323
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
79.0
77.05
85.03
84.33
87.08
24.22
20.26
21.54
18.81
28.95
23.11
25.04
21.10
35.84
28.31
30.03
29.56
36.89
36.01
-77.1
100.00
96.60
98.58
97.06
95.13
95.14
94.87
94.96
94.49
93.77
94.51
93.62
94.84
91.90
94.55
92.61
93.50
92.48
0.4
0.88
2.75
0.73
1.20
2.60
3.70
0.33
0.5
0.97
1.80
0.64
0.98
1.87
3.40
TaiMea (1b)
87.57
86.87
87.63
26.76
22.80
24.08
21.35
31.49
25.65
27.57
23.64
38.38
30.85
32.58
32.1
-38.84
PaiEta (2a)
-47.83
TaiEta (2b)
-40.20
Cd(PaiMe)2Cl2 (3a)
Cd(TaiMe)2Cl2 (3b)
Cd(PaiEt)2Cl2 (4a)
Cd(TaiEt)2Cl2 (4b)
Cd(PaiMe)2Br2 (5a)
Cd(TaiMe)2Br2 (5b)
Cd(PaiEt)2Br2 (6a)
Cd(TaiEt)2Br2 (6b)
Cd(PaiMe)2I2 (7a)
Cd(TaiMe)2I2 (7b)
Cd(PaiEt)2I2 (8a)
Cd(TaiEt)2I2 (8b)
Cd(PaiMe)I2‚DMF (9a)
Cd(TaiMe)I2‚DMF (9b)
1.3217
1.5141
1.8007
2.2078
1.3617
1.5319
1.8324
2.2117
1.3621
1.6124
1.8406
2.1329
1.3928
1.6341
1.8524
2.1539
1.6862
2.1086
2.6073
3.0894
2.2677
2.7368
3.0947
3.7783
1.6926
1.9939
2.4611
2.8538
2.3629
2.9726
3.2419
3.8125
1.481
1.895
2.4236
3.1137
4.703
6.1055
7.4167
8.5429
1.557
2.22
-237.93
-251.28
-246.09
-255.56
-219.92
-237.11
-233.13
-243.33
-197.97
-213.39
-216.52
-211.56
-189.95
-189.48
2.582
3.25
3.663
4.594
5.292
6.906
2.4915
3.2877
4.4718
5.2423
3.6938
5.1373
6.5326
7.7931
39.43
38.05
8296 Inorganic Chemistry, Vol. 46, No. 20, 2007

Cadmium(II) Complexes of (Arylazo)imidazoles
Table 4 (Continued)
rate of thermal
temp
c f t convn
Ea,
∆H*,
∆S*,
∆G*,c
compd
(K)
× 104 (s-1
)
kJ mol-1
kJ mol-1
J mol-1 K-1
kJ mol-1
Cd(PaiEt)I2‚DMF (10a)
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
2.5299
3.4926
4.3277
5.1922
1.7629
2.5321
3.0739
3.9238
2.7329
3.5639
4.3527
5.0169
2.4248
2.7935
3.1578
3.8129
2.8129
3.6137
4.2963
4.8732
2.3647
2.9851
3.3738
3.7635
36.85
35.93
31.39
22.93
28.31
23.14
34.31
33.38
28.85
20.40
25.78
20.60
-198.36
-204.28
-216.07
-245.75
-226.14
-244.93
93.42
94.26
93.24
93.63
93.17
93.59
Cd(TaiEt)I2‚DMF (10b)
[Cd(PaiMe)4](ClO4)2 (11a)
[Cd(TaiMe)4](ClO4)2 (11b)
[Cd(PaiEt)4](ClO4)2 (12a)
[Cd(TaiEt)4](ClO4)2 (12b)
a In toluene. b In acetonitrile. c At 298 K. d Reference 15.
Table 5. Selected List of Transition Wavelengths (Oscillator Strength) and the Major Contributions of Cd(PaiMe)2Cl2, [Cd(PaiEt)4](ClO4)2, and
Cd(TaiMe)I2‚DMF, Calculated in the Gas Phase by the TD-DFT Method
wavelength λ, nm
(oscillator strength f × 103)
major contribution
Cd(PaiMe)2Cl2 (3a)
611.2 (0.7)
567.4 (7.7)
(41.5%) H f L+1, (33.9%) H f L, (37.4%) H-1 f L+1, XLCT
(29.2%) H-1 f L, (14%) H-3 f L+1, (16.3%) H-1 f L, XLCT
(22.2%) H-5 f L+1, MLCT, (19.6%) H-4 f L+1, XLCT
(14%) H-5 f L+1, MLCT, (28.6%) H-4 f L+1, XLCT
(19.5%) H-9 f L, (16.6%) H-5 f L, XLCT
466.81 (8.1)
482.22 (4.4)
417.31 (19.0)
363.75 (0.4)
341.82 (572.6)
328.76 (3.9)
320.12 (43.7)
270.79 (7.6)
(46%) H-7 f L+1, ILCT [π(Ph) f π*(azo)]
(21%) H-6 f L+1, ILCT [π(Ph) f π*(azo)]
(28.8%) H-9 f L+1, (17.4%) H-8 f L [π(Ph) f π*(azo)]
(44.6%) H-11 f L [π(Ph) f π*(azo)]
(36%) H f L+2, ILCT [π(Ph) f π*(azo)]
Cd(TaiMe)I2‚DMF (9b)
877.38 (0.8)
839.36 (1.2)
742.55 (5.0)
729.13 (2.9)
550.01 (29.8)
504.86 (2.3)
467.56 (4.8)
363.50 (183.3)
323.88 (0.3)
289.51 (16.6)
273.97 (18.5)
269.55 (11.7)
(49.54%) H f L, XLCT
(49.56%) H-1 f L, XLCT
(46.8%) H-2 f L, XLCT
(47.2%) H-3 f L, XLCT
(48.2%) H-4 f L, XLCT
(45.5%) H-5 f L, XLCT
(37%) H-6 f L, XLCT + [π(DMF) f π*(azo)]
(30%) H-8 f L, (33%) H-10 f L, XLCT and [π(Ph) f π*(azo)]
(47%) H-9 f L [π(Ph) f π*(azo)]
(49.68%) H-1 f L+1, XLCT and [π(Ph) f π*(azo)]
(47.29%) H-3 f L+1 [π(Ph) f π*(azo)]
(35.8%) H-11 f L [π(Ph) f π*(azo)]
[Cd(PaiEt)4](ClO4)2 (12a)
464.12 (27.3)
407.64 (16.6)
398.71 (0.9)
368.90 (49.5)
353.43 (16.9)
347.31 (150.5)
330.01 (3.7)
312.22 (3.6)
(12%) H-2 f L+1, (11%) H-5 f L, π f π*
(19%) H f L, (15%) H-1 f L+4, (15%) H-1 f L+1, π f π*
(23%) H-1 f L+1, (28%) H-3 f L+1, π f π*
(18%) H-4 f L+1, (15%) H-3 f L+2, π f π* and n f π*
(32%) H-5 f L+1, (32%) H-7 f L, (40%) H-6 f L+3, π f π*, n f π*
(19%) H-8 f L+1, (27%) H-8 f L, π f π*, n f π*
(19%) H-6 f L, (32%) H-8 f L+3, π f π*, n f π*
(40%) H-6 f L+2, (25%) H-10 f L+2, π f π*, n f π*
orbitals of the Cl ligands (Figure 6) bonded to the Cd ion.
The occupied MOs other than H, such as H-1 to H-5, are
bonding orbitals comprised of mixtures of 3p(Cl), n(imida-
zole), and 4d(Cd). Further below (H-6, H-7, etc.) are π
orbitals of the (phenylazo)imidazole ligands. L (LUMO;
-2.75 eV) and L+1 (LUMO+1; -2.69 eV) are nearly
degenerate π* orbitals of the (phenylazo)imidazoles. In the
case of Cd(TaiMe)I2‚DMF, H (-5.17 eV) and H-1 (-5.25
Inorganic Chemistry, Vol. 46, No. 20, 2007 8297

Sarker et al.
To gain detailed insight into the charge transitions, TD-
DFT calculations were performed on the above complexes
in the gas phase (Table 5). The transitions at longer
wavelength are coming from X(Cl or I) f π*(azoimine)
charge-transfer transitions (abbreviated in Table 5 as XLCT)
along with a series of Cd-to-(phenylazo)imidazole charge-
transfer transitions (mainly low intensities). They are pre-
dicted in the range between 880 and 370 nm. Strong π-π*
transitions (H-8/H-7 to L+1 with charge-transfer character
are expected around 350-360 nm). The calculation of Cd-
(PaiMe)2Cl2 shows a series of Cl/Cd-to-(phenylazo)imidazole
charge-transfer transitions (mainly from H or H-6 to L and
L+1 with low intensities) predicted in the range between
610 and 360 nm. Strong π-π* transitions (H-7 to L and
H-6 to L+1) are expected at around 340 nm. These are
defined as intraligand charge-transfer transitions. Optical
transitions are predicted in the range <470 nm. Among them,
transitions that have a large oscillator strength (>0.1) are as
follows: 348 nm [H-1 (π) to L+3 (π*)]; 347 nm [H-8
(n/π) to L+1 (π*)]; 327 nm [H-11 (n) to L+2 (π*)]; 325
nm [H-10 (n) to L+3 (π*)].
Figure 4. Spectral changes of TaiMe in toluene upon repeated irradiation
at 365 nm at 3 min intervals at 25 °C. The inset shows spectra of the cis
and trans isomers of TaiMe.
An energy correlation diagram (Figure 7) of MOs shows
a decrease in the energies of the respective unoccupied
orbitals in the order of Cd(PaiMe)2Cl2 > Cd(PaiMe)I2‚DMF
<<< [Cd(PaiEt)4](ClO4)2. The occupied MOs show a small
increment in energy on going from Cd(PaiMe)2Cl2 to Cd-
(PaiMe)I2‚DMF, while a huge fall in energy is observed to
[Cd(PaiEt)4](ClO4)2. Besides, the energy difference (∆E )
ELUMO - EHOMO) between HOMO and LUMO follows the
ordering Cd(PaiMe)I2‚DMF (2.03 eV) < Cd(PaiMe)2Cl2
(2.64 eV) < [Cd(PaiEt)4](ClO4)2 (3.57 eV). The plots of ∆E
versus rates of isomerization and quantum yields are linearly
related (Figure 8). This implies the direct correlation between
the photophysical process and the activation energy barrier.
In the photochromic process, UV-light irradiation is
mandatory for the molecule in solution for a fixed time,
which will enforce isomerization from the more stable trans
isomer to the cis isomer. Irradiation in the UV region (360-
395 nm) may be responsible for the π f π* transition. The
MLCT or XLCT (X ) Cl or I) is a lower energetic transition,
which is capable of charge transfer to azoimidazole but its
energy is insufficient to perform a physical process like
isomerization. Conversely, the metalated ligand may perform
a charge transition in a secondary (MLCT or XLCT) process,
which is responsible for deactivation of excited-state species
and reduces the rate of the trans f cis change and quantum
yields. This is indeed observed (Table 3).
Figure 5. Eyring plots of thermal cis-to-trans isomerization of (a) TaiMe
and (b) [Cd(TaiMe)I2].
eV) are also closely spaced and consist mainly of the 5p
orbitals (Figure 6) of the I ligands bonded to the Cd ion.
The lower-energy orbitals (H-2 to H-6) are bonding orbitals
comprised of mixtures of 5p(I), n(imidazole), and 4d(Cd).
Further below, the orbitals H-7, H-8, etc., are localized
on the p-tolyl moiety of TaiMe. The LUMO (L) is a π*
orbital delocalized over the entire (phenylazo)imidazole
moiety, while L+1 and L+2 are also π* orbitals but rather
are localized on the phenyl and imidazole groups, respec-
tively. In [Cd(PaiEt)4](ClO4)2, four PaiEt ligands coordinate
to a Cd ion in a tetrahedral geometry. Each two of the four
ligands are crystallographically equivalent. The cationic
complex excluding the counteranions was subjected to the
calculation. Metal-centered orbitals contribute little to the
frontier orbitals (Figure 6), which are therefore dominated
by ligand orbitals. Then, the orbitals on the ligands are
perturbed by mutual interchromophoric interactions within
the complex. As a consequence, a significant mixing of n
and π orbitals is recognized. The first three occupied orbitals,
H (-10.97 eV), H-1 (-10.99 eV), and H-2 (-11.05 eV),
are energetically close to each other, and all of these MOs
are mainly constituted of ligand orbitals. The L (-7.40 eV)
and L+1 (-7.40 eV) orbitals are degenerate as are the L+2
(-7.18 eV) and L+3 (-7.16 eV) levels. These MOs are
constituted of ligand π* levels.
3. Experimental Section
3.1. Materials. CdCl2, CdBr2, and CdI2 were obtained from Loba
Chemicals, Bombay, India. 1-Alkyl-2-(arylazo)imidazoles were
synthesized by a reported procedure.15 All other chemicals and
solvents were reagent-grade as received.
3.2. Physical Measurements. Microanalytical data (C, H, and
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
spectrophotometer; IR spectra (KBr disk, 4000-200 cm-1) from a
8298 Inorganic Chemistry, Vol. 46, No. 20, 2007

Cadmium(II) Complexes of (Arylazo)imidazoles
Figure 6. Some MO pictures (HOMO, HOMO-1, LUMO, and LUMO+1) of Cd(PaiMe)2Cl2, Cd(PaiMe)I2‚DMF, and [Cd(PaiEt)4](ClO4)2.
Perkin-Elmer RX-1 FTIR spectrophotometer; photoexcitation car-
ried out using a Perkin-Elmer LS-55 spectrofluorimeter; 1H NMR
spectra from a Bruker (AC) 300 MHz FTNMR spectrometer.
3.3. Synthesis of [Cd(TaiMe)2I2] (7b). TaiMe (140 mg, 0.7
mmol) in MeOH (20 mL) was added dropwise to an acetonitrile
solution (5 mL) of CdI2 (125 mg., 0.34 mmol), which was refluxed
for 2 h. An orange-yellow precipitate appeared. The precipitate was
collected by filtration, washed with cold MeOH, and dried over
CaCl2 in vacuo. The yield was 168 mg (64%). Other complexes
were prepared under identical conditions, and the yield varied in
the range 60-70%.
Microanalytical data. Calcd for C20H20N8Br2Cd (5a): C, 37.24;
H, 3.10; N, 17.38. Found: C, 37.15; H, 3.00; N, 17.46. Calcd for
C22H24N8Br2Cd (5b): C, 39.26; H, 3.57; N, 16.66. Found: C, 39.12;
H, 3.60; N, 16.49. Calcd for C22H24N8Br2Cd (6a): C, 39.26; H,
3.57; N, 16.66. Found: C, 39.21; H, 3.52; N, 16.59. Calcd for
C24H28N8Br2Cd (6b): C, 41.12; H, 4.00; N, 15.99. Found: C, 41.20;
H, 4.10; N, 16.10. Calcd for C20H20N8I2Cd (7a): C, 32.50; H, 2.71;
N, 15.17. Found: C, 32.40; H, 2.65; N, 15.29. Calcd for C22H24N8I2-
Cd (7b): C, 34.45; H, 3.13; N, 14.61. Found: C, 34.52; H, 3.10;
N, 14.70. Calcd for C22H24N8I2Cd (8a): C, 34.45; H, 3.13; N, 14.61.
Found: C, 34.56; H, 3.18; N, 14.58. Calcd for C24H28N8I2Cd (8b):
C, 36.25; H, 3.52; N, 14.10. Found: C, 36.32; H, 3.60; N, 14.20.
Calcd for C13H17N5 OI2Cd (9a): C, 24.94; H, 2.72; N, 11.19.
Found: C, 25.00; H, 2.80; N, 11.10. Calcd for C14H19N5OI2Cd
(9b): C, 26.27; H, 2.97; N, 10.95. Found: C, 26.35; H, 3.05; N,
11.04. Calcd for C14H19N5OI2Cd (10a): C, 26.27; H, 2.97; N, 10.95.
Found: C, 26.39; H, 3.00; N, 11.00. Calcd for C15H21N5OI2Cd
(10b): C, 27.55; H, 3.21; N, 10.71. Found: C, 27.60; H, 3.25; N,
10.80.
3.4. X-ray Diffraction Study. The crystallographic data are
shown in Table 6. A suitable single crystal of the complex (0.30 ×
0.25 × 0.20 mm) was mounted on a CrysAlis CCD, Oxford
Diffraction Ltd. diffractometer equipped with graphite-monochro-
mated Mo KR (λ ) 0.710 73 Å) radiation. The unit cell parameters
and crystal-orientation matrices were determined by least-squares
refinements of all reflections. The intensity data were corrected
for Lorentz and polarization effects, and an empirical absorption
correction was also employed using the SAINT program.31 Data
were collected by applying the condition I > 2σ(I). All of these
structures were solved by direct methods followed by successive
Fourier and difference Fourier syntheses. Full-matrix least-squares
refinements on F 2 were carried out using SHELXL-97 with
anisotropic displacement parameters for all non-H atoms. H atoms
were constrained to ride on the respective C or N atoms with
isotropic displacement parameters equal to 1.2 times the equivalent
isotropic displacement of their parent atom in all cases. Complex
(31) Bruker. SMART and SAINT; Bruker AXS Inc.: Madison, WI, 1998.
Inorganic Chemistry, Vol. 46, No. 20, 2007 8299

Sarker et al.
Figure 7. Energy correlation diagram for MOs of Cd(PaiMe)2Cl2, Cd(TaiMe)I2‚DMF, and [Cd(PaiEt)4](ClO4)2.
Table 6. Summarized Crystallographic Data for 9b
compound
empirical formula
fw
[Cd(TaiMe)I2(C3H6NO)] (9b)
C14H18CdI2N5O
638.53
T (K)
293(2)
cryst syst
space group
cryst size (mm3)
a (Å)
triclinic
P1h
0.30 × 0.25 × 0.20
8.244(2)
b (Å)
10.408(3)
c (Å)
13.512(4)
R (deg)
83.63(3)
86.59(3)
65.35(3)
1047.0(5)
â (deg)
γ (deg)
V (Å3)
Z
2
λ (Å)
0.710 73
4.000
2.96-25.00
µ(Mo KR) (mm-1
θ range (deg)
hkl range
)
-9 < h < 9; -12 < k < 12; -16 < l < 16
D
calc (mg m-3
)
2.025
208
9518
refined param
total reflns
unique reflns
3626
R1a [I > 2σ(I)]
0.0367
0.0897
1.038
wR2b
GOF
2
2
2
a R1 ) ∑||Fo| - |Fc||∑|Fo|. b wR2 ) [∑w(Fo - Fc )2/∑w(Fo )2]1/2, w
2
2
) 1/[σ2(Fo)2+ (0.0415P)2 + (1.5065P)] where P ) ((Fo + 2Fc )/3.
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-
rimeter was used as an excitation light, with a slit width of 10 nm.
Figure 8. Correlation between ∆E ()ELUMO - EHOMO) and (a) the rates
(32) Sheldrick, G. M. SHELXS 97, Program for the Solution of Crystal
Structure; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(33) Sheldrick, G. M. SHELXL 97, Program for the Solution of Crystal
Structure; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(34) Spek, A. L. PLATON, Molecular Geometry Program; University of
Utrecht: Utrecht, The Netherlands, 1999.
of photoisomerization and (b) the quantum yields.
neutral atom scattering factors were used throughout for all cases.
All calculations were carried out using SHELXS 97,32 SHELXL97,33
PLATON 99,34 and ORTEP-335 programs.
8300 Inorganic Chemistry, Vol. 46, No. 20, 2007

Cadmium(II) Complexes of (Arylazo)imidazoles
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 the initial trans-
to-cis isomerization rates (ν) in a well-stirred solution within the
above instrument using the equation
∆G* ) Ea - RT - T∆S* and ∆S* ) ln A - 1 - ln(kBT/h)/R
where kB and h are Boltzmann’s and Plank’s constants, respectively.
3.6. DFT Calculations. DFT calculations were carried out using
X-ray crystallographic parameters of Cd(PaiMe)2Cl2 (3a), Cd-
(PaiEt)4](ClO4)2 (12a), and Cd(TaiMe)I2‚DMF (9b). A Gaussian
03w package36 was run on a personal computer. The functional
B3LYP37 and basis set LanL2DZ38 were chosen for the calculations.
The electronic spectrum was calculated with the TD-DFT method.
For nonmetallic atoms, diffuse and polarization functions were used.
Natural bond order (NBO) calculations were performed with the
NBO code included in Gaussian 03.
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 irradiation
wavelength. The value of I0 was obtained by using azobenzene (φ
) 0.11 for ππ* excitation26) under the same irradiation conditions.
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 (T) in the range from 298
to 313 K. The activation energy (Ea) and the frequency factor (A)
were obtained from the Arrhenius plot
4. Conclusion
Cadmium(II) halide complexes of 1-alkyl-2-(arylazo)-
imidazoles are described. One of the complexes has been
characterized by a single-crystal X-ray structure study.
Photochromism of the complexes is examined by UV-light
irradiation in a MeCN solution, and the results are compared
with free-ligand data. 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 energy (Ea) of the cis-to-trans
isomerization has been calculated. The slow rate of isomer-
ization in complexes may be due to a higher rotor volume
than that of free ligands.
ln k ) ln A - Ea/RT
where k is the measured rate constant, R is the gas constant, and T
is the temperature. The values of the activation free energy (∆G*)
and the activation entropy (∆S*) were obtained through the
relationships
Acknowledgment. Financial support from the Council
of Scientific and Industrial Research (CSIR), New Delhi,
India, is gratefully acknowledged. We thank the National
Single-Crystal Facility Centre, IIT, Bombay, Mumbai, India,
for help regarding X-ray crystallographic data collection.
(35) Farrugia, L. J. ORTEP-3 for windows. J. Appl. Crystallogr. 1997,
30, 565.
(36) 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.
Supporting Information Available: X-ray crystallographic data
for the structures in CIF format. This material is available free of
charge via the Internet at http://pubs.acs.org. This file has also been
deposited with the Cambridge Crystallographic Data Centre as
CCDC 635225. 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 or www:htpp://
www.ccdc.cam.ac.uk).
IC7012073
(37) B3LYP: Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(38) LanL2DZ: (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270.
(b) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (c) Hay, P.
J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299.
Inorganic Chemistry, Vol. 46, No. 20, 2007 8301