Synthesis, structure, photochromism and DFT calculations of copper(I)-triphenylphosphine halide complexes of thioalkylazoimidazoles

Article abstract of DOI:10.1016/j.poly.2010.11.036

[Cu(SRaaiNR′)(PPh₃)X] complexes are synthesized by the reaction of CuX (X = Cl, Br, I), triphenylphosphine and 1-alkyl-2-[(o-thioalkyl) phenylazo]imidazole (SRaaiNR′). The single crystal X-ray structure of [Cu(SEtaaiNH)(PPh₃)I] (SEta

Full text of DOI:10.1016/j.poly.2010.11.036

Polyhedron 30 (2011) 614–623
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, structure, photochromism and DFT calculations of
copper(I)-triphenylphosphine halide complexes of thioalkylazoimidazoles
Gunomoni Saha a,1, Papia Datta a, Kamal Krishna Sarkar b, Rajat Saha c, Golam Mostafa c,
Chittaranjan Sinha a,
⇑
a Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700 032, India
b Department of Chemistry, Mahadevananda Mahavidyalaya, Barrackpore, Monirampore, Kolkata 700 120, India
c Department of Physics, Jadavpur University, Kolkata 700 032, India
a r t i c l e i n f o
a b s t r a c t
[Cu(SRaaiNR0)(PPh3)X] complexes are synthesized by the reaction of CuX (X = Cl, Br, I), triphenylphos-
phine and 1-alkyl-2-[(o-thioalkyl)phenylazo]imidazole (SRaaiNR0). The single crystal X-ray structure of
[Cu(SEtaaiNH)(PPh3)I] (SEtaaiNH = 2-[(o-thioethyl)phenylazo]imidazole) shows a distorted tetrahedral
geometry of the copper center with bidentate, N(azo), N(imidazole) chelation of SEtaaiNH and coordina-
tion from PPh3 and iodine. These complexes show a trans-to-cis isomerization upon irradiation with UV
light. The reverse transformation, cis-to-trans isomerization, is very slow with visible light irradiation and
Article history:
Received 30 September 2010
Accepted 25 November 2010
Available online 7 December 2010
Keywords:
Copper(I)-thioalkylazoimidazole-halide
X-ray structure
Photochromism
Electrochemistry
is thermally accessible. The quantum yields (/t?c) of the trans-to-cis isomerization of [Cu(SRa-
aiNR0)(PPh3)X] are lower than the free ligand values. This is due to the increased mass and rotor volume
of the complexes compared to the free ligand data. The rate of isomerization follows the order: [Cu
(SRaaiNR0)(PPh3)Cl] < [Cu(SRaaiNR0)(PPh3)Br] < [Cu(SRaaiNR0)(PPh3)I]. The activation energy (Ea) of the
cis-to-trans isomerization is calculated by a controlled temperature reaction. DFT computation of repre-
sentative complexes has been used to determine the composition and energy of the molecular levels.
Ó 2010 Elsevier Ltd. All rights reserved.
DFT computation
1. Introduction
coordination site. This family of compounds has been extensively
used as ligands to synthesize transition and non-transition metal
Azobenzene displays reversible photoisomerization and the
derivatives are very useful as an optical response function (trigger
and switching) molecule, because the molecular shape and polarity
greatly change on optical illumination [1–7]. This has inspired
changing either or both aryl rings of azobenzene with heterocycles
and to explore their photophysical properties as well as the influ-
ence of metal coordination or protonation. (Arylazo)heterocycles
which bear pyridine, pyrimidine, imidazole or pyrazole rings have
been extensively utilized in the field of coordination chemistry
owing to their chelation by N(heterocycle) and N(azo) to a metal
ion [8–12]. Many of them also exhibit trans-to-cis (Z–E) photoiso-
merization, and the coordination ability of the heterocyclic moiety
can be changed by the photoisomerization. This activity has im-
mense application in the design of light driven on-off switching
[13,14]. Arylazoimidazoles constitute an interesting class of hetero-
cyclic azo compounds as a potential switching group in biological
applications and in coordination chemistry, since imidazole is a
ubiquitous and essential group in biology, especially as a metal
complexes [15–22]. The photochromism of 1-alkyl-2-(ary-
lazo)imidazole (RaaiR0) [10,11] and some complexes like Hg(II)-
[23] Cd(II)- [24] and Pd(II)-azoimidazoles [25] have been reported
by our group. To extend the chemistry of azoimidazoles we have
synthesized 1-alkyl-2-[(o-thioalkyl)phenylazo]imidazoles (SRa-
aiNR0) (2 and 3) [26]. The reaction of CuX with SRaaiNR0 in the pres-
ence of PPh3 has isolated [Cu(SRaaiNR0)(PPh3)X] (4–9). The structure
is confirmed by a single crystal X-ray diffraction study in one case
and also by other spectroscopic studies. The photochromic property
of the complexes is also examined. Quantum chemical calculations
using density functional theory (DFT) have been carried out to ex-
plain the structural, electronic, spectroscopic and thermodynamic
properties of the different compounds. Time-dependent density
functional theory (TD-DFT) is opening new perspectives in this field
[27,28], so DFT computations have been used to correlate the UV–
Vis spectral and redox properties of the complexes.
2. Results and discussion
2.1. Synthesis and formulation
⇑
Corresponding author.
E-mail address: c_r_sinha@yahoo.com (C. Sinha).
Present address: Department of Chemistry, Barrackpore Rastraguru Surendranath
The 1-alkyl-2-[(o-thioalkyl)phenylazo]imidazoles (SRaaiNR0) (2
and 3) are synthesized by the reported procedure [26] (Scheme
1
College, Barrackpore, Kolkata 700 120, India.
0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.11.036

G. Saha et al. / Polyhedron 30 (2011) 614–623
615
1). The reaction of CuX and the appropriate ligand, SRaaiNR0 in a
1:1 mole ratio in methanol, followed by the addition of one equiv-
alent of PPh3 has isolated [Cu(SRaaiNR0)(PPh3)X] (4–9). The reac-
tion was also carried out with [Cu(PPh3)X]4 and SRaaiNR0, and
the same complex was isolated. However, the process is not fast
and we followed the reaction in situ. The complexes are purified
by crystallization, on slow evaporation of a solution in acetoni-
trile–methanol (1:1, v/v). Microanalytical data have confirmed
the composition of the complexes. The complexes are sufficiently
soluble in common organic solvents, viz. methanol, ethanol, chloro-
form, dichloromethane and acetonitrile, but are insoluble in hydro-
carbons (hexane, benzene, and toluene). They are non-conducting
in methanol/acetonitrile solution.
The 1H NMR spectral data (Supplementary Material, Table S1)
have been used to determine the stereochemistry of the com-
plexes. The aryl and imidazole protons are downfield shifted on
coordination of the ligand to the Cu(I) center compared to the free
ligand data [26]. This implies a significant bonding interaction of
the metal ion with the ligand, which effectively influences the li-
gand’s electron density. The S–R and N(1)–R0 groups show the
usual spin–spin interaction pattern in the upper field portion of
the aliphatic region in the NMR spectra.
2.3. Crystal structure of [Cu(SEtaaiNH)(PPh3)I] (9a)
The molecular structure of [Cu(SEtaaiNH)(PPh3)I] (9a) is shown
in Fig. 2. The bond parameters are listed in Table 1. The chelating
ligand, SEtaaiNH, acts as a N,N0-donor (N refers to N(imidazole)
and N0 refers to N(azo)), and the two other donor centers are
PPh3 and I. The –S–Et group remains uncoordinated and away from
the metal center. The atomic arrangements Cu, N(4), N(3), C(3),
N(1), constitute a chelate plane with a deviation < 0.02 Å. Cu(I) is
at the center of a distorted tetrahedron. The pendant aryl ring
makes a dihedral angle of 2.3(5)° with the chelated azoimidazole
ring. The acute bite angle, Cu(N, N0), of 77.42(18)° is extended by
SEtaaiNH on coordination to Cu(I) and is comparable with reported
results in the series of chelated arylazoimidazole complexes of d10
metal complexes [29,9,30] The small chelate angle may be one of
the reasons for geometrical distortion. The Cu–N(azo) bond length
(2.163(5) Å), is longer than the Cu(I)–N(imidazole) bond length
(2.088(5) Å), which reflects the stronger interaction of Cu(I) with
2.2. Spectral studies
Infrared spectra of the complexes exhibit
1412–1431 and 1580–1597 cmꢀ1, respectively. These two specific
frequencies, (N@N) and
(C@N), are red shifted by 10–12 cmꢀ1
t(N@N) and t(C@N) at
t
t
in the complexes compared to the free ligand values. This supports
coordination of azo-N and imine-N to Cu(I).
The UV–Vis spectra of the complexes exhibit three or four high
intense transitions (
e
ꢁ 104 Mꢀ1 cmꢀ1) at 450–465, 400–420, 355–
365 and 245–260 nm, along with a weak transition at 600–620 nm
(e
ꢁ 103 Mꢀ1 cmꢀ1) (Fig. 1). The free ligand SRaaiNR0 shows intral-
igand charge transferences, n–p⁄ and
p–
p⁄, at 370–380 and
250–260 nm respectively [26]. Thus the transitions > 400 nm are
assigned to Cu(I) ? p⁄(azo). The transitions are characterized
based on an electronic structure calculation using an optimized
geometry by the DFT computation technique (vide infra).
N(imidazole) compared to
N (azo). The N@N distance is
4
5
5
4
N
N
PPh
N
N
R'
+
R'
Cu
N2
X
N
i, ii
N
S
N
N
R
S
11
10
S
11
R
R
8
8
10
9
9
3
1
2
SRaaiNR/ (2, 3)
[Cu((SRaaiNR/)(PPh3)X] (4 - 9)
i) imidazole in pH, 7; ii) NaH in THF and R/I
R = Me, R/ = H (2a), R = Me, R/ = Me (2b);
R = Me, R/ = Et (2c), R = Et, R/ = H (3a);
R = Et, R/ = Me (3b), R = Et, R/ = Et (3c)
Abbreviations of a, b, c remain the same.
X = Cl (4, 5,); Br (6, 7,); I (8, 9)
[Cu(SMeaaiNH)(PPh3)Cl] (4a), [Cu(SMeaaiNMe)(PPh3)Cl] (4b), [Cu(SMeaaiNEt)(PPh3)Cl] (4c),
[Cu(SEtaaiNH)(PPh3)Cl] (5a), [Cu(SEtaaiNMe)(PPh3)Cl] (5b), [Cu(SEtaaiNEt)(PPh3)Cl] (5c)
[Cu(SMeaaiNH)(PPh3)Br] (6a), [Cu(SMeaaiNMe)(PPh3)Br] (6b), [Cu(SMeaaiNEt)(PPh3)Br] (6c),
[Cu(SEtaaiNH)(PPh3)Br] (7a), [Cu(SEtaaiNMe)(PPh3)Br] (7b), [Cu(SEtaaiNEt)(PPh3)Br] (7c)
[Cu(SMeaaiNH)(PPh3)I] (8a), [Cu(SMeaaiNMe)(PPh3)I] (8b), [Cu(SMeaaiNEt)(PPh3)I] (8c),
[Cu(SEtaaiNH)(PPh3)I] (9a),
[Cu(SEtaaiNMe)(PPh3)I]
(9b),
[Cu(SEtaaiNEt)(PPh3)I] (9c)
Scheme 1.

616
G. Saha et al. / Polyhedron 30 (2011) 614–623
Fig. 1. UV–Vis spectrum of [Cu(SMeaaiNEt)(PPh3)Cl] (4c) in acetonitrile.
1.286(6) Å. This is elongated by a small amount compared to that
of the free ligand value (1.252(1) Å) [10].
2.4. Electrochemistry
C–Hꢂ ꢂ ꢂ
p
and
p
ꢂ ꢂ ꢂ
p
interactions are observed in the packing of
The redox activity of the compounds is examined by the cyclic
voltammetric technique. Fig. 5 shows the cyclic voltammograms
of the copper(I) complexes in MeCN at a Pt-disk milli electrode
in the potential range +1.5 to ꢀ1.8 V versus a SCE reference elec-
trode (Supplementary material, Table S3). An oxidative response
is observed at 0.4 V. The quasireversiblity is proved by the peak-
the crystal to generate 1D (Fig. 3) and 2D (Fig. 4) supramolecular
structures. The interactions are observed between the Cu(N, N0)
plane and the imidazole ring (Cg(2)) of two adjacent molecules
(Cu(N, N0)ꢂ ꢂ ꢂCg(2)(imidazole), 3.721 Å; symmetry, ꢀx, ꢀy, ꢀz and
x, y, z respectively). The
Cg(3) at 3.562(6) Å (Cg(3): C(4)–C(5)–C(6)–C(7)–C(8)–C(9); sym-
pꢂ ꢂ ꢂp interactions are observed in Cg(2)–
to-peak separation (DEp > 100 mV) and the response is assigned
metry, 1 + x, y, z). The phenyl rings of PPh3 interact with the part-
to Cu(II)/Cu(I). On scanning to the ꢀve direction up to ꢀ1.8 V, we
ners of a neighboring molecule and the C–Hꢂ ꢂ ꢂ
p distance is 2.93 Å;
observe an irreversible response Epc at ꢀ0.4 V and a quasireversible
angle 159°, symmetry, x, ꢀ1 + y, z and 2.99 Å; angle 154°, symme-
try, 1 + x, y, z. The pendant aryl-SR group also interacts with C(11)–
H(9), at a distance of 2.96 Å, angle 140°, symmetry, 2 ꢀ x, ꢀy, 1 ꢀ z
(Supplementary Material, Table S2). These interactions increase
response at ꢀ1.1 to ꢀ1.3 V (
DEP > 140 mV). These may be assigned
to the reduction of the azo group [(–N@N–)/(–N@N–)ꢀ] of the che-
lated ligands. The voltammograms also show a sharp anodic part at
ꢁꢀ0.2 V, possibly due to the Cu(I)/Cu(0) couple [26,30]. Reduced
Cu(0) is absorbed on the electrode surface, as evidenced from the
the intermolecular attraction to generate a p-network (Fig. 4).
Fig. 2. The ORTEP diagram of [Cu(SEtaaiNH)(PPh3)I] (9a) with 30% probability.

G. Saha et al. / Polyhedron 30 (2011) 614–623
617
Table 1
Bond distances and bond angles of [Cu(SEtaaiNH)(PPh3)I] (9a).
X-ray
Theoa
X-ray
Theoa
1.403
1.88766
1.88766
1.888
Bond distance (Å)
Cu(1)–N(1)
Cu(1)–N(4)
Cu(1)–I(1)
Cu(1)–P(1)
N(3)–N(4)
2.088(5)
2.163(5)
2.5596(9)
2.2108(15)
1.286(6)
2.12168
2.14459
2.63375
2.40883
1.31151
N(1)–C(3)
P(1)–C(12)
P(1)–C(18)
P(1)–C(24)
C(4)–N(4)
1.292(12)
1.814(8)
1.778(4)
1.831(8)
1.401(7)
1.41544
Angles (°)
I(1)–Cu(1)–P(1)
I(1)–Cu(1)–N(1)
I(1)–Cu(1)–N(4)
P(1)–Cu(1)–(1)
P(1)–Cu(1)–N(4)
119.65(5)
110.9872
117.5242
114.9913
108.7570
122.6451
N(1)–Cu–N(4)
77.42(18)
114.40(17)
111.10(18)
119.02(18)
78.13
108.5
103.4
112.7
111.02(15)
110.48(13)
112.66(14)
118.01(14)
C(12)–P(1)–Cu(1)
Cu(1)–P(1)–C(18)
Cu(1)–P(1)–C(24)
a
Theoretically calculated data are obtained from the optimized geometry of the complex using the DFT computation technique.
Fig. 3. 1D pattern due to C–Hꢂ ꢂ ꢂp interactions of [Cu(SEtaaiNH)(PPh3)I] (9a).
narrow width of the anodic response with a large peak current.
Carrying out the experiment with a back scan going to a negative
potential of ꢀ0.6 V does not produce such a large anodic current
at ꢀ0.2 V.
observed under thermal conditions (vide supra). Perfect recovery
of the trans form is not achieved either by photoirradiation or by a
thermal reaction process, which indicates either permanent photo-
chemical change or irreversible phototransformation to some
unidentified product(s). The later possibility may be discouraged
as the spectral pattern remains unchanged even after 10 h visible
light exposure. We do not rule out the possibility of coordination
of solvent (CH3CN) to copper(I) upon light irradiation. The com-
plexes [Cu(SMeaaiNH)(PPh3)Cl] (4a), [Cu(SEtaaiNH)(PPh3)Cl] (5a),
[Cu(SMeaaiNH)(PPh3)Br] (6a), [Cu(SEtaaiNH)(PPh3)Br] (7a),
[Cu(SMeaaiNH)(PPh3)I] (8a) and [Cu(SEtaaiNH)(PPh3)I] (9a) do not
exhibit photochromism under identical conditions. The metal ion
and its oxidation state, nature of the ligand, presence of other coor-
dinating ligand(s), etc. play an important role in the photoisomer-
ization of the coordinated ligand in the complexes [32–35]. It is
proposed that UV-light irradiation may cleave the Cu–N(azo) bond
to make a free rotatory azo-aryl group which isomerizes to the cis-
form. Because of the long Cu(I)–N(azo) distance (vide supra), the
molecule may exhibit photophysical activation via cleavage of this
bond followed by rotation to introduce photoisomerization. The
2.5. Photo response of [Cu((SRaaiNR0)(PPh3)X]
The effect of UV-light irradiation to an acetonitrile solution of
[Cu((SRaaiNR0)(PPh3)X] is shown in Fig. 6 (Scheme 2). The irradia-
tion wavelength and its quantitative effect are given as Supplemen-
tary material, Table S4. The irradiation wavelength is ascribed to the
p–
p⁄ transition of the azo moiety in the trans form, at 364 nm
(14 530 Mꢀ1 cmꢀ1) (4b), 361 nm (11 421 Mꢀ1 cmꢀ1) (4c), 357 nm
(8842 Mꢀ1ꢀc1mꢀ1) (5b), 365 nm (18 950 Mꢀ1 cmꢀ1) (5c), 358 nm
(12 220 M cmꢀ1) (6b), 359 nm (14 561 Mꢀ1 cmꢀ1) (6c), 361 nm
(11 930 Mꢀ1 cmꢀ1) (7b), 363 nm (13 479 Mꢀ1 cmꢀ1) (7c), 363 nm
(13 340 Mꢀ1 cmꢀ1) (8b), 364 nm (10 832 Mꢀ1 cmꢀ1) (8c), 360 nm
(12 341 Mꢀ1 cmꢀ1) (9b), 362 nm (13 830 Mꢀ1 cmꢀ1) (9c). The p⁄
p–
transition band decreased in intensity after photoirradiation, indi-
cating the photoisomerization of the trans form to the cis form
(trans-to-cis) [10,31]. The reverse process, a cis-to-trans change is
quantum yields were measured for the trans-to-cis (/t?c
)

618
G. Saha et al. / Polyhedron 30 (2011) 614–623
Fig. 4. C–Hꢂ ꢂ ꢂp and p–p stacked 2D diagram of [Cu(SEtaaiNH)(PPh3)I] (9a).
properties, predicting the reaction mechanism and redox proper-
ties of complex compounds, bio-molecules, etc. The DFT calcula-
tions of [Cu(SEtaaiNH)(PPh3)I] (9a) have been performed using
the GAUSSIAN 03 analyses package. Structural agreement has been
observed from a comparison of bond distances and angles between
the calculated and X-ray determined structures (Table 1). The orbi-
tal energies, along with contributions from the ligands and metal,
are given as Supplementary material, and Fig. 8 depicts selected
occupied and unoccupied frontier orbitals in the gas phase calcula-
tion. The highest occupied molecular orbital (HOMO) (energy
ꢀ4.38 eV) is dominated by I character (85%), while Cu contributes
only 12%. HOMO ꢀ 1 (-4.48 eV) also bears 81% character from the I
group, while HOMO ꢀ 2, HOMO ꢀ 3, etc. are dominated by func-
tions of Cu (>50%). The LUMO (lowest unoccupied MO, ꢀ2.70 eV)
has 90% characteristics of SEtaaiNH, in which the azo group shares
a half portion (45%). LUMO + 1, LUMO + 2 and LUMO + 3 are dom-
inated by (>75%) functions from PPh3. The calculated transitions
are grouped in Table 2. The intensity of these transitions has been
assessed from the oscillator strength (f). The band at 687 nm (f,
0.0746) is an admixture of MLCT and I ? p⁄(SEtaaiNH) transition,
Fig. 5. Cyclic voltammogram of [Cu(SEtaaiNEt)(PPh3)Cl] (5c) in MeCN.
photoisomerization of these compounds in MeCN on irradiation
with UV wavelength light (Supplementary Material, Table S4). In
general, an increase in the mass of a molecule reduces the rate of
isomerization, trans-to-cis [32,36,37]. In the complexes, the /t?c
values are significantly less than that of the free ligand data [38].
Thermal cis-to-trans isomerization of the ligands and complexes
were followed by UV–Vis spectroscopy in MeCN at various temper-
atures, 298–313 K. The Eyring plots (Fig. 7) in the range 298–313 K
gave a linear graph, from which the activation energy is obtained
(Supplementary Material, Table S5). In the complexes, the Ea values
are severely reduced, which means faster cis-to-trans thermal isom-
while 427 nm (f, 0.1162), an intense band, is defined as a
p
(SEta-
aiNH) ? p⁄(SEtaaiNH) transition.
A significant transition at
337 nm (f, 0.013) is assigned to a MLCT (d(Cu) ? p⁄(SEtaaiNH))
transition. An intense band at 331 nm (f, 0.1153) is also an admix-
ture of I ? p⁄(SEtaaiNH) and I ? p⁄(PPh3) transitions, while bands
at 329 and 326 nm are assigned to p
(PPh3) ? p⁄(SEtaaiNH) transi-
tions. Other transitions in the UV region are mainly due to ligand to
ligand charge transfer transitions. Thus, the excitation may lead to
charge population at the chelated SEtaaiNH dominated function,
that may be manifested by cleavage of the longer bond, Cu–
N(azo), followed by immediate rotation about the –N@N– bond.
This may lead to trans-to-cis isomerization. This is indeed
observed.
erization of the complexes. The entropies of activation (D
S⁄) are
more negative in the complexes than that of the free ligand. This
is also in support of an increase in the rotor volume of the
complexes.
The cyclic voltammetric behavior of the complexes is readily
accountable from the DFT calculations. Because of the higher metal
(Cu) function in the occupied MOs, the complexes show a metal
oxidation Cu(II)/Cu(I) couple. The unoccupied MOs are significantly
dominated by the azoimine function, thus reduction may refer to
2.6. Electronic structure of [Cu(SEtaaiNH)(PPh3)I] (9a)
Density functional theory (DFT) calculations have been useful to
describe the electronic configuration, physical, chemical, catalytic

G. Saha et al. / Polyhedron 30 (2011) 614–623
619
decreasing
b
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.5
0.0
trans
500
cis
300
400
600
isobestic point
338 nm
Wavelength (nm)
increasing
300
350
400
450
500
550
600
Wavelength (nm)
decreasing
a
0.8
0.8
0.6
trans
0.4
0.6
0.4
0.2
0.0
cis
0.2
0.0
300 400 500 600
Wavelength (nm)
isobestic point
338 nm
increasing
300
350
400
450
500
550
600
Wavelength (nm)
Fig. 6. Spectral changes of (a) [Cu(SMeaaiNEt)(PPh3)Cl] (4c) (irradiation at 361 nm) and (b) [Cu(SEtaaiNMe)(PPh3)Cl] (5b) (irradiation at 357 nm) at 6 min interval at 25 °C in
acetonitrile. Inset figure shows spectra of the cis and trans isomers of the complex.
-13.95
(a)
(b)
-14.00
-14.05
-14.10
-14.15
-14.20
-14.25
-14.30
N
N
N
N
N
N
R
R
(c)
N
N
SR
SR
trans
cis
N
N
N
N
N
N
Cu
Cu
N
R
R
N
Cu
N
R
N
N
N
SR
0.00320
0.00324
0.00328
1/T
0.00332
0.00336
SR
SR
Fig. 7. Eyring plots of Z-to-E thermal isomerization of (a) [Cu(SMeaaiNEt)(PPh3)Cl]
(4c), (b) [Cu(SMeaaiNEt)(PPh3)Br] (6c) and (c[Cu(SMeaaiNEt)(PPh3)Br] (8c) at
different temperatures.
E- or trans-
Z- or cis-
Scheme 2. Photochromism of free and coordinated SRaaiNR.

620
G. Saha et al. / Polyhedron 30 (2011) 614–623
Fig. 8. Contour plots of some MOs with their energy and composition for [Cu(SEtaaiNH)(PPh3)I] (9a).
electron accommodation at the azo dominated orbital of the li-
gand. So the assignment of azo reductions is justified.
3. Experimental
Materials: Imidazole, different aromatic amines, CuX (X = Cl, Br,
I) and triphenyl phosphine were purchased from E. Merck India. All
other chemicals and solvents were of reagent grade and used as re-
ceived. 1-Alkyl-2-(arylazo)imidazoles (RaaiR0) were prepared by a
reported procedure [39]. The syntheses of the ligands were carried
out following the coupling of the o-(thioalkyl)phenyldiazonium ion
(obtained by diazotization of o-(thioalkyl)aniline) with imidazole
at pH 7, followed by N(1)-alkylation using alkyl iodide in the pres-
ence of NaH in dry THF under dry and inert conditions [26].
Table 2
Selected list of transitions obtained from TD-DFT calculations in the gas phase of
[Cu(SEtaaiNH)(PPh3)I] (9a).
Excited Wavelength Oscillator Transitions
state
Assignment
(nm)
strength
3
6
18
687
427
337
0.0746
0.1162
0.0130
(61%) H ꢀ 2 ? L
(63%) H ꢀ 5 ? L
(39%) H ꢀ 11 ? L
(21%) H ꢀ 12 ? L
XLCT(Major) + MLCT
ILCT
YLCT(Major)+MLCT
MLCT(Major)+XLCT
21
22
331
329
0.1153
0.0104
(56%) H ꢀ 1 ? L + 4 XLCT + XYCT
3.1. Physical measurements
(66%) H ꢀ 10 ? L
(12%) H ꢀ 8 ? L
(41%) H ꢀ 11 ? L
(15%) H ? L + 6
YLCT
YLCT
23
326
0.0184
YLCT(Major)+MLCT
XLCT
Microanalytical data (C, H, N) were collected on a Perkin–Elmer
2400 CHNS/O elemental analyzer. Spectroscopic data were ob-
tained using the following instruments: UV–Vis spectra, Perkin–El-
mer; model Lambda 25; IR spectra (KBr disk, 4000–450 cmꢀ1),
Perkin–Elmer; model RX-1; 1H NMR spectra, Bruker (AC)
300 MHz FTNMR spectrometer. Electrochemical measurements
were performed using a computer-controlled PAR model 250 Vers-
aStat electrochemical instruments with Pt-disk electrodes. All
40
44
285
275
0.0136
0.0358
(54%) H ꢀ 2 ? L + 5 XYCT + MYCT
(64%) H ꢀ 3 ? L + 1 MYCT + MYCT
(23%) H ꢀ 2 ? L + 8
46
47
50
268
266
261
0.0157
0.0128
0.0379
(66%) H ꢀ 2 ? L + 9 XLCT + MLCT
(68%) H ꢀ 3 ? L + 2 MYCT
(67%) H ꢀ 3 ? L + 3 MYCT + YLCT + MLCT
(11%) H ꢀ 4 ? L + 3

G. Saha et al. / Polyhedron 30 (2011) 614–623
621
4.79; N, 9.28%. FT-IR (KBr disc, cmꢀ1):
m
(N@N), 1425;
m
(C@N),
Table 3
1591. UV–Vis spectral data in CH3CN (kmax (nm) (10ꢀ3
ꢀ
Summarized crystallographic data for [Cu(SEtaaiNH)(PPh3)I] (9a).
(dm3 molꢀ1 cmꢀ1): 623 (0.73), 456 (5.24), 405 (8.09), 361 (11.42),
259 (15.17). Anal. Calc. for [Cu(SEtaaiNH)(PPh3)Cl] (5a),
29H27CuN4PSCl: C, 58.68; H, 4.91; N, 9.44. Found: C, 58.79; H,
4.88; N, 9.38%. FT-IR (KBr disc, cmꢀ1):
(N@N), 1432; (C@N),
1587. UV–Vis spectral data in CH3CN (kmax (nm) (10ꢀ3
(dm3 molꢀ1 cmꢀ1): 612 (0.32), 455 (5.15), 417 (9.49), 363 (12.25),
251 (20.53). Anal. Calc. for [Cu(SEtaaiNMe)(PPh3)Cl] (5b),
30H29CuN4PSCl: C, 59.30; H, 4.81; N, 9.22. Found: C, 59.44; H,
4.84; N, 9.29%. FT-IR (KBr disc, cmꢀ1):
(N@N), 1424; (C@N),
1584. UV–Vis spectral data in CH3CN (kmax (nm) (10ꢀ3
(dm3 molꢀ1 cmꢀ1): 617 (0.61), 462 (3.39), 403 (5.89), 357 (8.84),
259 (12.88). Anal. Calc. for [Cu(SEtaaiNEt)(PPh3)Cl] (5c),
31H31CuN4PSCl: C, 59.81; H, 5.02; N, 8.99. Found: C, 59.89; H,
5.09; N, 8.78%. FT-IR (KBr disc, cmꢀ1):
(N@N), 1418; (C@N),
Empirical formula
Formula weight
T (K)
Crystal size (mm)
Crystal system
Space group
a (Å)
C29H26N4PSICu
685.01
298(2)
C
0.16 ꢃ 0.14 ꢃ 0.12
m
m
triclinic
ꢀ
ꢀ
P1
8.2050(2)
9.5070(3)
21.7100(7)
89.447(2)
84.103(2)
66.357(2)
1542.21(8)
1.54184
b (Å)
c (Å)
C
a
(°)
m
m
b (°)
ꢀ
c
(°)
V (Å3)
Radiation (k, Å)
Z
l
2
C
(Cu K
a
) (mmꢀ1
)
10.159
m
m
h Range for data collection (°)
Index ranges
4.10–73.38
ꢀ10 6 h 6 10, ꢀ11 6 k 6 11,
ꢀ13 6 l 6 26
1.473
335
682
6334
5289
0.0580
0.1456
1591. UV–Vis spectral data in CH3CN (kmax (nm) (10ꢀ3
e
(dm3 molꢀ1 cmꢀ1): 608 (0.83), 465 (11.23), 420 (14.43), 365
Dcalc (Mg mꢀ3
)
(18.95), 252 (27.32). Anal. Calc. for [Cu(SMeaaiNH)(PPh3)Br] (6a),
Refine parameters
F(0 0 0)
Total reflections
C
28H25CuN4PSBr: C, 53.89; H, 4.04; N, 9.02. Found: C, 53.78; H,
4.06; N, 9.08%. FT-IR (KBr disc, cmꢀ1):
m
(N@N), 1428; (C@N),
m
Unique reflections
1582. UV–Vis spectral data in CH3CN (kmax (nm) (10ꢀ3
ꢀ
a
(dm3 molꢀ1 cmꢀ1): 600 (0.54), 460 (7.46), 411 (10.31), 359
(13.51), 242 (21.24). Anal. Calc. for [Cu(SMeaaiNMe)(PPh3)Br]
(6b), C29H27CuN4PSBr: C, 54.59; H, 4.27; N, 8.78. Found: C, 54.51;
R1 [I > 2
r
(I)]
b
wR2
Goodness of fit
1.116
ꢀ0.713 and 0.777
Largest difference in peak and hole
H, 4.29; N, 8.87%. FT-IR (KBr disc, cmꢀ1):
m
(N@N), 1422;
1586. UV–Vis spectral data in CH3CN (kmax (nm) (10ꢀ3
(dm3 molꢀ1 cmꢀ1): 598 (0.29), 455 (4.98), 402 (9.32), 358 (12.22),
249 (19.32). Anal. Calc. for [Cu(SMeaaiNEt)(PPh3)Br] (6c),
30H29CuN4PSBr: C, 55.26; H, 4.48; N, 8.52. Found: C, 55.38; H,
4.43; N, 8.59%. FT-IR (KBr disc, cmꢀ1):
(N@N), 1429; (C@N),
1581. UV–Vis spectral data in CH3CN (kmax (nm) (10ꢀ3
(dm3 molꢀ1 cmꢀ1): 621 (0.59), 451 (7.84), 405 (9.43), 363 (13.48),
261 (21.78). Anal. Calc. for [Cu(SEtaaiNH)(PPh3)Br] (7a),
29H27CuN4PSBr: C, 54.59; H, 4.27; N, 8.78. Found: C, 54.53; H,
4.28; N, 8.83%. FT-IR (KBr disc, cmꢀ1):
(N@N), 1415; (C@N),
m(C@N),
(e Åꢀ3
)
ꢀ
a
R =
wR2
R
¼
||Fo| ꢀ |Fc||/
R|Fo|.
h
i
1=2
P
P
P
P
2
2
2 1=2
wðF2o ꢀ Fc2Þ = wðFo2Þ
,
wR2 ¼ ½ wðF2o ꢀ Fc2Þ2= wðF2o Þ ꢄ
,
b
w = 1/[
r
2(Fo)2 + (0.0484P)2 + (3.4781P)], where P ¼ ðFo2 þ 2F2c Þ=3.
C
m
m
ꢀ
measurements were carried out under a nitrogen environment at
298 K with reference to SCE in acetonitrile, using [nBu4N][ClO4]
as a supporting electrolyte. The reported potentials are uncor-
rected for junction potential. Room temperature (298 K) magnetic
susceptibility was measured using Sherwood Scientific Cambridge,
UK at 298 K.
C
m
m
1593. UV–Vis spectral data in CH3CN (kmax (nm) (10ꢀ3
ꢀ
(dm3 molꢀ1 cmꢀ1): 601 (1.23), 457 (4.34), 402 (8.43), 360 (12.65),
252 (25.67). Anal. Calc. for [Cu(SEtaaiNMe)(PPh3)Br] (7b),
3.2. Synthesis of complexes
C
30H29CuN4PSBr: C, 55.26; H, 4.48; N, 8.52. Found: C, 55.35; H,
4.42; N, 8.55%. FT-IR (KBr disc, cmꢀ1):
(N@N), 1417; (C@N),
1584. UV–Vis spectral data in CH3CN (kmax (nm) (10ꢀ3
(dm3 molꢀ1 cmꢀ1): 601 (0.68), 462 (5.32), 415 (9.54), 361 (11.93),
249 (26.41). Anal. Calc. for [Cu(SEtaaiNEt)(PPh3)Br] (7c),
31H31CuN4PSBr: C, 55.82; H, 4.68; N, 8.40. Found: C, 55.91; H,
4.65; N, 8.31%. FT-IR (KBr disc, cmꢀ1):
(N@N), 1421; (C@N),
1580. UV–Vis spectral data in CH3CN (kmax (nm) (10ꢀ3
(dm3 molꢀ1 cmꢀ1): 621 (0.64), 459 (5.63), 406 (8.48), 363 (13.48),
260 (22.32). Anal. Calc. for [Cu(SMeaaiNH)(PPh3)I] (8a),
C28H25CuN4PSI: C, 50.12; H, 3.76; N, 8.35. Found: C, 50.22; H,
3.75; N, 8.31%. FT-IR (KBr disc, cmꢀ1):
(N@N), 1427; (C@N),
1587. UV–Vis spectral data in CH3CN (kmax (nm) (10ꢀ3
(dm3 molꢀ1 cmꢀ1): 618 (0.71), 465 (5.31), 416 (9.48), 366 (13.12),
254 (26.54). Anal. Calc. for [Cu(SMeaaiNMe)(PPh3)I] (8b),
29H27CuN4PSI: C, 50.85; H, 3.97; N, 8.12. Found: C, 50.71; H,
3.99; N, 8.17%. FT-IR (KBr disc, cmꢀ1):
(N@N), 1428; (C@N),
1590. UV–Vis spectral data in CH3CN (kmax (nm) (10ꢀ3
(dm3 molꢀ1 cmꢀ1): 610 (0.74), 460 (4.48), 400 (7.90), 363 (13.83),
244 (19.48). Anal. Calc. for [Cu(SMeaaiNEt)(PPh3)I] (8c),
C30H29CuN4PSI: C, 51.54; H, 4.18; N, 8.01. Found: C, 51.65; H,
4.16; N, 8.08%. FT-IR (KBr disc, cmꢀ1):
(N@N), 1431; (C@N),
1589. UV–Vis spectral data in CH3CN (kmax (nm) (10ꢀ3
(dm3 molꢀ1 cmꢀ1): 609 (0.51), 463 (5.75), 405 (9.46), 364 (10.83),
243 (23.48). Anal. Calc. for [Cu(SEtaaiNH)(PPh3)I] (9a),
29H27CuN4PSI: C, 50.85; H, 3.97; N, 8.12. Found: C, 50.78; H,
3.94; N, 8.15%. FT-IR (KBr disc, cmꢀ1):
(N@N), 1422; (C@N),
m
m
3.2.1. [Cu(SEtaaiNH)(PPh3)I] (9a)
ꢀ
1-Methyl-2-[o-(thiomethyl)phenylazo]imidazole (SMeaaiNMe)
(200 mg, 0.86 mmol) in methanol (10 ml) was added dropwise to
a methanolic solution (10 ml) of CuI (112 mg, 0.88 mmol), followed
by the addition of PPh3 (236 mg, 0.9 mmol) at 298 K. The brown-
red solution was stirred for 10 min. The resulting solution was then
refluxed for 3 h. It was then filtered and left undisturbed for a
week. Dark brown crystalline compounds separated out. The crys-
tals were filtered, washed with water, cold methanol and dried in
vacuo. The yield was 368 mg (67%).
All the other complexes were prepared by the same procedure.
The yields varied from 60% to 75% and microanalytical and spectral
data of the complexes are given herewith.
Anal. Calc. for [Cu(SMeaaiNH)(PPh3)Cl] (4a), C28H25CuN4PSCl: C,
58.08; H, 4.35; N, 9.67. Found: C, 58.12; H, 4.44; N, 9.59%. FT-IR
C
m
m
ꢀ
m
m
ꢀ
C
m
m
(KBr disc, cmꢀ1):
m
(N–H), 3007;
UV–Vis spectral data in CH3CN (kmax (nm) (10ꢀ3
(dm3 molꢀ1 cmꢀ1): 606 (0.43), 459 (6.37), 411 (8.93), 358 (15.58),
254(24.91). Anal. Calc. for [Cu(SMeaaiNMe)(PPh3)Cl] (4b),
29H27CuN4PSCl: C, 58.68; H, 4.91; N, 9.44. Found: C, 58.79; H,
4.97; N, 9.34%. FT-IR (KBr disc, cmꢀ1):
(N@N), 1429; (C@N),
m
(N@N), 1430;
m
(C@N), 1597.
ꢀ
ꢀ
C
m
m
m
m
ꢀ
1583. UV–Vis spectral data in CH3CN (kmax (nm) (10ꢀ3
ꢀ
(dm3 molꢀ1 cmꢀ1): 602 (1.15), 468 (6.39), 417 (11.32), 364
(14.53), 262 (21.54). Anal. Calc. for [Cu(SMeaaiNEt)(PPh3)Cl] (4c),
C
C30H29CuN4PSCl: C, 59.30; H, 4.81; N, 9.22. Found: C, 59.46; H,
m
m

622
G. Saha et al. / Polyhedron 30 (2011) 614–623
1592. UV–Vis spectral data in CH3CN (kmax (nm) (10ꢀ3
ꢀ
3.5. Computational methods
(dm3 molꢀ1 cmꢀ1): 601 (1.32), 458 (10.32), 403 (14.46), 361
(19.43), 249 (31.43). Anal. Calc. for [Cu(SEtaaiNMe)(PPh3)I] (9b),
The density functional theory (DFT) calculations on the crystal
structure of [Cu(SEtaaiNH)(PPh3)I] (9a) was carried out using the
GAUSSIAN 03w program package [45], with the aid of the GAUSSVIEW
visualization program [46]. The hybrid functional by Becke,
B3LYP [45], was used, which includes a mixture of Hartree–Fock
exchange with DFT exchange–correlation. For C, H, N, and I, we
used MIDI! basis functions [48] as this basis set is applicable to
all these elements and includes polarization functions. For Cu, we
used the LanL2DZ basis set, including Los Alamos Effective Core
Potentials [49–51].
C
30H29CuN4PSI: C, 51.54; H, 4.18; N, 8.01. Found: C, 51.62; H,
4.20; N, 8.10%. FT-IR (KBr disc, cmꢀ1):
(N@N), 1418; (C@N),
1595. UV–Vis spectral data in CH3CN (kmax (nm) (10ꢀ3
(dm3 molꢀ1 cmꢀ1): 611 (0.23), 463 (5.42), 412 (9.32), 360 (12.34),
251 (22.54). Anal. Calc. for [Cu(SEtaaiNEt)(PPh3)I] (9c),
31H31CuN4PSI: C, 52.15; H, 4.38; N, 7.85. Found: C, 52.24; H,
4.34; N, 7.89%. FT-IR (KBr disc, cmꢀ1):
(N@N), 1412; (C@N),
m
m
ꢀ
C
m
m
1587. UV–Vis spectral data in CH3CN (kmax (nm) (10ꢀ3
ꢀ
(dm3 molꢀ1 cmꢀ1): 603 (0.59), 461 (5.40), 409 (9.41), 362 (13.83),
245 (27.84).
4. Conclusion
3.3. X-ray crystal structure analysis of [Cu(SEtaaiNH)(PPh3)I] (9a)
1-Alkyl-2-{(o-thioalkyl)phenylazo}imidazole (SRaaiNR0) acts as
a bidentate N(imidazole), N(azo) chelator to a Cu(I) center. The
reaction of CuX and PPh3 followed by the addition of SRaaiNR0 iso-
lated complexes of the composition [Cu(SRaaiNR0)(PPh3)X] (X = Cl,
Br, I). One of the complexes was structurally characterized by a sin-
gle crystal X-ray diffraction study. The complexes show trans-to-cis
isomerization of the coordinated azoimidazole upon light irradia-
tion in the UV range. Quantum yields (/t?c) of photoisomerization
were calculated, and the free ligand shows a higher / value than
the Cu(I) complexes. The cis-to-trans isomerization is a thermally
induced process. The activation energy (Ea) of the cis-to-trans isom-
erization was calculated by a controlled temperature experiment.
DFT calculations determine the electronic structure and explain
the spectra and cyclic voltammetric properties of the complexes.
Details of the crystal analyses, data collection and structure
refinement data are given in Table 3. Single crystal data collections
were performed with a Siemens SMART CCD diffractometer using
fine focus sealed graphite-monochromatized Cu Ka radiation. Data
were corrected for Lorentz polarization effects and for linear decay.
Semi-empirical absorption corrections based on -scans were ap-
plied. The structure was solved by direct methods using SHELXS-97
and successive difference Fourier syntheses [40]. All non-hydrogen
atoms were refined anisotropically. The hydrogen atoms were
fixed geometrically and refined using the riding model. The final
difference Fourier map of residual electron density was carried
out using SHELXL-97 [41]. The structures were drawn using ORTEP
32 [42] and PLATON-99 [43] programs.
-
Acknowledgment
3.4. Photometric measurements
Financial support from Department of Science and Technology,
New Delhi is gratefully acknowledged.
Absorption spectra were taken with a Perkin–Elmer Lambda 25
UV–Vis spectrophotometer in a 1 ꢃ 1 cm quartz optical cell main-
tained at 25 °C with a Peltier thermostat. The light source of a Per-
kin–Elmer LS 55 spectrofluorimeter was used as the excitation
light, with a slit width of 10 nm. An optical filter was used to cut
off overtones when necessary. The absorption spectra of the cis iso-
mers were obtained by extrapolation of the absorption spectra of a
cis-rich mixture, for which the composition is known from 1H NMR
integration. Quantum yields (/) were obtained by measuring initial
Appendix A. Supplementary data
CCDC 792771 contains the supplementary crystallographic data
for [Cu(SEtaaiNH)(PPh3)I]. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: de-
posit@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at doi:10.1016/
j.poly.2010.11.036.
trans-to-cis isomerization rates (
m) in a well-stirred solution within
the above instrument using the equation:
m
¼ ð/I0=VÞð1 ꢀ 10ꢀAbs
Þ
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