Ir(I)-carbonyl complexes of coumarinazoimidazoles: Synthesis structure, electrochemistry, photophysical properties and DFT calculations

Article abstract of DOI:10.1016/j.poly.2011.03.021

The reactions of Vaska's complex [IrCl(CO)(PPh₃)₂] with 2-(coumaryl-6-azo)imidazole (CZ-H) and its derivatives (CZ-X) have synthesized [Ir(CZ)(CO)(PPh₃)₂] and [Ir(CZ-X)(CO)(PPh ₃)₂]. All th

Full text of DOI:10.1016/j.poly.2011.03.021

Polyhedron 30 (2011) 1516–1523
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Ir(I)–carbonyl complexes of coumarinazoimidazoles: Synthesis structure,
electrochemistry, photophysical properties and DFT calculations
Papia Datta a,1, Dibakar Sardar a,2, Partha Mitra b, Chittaranjan Sinha a,
⇑
a Department of Chemistry, Jadavpur University, Kolkata 700 032, India
b Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata 700 032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The reactions of Vaska’s complex [IrCl(CO)(PPh3)2] with 2-(coumaryl-6-azo)imidazole (CZ-H) and its
derivatives (CZ-X) have synthesized [Ir(CZ)(CO)(PPh3)2] and [Ir(CZ-X)(CO)(PPh3)2]. All the complexes
have been characterized by FT-IR, UV–Vis, 1H NMR and FAB-MS spectroscopy. The structural confirma-
tion has been done in one case, by a single crystal X-ray diffraction study, which shows a distorted square
pyramidal geometry around the central Ir atom. The complexes are emissive at room temperature. The
cyclic voltammetry of the complexes shows a metal centered irreversible oxidation and ligand centered
quasireversible reduction couples. To get an insight into the electronic structure, absorption spectra and
electrochemical properties, detailed calculations on all three complexes have been performed at the DFT
level.
Received 14 December 2010
Accepted 3 March 2011
Available online 17 March 2011
Keywords:
Coumarinazoimidazoles
Ir(I)–carbonyl complex
Fluorescence
Electrochemistry
DFT calculation
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
they are very much promising in the field of OLED [19] and optical
sensors [20–22]. Besides, iridium complexes are important due to
Coumarin (2H-1-benzopyran-2-one) and its derivatives are nat-
urally available compounds in a variety of plants, such as beans,
lavender, sweet clover grass and licorice, and also in food plants
such as strawberries, apricots, cherries and cinnamon. The impor-
tance of coumarin and its derivatives in biology and medicine are
remarkable [1–10] due to their blood-thinning, anti-fungicidal,
anti-tumor and anticoagulant activities. Photophysics and photo-
chemistry of coumarin derivatives are significant [11–14] and
considerable effort has now been given to the design of metal–cou-
marin complexes. These complexes display interesting excited-
state properties and are useful in designing artificial photosyn-
thetic systems, chemical sensors and molecular level devices. The
functionalization of coumarin via appending an arylazo (Ar–
N@N–) group is of current interest in the design of efficient mole-
cules of photo-biological interest. The azo (–N@N–) linkage is used
to mediate electronic communication between the photo-redox ac-
tive group [15–18]. Anchoring of the azoimidazole motif to the
coumarin backbone may synthesize 2-(coumaryl-6-azo)imidazole
(CZ-H) and substituted coumarinazoimidazole (CZ-X) derivatives
(Scheme 1). Luminescent iridium complexes containing coumarin
functionalized molecules have been reported recently [19,20] and
their application in different fields, viz. photocatalysis, [23,24]
photo-electrochemistry [25,26], biological labeling [27] and elec-
troluminescence (EL) [28,29]. Vaska’s complex, [IrCl(CO)(PPh3)2],
and other Ir(I)–carbonyl compounds [30–36] are known for their
catalytic oxidation processes [37–46] due to the presence of irid-
ium in a low oxidation state. The present study is focused on the
synthesis, structural characterization of Ir(I)–carbonyl complexes
of coumarinazoimidazoles (CZ-H and CZ-X) and interpretation of
their electronic structure, absorption characteristics and redox
properties has been done by DFT computation of an optimized
geometry.
2. Experimental
2.1. Materials and physical measurements
IrCl3ꢀ3H2O was purchased from Arora-Matthey, India and was
used as received. Vaska’s complex was prepared by the reported
procedure [47]. The chemicals used for the syntheses were of ana-
lytical grade and solvents were dried before use [48]. The solution
spectral studies were carried out using spectroscopic grade sol-
vents obtained from Lancester, UK.
Microanalyses (C, H, N) were performed using a Perkin–Elmer
2400 CHN elemental analyzer. Spectroscopic measurements were
carried out using the following instruments: UV–Vis spectra,
Lambda 25 Perkin–Elmer; FT-IR spectra (KBr disk, 4000–
450 cmꢁ1), RX-1 Perkin–Elmer; 1H NMR spectra in CDCl3, Bruker
⇑
Corresponding author. Fax: +91 033 2413 7121.
E-mail address: c_r_sinha@yahoo.com (C. Sinha).
Address: Calcutta Institute of Engineering and Management, 24/1A Chandi Ghosh
1
Road, Kolkata 700 040, India.
2
Address: Dinabandhu Andrews College, Garia, Kolkata 700 084, India.
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.03.021

P. Datta et al. / Polyhedron 30 (2011) 1516–1523
1517
R
4
5
N
N'
N
N
H
Et3N
3
1
2
N
+
PPh3
CO
[Ir(PPh3)2(CO)Cl]
N
Ir
2
N'
Dry THF
N2 Atmosphere
N
6
Ph3P
7
12
11
14
[Ir(CZ-X)(CO)(PPh3)2]
8
9
13
O
R = H, CZ-H (1a)
R = Me, CZ-4-Me-H (1b),
R = Ph, CZ-4-Ph-H (1c)
10
O
1 (CZ-X)
Scheme 1. The ligands and the complexes.
300 MHz FT-NMR spectrometers in the presence of TMS as an
internal standard. FAB-MS data were collected from a Jeol-AX
500 instrument. Electrochemical measurements were carried out
with a computer controlled EG & G PARC VersaStat (model 250)
instrument using a Pt-disk working electrode and a Pt-wire auxil-
iary electrode, [n-Bu4N][ClO4] supporting electrolyte under an in-
ert (dry N2) environment at a scan rate 50 mV sꢁ1. The solution
was IR compensated and the results were collected at 298 K. The
reported results were referenced to SCE in MeCN and were uncor-
rected for junction potential. Emission was examined by a LS 55
Perkin–Elmer spectrofluorimeter at room temperature (298 K) in
acetonitrile solution under degassed conditions.
The fluorescence quantum yield of the complexes was deter-
mined using anthracene as a reference [49]. The complexes and
the reference dye were excited at the same wavelength, maintain-
ing nearly equal absorbance (ꢂ0.1), and the emission spectra were
recorded. The area of the emission spectrum was integrated using
the software available in the instrument and the quantum yield
was calculated according to the following equation (Eq. (1)):
azole (CZ-4-Ph-H) (1c), were prepared by a similar procedure using
4-methyl imidazole and 4-phenyl imidazole, respectively.
2.2.1.1. Microanalytical and spectral data of 1. CZ-H (1a): Anal. Calc.
for C12H8N4O2: C, 60.00; H, 3.33; N, 23.33. Found: C, 60.03; H, 3.29;
N, 23.31%. FT-IR (KBr disc) m
max/cmꢁ1: 1737 (COO), 1530 (C@N),
1405 (N@N). 1H NMR (DMSO-d6, Me4Si), d (ppm): 6.62 (m, 1H),
6.59 (m, 1H), 7.34 (s, 1H), 7.50 (s, 1H), 8.07 (m, 1H), 8.22 (s, 1H),
8.25 (d, 1H, J = 9.2 Hz), 13.06 (bs, 1H); MS(FAB+), m/z:
241(M+H)+. UV–Vis data in DMF kmax/nm (10ꢁ3 /Mꢁ1 cmꢁ1): 373
e
(13.47), 273 (6.92); kexitation 373 nm; kemission 442 nm; quantum
yield (/): 0.0012.
CZ-4-Me-H(1b): Anal. Calc. for C13H10N4O2: C, 61.42; H, 3.94; N,
22.05. Found: C, 61.40; H, 3.97; N, 22.09%. FT-IR (KBr disc) mmax
/
cmꢁ1: 1723 (COO), 1561 (C@N), 1420 (N@N). 1H NMR (DMSO-d6,
Me4Si), d (ppm): 2.51 (d, 3H, J = 9.6 Hz), 6.55 (m, 1H), 6.58 (m,
1H), 7.54 (q, 1H), 7.98 (m, 1H), 8.17 (s, 1H), 8.22 (d, 1H,
J = 8.7 Hz), 12.88 (bs, 1H); MS(FAB+), m/z: 255 (M+H)+. UV–Vis data
in DMF kmax/nm (10ꢁ3 /Mꢁ1 cmꢁ1): 374 (18.57), 273 (17.62), 263
e
(20.19); kexitation 374 nm; kemission 428 nm; quantum yield (/):
0.0021.
/S=/R ¼ ½AS=ARꢃ ꢄ ½ðAbsÞR=ðAbsÞSꢃ ꢄ ½gS2
=
gR2
ꢃ
ð1Þ
Here /S and /R are the fluorescence quantum yields of the sam-
ple and reference, respectively. AS and AR are the areas under the
fluorescence spectra of the sample and the reference respectively,
(Abs)S and (Abs)R are the respective optical densities of the sample
and the reference solution at the wavelength of excitation, and gS
CZ-4-Ph-H (1c): Anal. Calc. for C18H12N4O2: C, 68.35; H, 3.8; N,
17.7. Found: C, 68.4; H, 3.8; N, 17.8%. FT-IR (KBr disc) m :
max/cmꢁ1
1721 (COO), 1561 (C@N), 1431 (N@N). 1H NMR (DMSO-d6, Me4Si),
d (ppm): 6.60 (d, 1H, J = 7.0 Hz), 6.62 (d, 1H, J = 6.9 Hz), 7–29–7.61
(5h, m), 7.91 (s, 1H), 7.98 (d, 1H, J = 7.5 Hz), 8.25 (s, 1H), 8.27 (d,
1H, J = 8.5 Hz), 13.27 (bs, 1H); MS(FAB+), m/z: 317 (M+H)+. UV–
and gR are the values of refractive index for the respective solvent
used for the sample and reference.
Vis data in DMF kmax/nm (10ꢁ3 /Mꢁ1 cmꢁ1): 407 (21.15), 370 (s),
e
266 (22.59), 259 (23.25); kexitation 370 nm; kemission 443 nm; quan-
tum yield (/): 0.0024.
2.2. Synthesis
2.2.1. 2-(Coumaryl-6-azo)imidazole (CZ-H) (1a)
2.2.2. Preparation of [Ir(CZ)(CO)(PPh3)2] (2a)
The coumaryldiazonium ion was prepared by adding a NaNO2
(0.429 g, 6.21 mmol) solution to a 1 N HCl (50 cm3) solution of
6-aminocoumarin (1 g, 6.21 mmol) at 0–5 °C under ice cold condi-
tions. The yellow solution of the coumarin-6-diazoium ion was
filtered and the filtrate was used for a coupling reaction with imid-
azole. To an aqueous solution of imidazole (0.422 g, 6.21 mmol)
and Na2CO3 (2.65 g, 0.025 mmol) at 0–5 °C, the coumarin-6-diazo-
ium ion was added dropwise with continuous stirring. The orange
compound was precipitated, filtered, washed with cold water and
extracted with 2 N HCl (50 cm3). The solution was then neutralized
with Na2CO3, and the pH was regulated to 7. The precipitate of CZ-
H (1a) was filtered, washed and dried, yield 1.2 g (80.5%).
2-(Coumaryl-6-azo)imidazole (CZ-H) (50 mg, 0.21 mmol) was
dissolved in dry THF (10 ml), followed by the addition of Et3N,
and the solution was then stirred for 15 min under ambient condi-
tions in a nitrogen atmosphere. To the resulting solution Vaska’s
complex, [IrCl(CO)(PPh3)2], (165 mg, 0.21 mmol) was added and
stirring was continued for another 12 h under a nitrogen atmo-
sphere. The remaining solvent was then removed under reduced
pressure in a rotary evaporator. The crude product was washed
with water followed by diethylether and the dried mass was puri-
fied by column chromatography on a column of neutral alumina
using a benzene–acetonitrile mixture (1:5 v/v) for elution of the
expected product. The crystallization was carried out by diffusion
of a dichloromethane solution of the complex layered with hexane
to give single crystals of [Ir(CZ)(CO)(PPh3)2], yield 115 mg (55%).
Other coumarinazoimidazoles, 2-(coumaryl-6-azo)-4-methyl-
imidazole (CZ-4-Me-H) (1b), 2-(coumaryl-6-azo)-4-phenyl-imid-

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P. Datta et al. / Polyhedron 30 (2011) 1516–1523
Microanalytical data: Anal. Calc. for C49H37N4O3P2Ir (2a): C,
59.82; H, 3.76; N, 5.70. Found: C, 59.79; H, 3.78; N, 5.68%. FT-IR
corrected for Lorentz and polarization effects. Data reduction was
carried out using the ‘Bruker SAINT’ program. The structure was
solved by the direct method using SHELXS97 (Sheldrick, 2008) and
successive difference Fourier syntheses. All non-hydrogen atoms
were refined anisotropically. Full matrix least squares refinements
on F02 were carried out using SHELXL97 (Sheldrick, 2008) with aniso-
tropic displacement parameters for all non-hydrogen atoms.
Hydrogen atoms were constrained to ride on their respective car-
bon or nitrogen atoms with anisotropic displacement parameters
equal to 1.2 times the equivalent isotropic displacement of their
parent atom in all cases. All calculations were carried out using
SHELXS97 (Sheldrick, 2008) [50], SHELXL97 (Sheldrick, 2008) [50], PLA-
TON [51], ORTEP-3 [52] programs.
(KBr disc) m
max/cmꢁ1: 1993 (CO), 1735 (COO), 1560 (C@N), 1374
(N@N). 1H NMR (CD3CN, Me4Si), d (ppm): 7.55 (1H, s), 7.40 (1H,
s), 8.25 (1H, s), 6.60 (1H, d, J = 8.4 Hz), 6.58 (1H, d, 8.9 Hz), 7.90
(1H, m), 8.18 (1H, b, J = 7.2 Hz). 7.38–5.50 (15H, m). FAB-MS, m/
z: 984 (M+H)+.
[Ir(CO)(PPh3)2(CZ-4-Me)] (2b) (50% yield) and [Ir(-
CO)(PPh3)2(CZ-4-Ph)] (2c) (48% yield) were isolated by following
identical procedures.
Microanalytical data: Anal. Calc. for C50H39N4O3P2Ir (2b): C,
60.18; H, 3.91; N, 5.61. Found: C, 60.20; H, 3.88; N, 5.63%. FT-IR
(KBr disc) m
max/cmꢁ1: 1995 (CO), 1735 (COO), 1559 (C@N), 1375
(N@N). 1H NMR (CD3CN, Me4Si), d (ppm): 2.45 (3H, d, J = 9.5),
7.60 (1H, s), 8.20 (1H, s), 6.54 (1H, m), 6.59 (1H, m), 7.97 (1H,
m), 8.24 (1H, d, J = 7.6), 7.35–7.55 (15H, m). FAB-MS, m/z = 998
(M+H)+.
2.4. Theory and computational methods
Density functional calculations (DFT) of the complexes 2a, 2b
and 2c were carried out within the GAUSSIAN 03 program package
[53] using Becke’s three-parameter B3LYP exchange-correlation
functional together with the 6-31G(d) basis set for C, H, and N
atoms and the ‘‘double’’ quality LanL2DZ basis set [54] for the Ir
element. In the framework of the density functional theory (DFT)
approach, the B3LYP hybrid functional [55], one of the most pre-
ferred methods since it has proved its ability at reproducing vari-
ous molecular properties including structural parameters, was
used. The combined use of the B3LYP, consisting of a hybrid ex-
change functional as defined by Becke’s three-parameter equation,
and the Lee–Yang–Par correlation and standard split valence basis
set 6-31G(d) is a common technique to provide an excellent com-
promise between accuracy and computational efficiency of proper-
ties for large and medium-size molecules [56–62]. The ground-
state geometries were obtained in the gas phase by full geometry
optimization and the optimum structures, located as stationary
points on the potential energy surfaces, were verified by the ab-
sence of imaginary frequencies. Using the respective optimized S0
geometries we performed time dependent density functional the-
ory (TD-DFT) calculations at the B3LYP level to predict the absorp-
tion characteristics [63].
Microanalytical data: Anal. Calc. for C55H41N4O3P2Ir (2c): C,
62.32; H, 3.87; N, 5.29. Found: C, 62.34; H, 3.90; N, 5.26%. FT-IR
(KBr disc) m
max/cmꢁ1: 2008 (CO), 1740 (COO), 1553 (C@N), 1379
(N@N). 1H NMR (CD3CN, Me4Si), d (ppm): 7.22–7.50 (20H, m),
7.98 (1H, s), 8.22 (1H, s), 6.58 (1H, d, 8.8 Hz), 6.61 (1H, d,
J = 7.1 Hz), 7.96 (1H, m), 8.28 (1H, d, J = 7.0). FAB-MS, m/z = 1060
(M+H)+.
2.3. X-ray crystallography
Crystals were grown by slow diffusion of a CH2Cl2 solution of
the complex into hexane.
A suitable single crystal of [Ir(-
CO)(PPh3)2(CZ-4-Ph)] (2c) was mounted on a CCD diffractometer
equipped with fine-focus sealed tube graphite monochromated
Mo Ka (k = 0.71073 Å) radiation. Unit cell parameters were deter-
mined from the least-squares method with the data in the appro-
priate index range. A summary of the crystallographic data and
structure refinement parameters are given in Table 1. Data were
Table 1
Selected crystallographic data for [Ir(CZ-4-Ph)(CO)(PPh3)2] (2c).
[Ir(CZ-4-Ph)(CO)(PPh3)2] (2c)
3. Result and discussion
Empirical formula
C55H41IrN4O3P2
Formula weight (g mꢁ1
)
1060.06
monoclinic
C2/c
37.033(3)
13.1989(10)
22.0736(17)
113.475(3)
9896.5(14)
8
3.1. Synthesis and formulation
Crystal system
Space group
a (Å)
b (Å)
c Å)
The ligands and complexes synthesized in this work are sum-
marized in Scheme 1. Coumarinazoimidazoles (CZ-X), viz. 2-
(coumaryl-6-azo)imidazole (CZ-H), 2-(coumaryl-6-azo)-4-methyl-
imidazole (CZ-4-Me-H), 2-(coumaryl-6-azo)-4-phenyl-imidazole
(CZ-4-Ph-H), were prepared following the reported procedure
[64–66], and characterization data are summarized in Section 2.
The imidazole group N(1)–H in the ligand dissociates in basic med-
ium to act as a monoanionic bidentate, N(imidazole) (N), N(azo)
(N0), chelating agent. The complexes [Ir(CO)(PPh3)2(CZ-X)] (2) were
synthesized by reacting Vaska’s complex, [IrCl(CO)(PPh3)2] with
CZ-X in presence of Et3N under an inert atmosphere at room tem-
perature. The complexes are non-electrolytes and from the mag-
netic susceptibility measurements it was established that the
complexes are diamagnetic, indicating a +1 oxidation state of irid-
ium in the complexes.
b (°)
V (Å3)
Z
T (K)
296(2)
1.423
0.71073
Dcalc (Mg/m3)
0
k (AÅ) [Mo Ka]
Index range
ꢁ36 6 h 6 33; 0 6 k 6 13;
0 6 l 6 21
2.40–41.34
2.809
2h (°)
Absorption coefficient (mmꢁ1
Total reflection collected
Unique reflections
)
37 518
5068
586
1.457 and ꢁ0.424
Refined parameters
Largest difference in peak and hole
(e Åꢁ3
)
(I))
Ra (I > 2
wRb
r
0.0298
0.0670
0.993
3.2. Molecular structure of [Ir(CZ-4-Ph)(CO)(PPh3)2] (2c)
GOFc
a
R =
wR = [
R
|F0 ꢁ Fc|/
RF0.
A perspective view of the molecular structure of 2c is depicted
in Fig. 1. The bond distances and angles are listed in Table 2. Five
coordinated compounds may adopt a trigonal bipyramidal or
square pyramidal geometry, both type of structures being known
R
w(F0 ꢁ Fc2)/
R
wF0
]
where w = 1/[r
2(F02) + (0.0337P)2], where
b
2
4 1/2
P = (F02 + 2Fc2)/3.
GOF (Goodness-of-fit) is defined as [w(F0 ꢁ Fc2)/(n0 ꢁ np)]1/2 where n0 and np
c
2
denote the number of data and variables, respectively.

P. Datta et al. / Polyhedron 30 (2011) 1516–1523
1519
Fig. 1. X-ray structure of [Ir(CZ-4-Ph)(CO)(PPh3)2] (2c).
Table 2
Selected bond lengths (Å) and angles (°) for [Ir(CZ-4-Ph)(CO)(PPh3)2] (2c) with estimated standard deviations in the parentheses.
Bond lengths (Å)
Experimental
Calculated
Bond angles (°)
Experimental
Calculated
Ir(1)–N(1)
Ir(1)–N(4)
Ir(1)–P(1)
Ir(1)–P(2)
Ir(1)–C(19)
N(1)–C(28)
N(3)–N(4)
2.078(5)
2.165(5)
2.3492(16)
2.3599(16)
1.856(7)
1.374(7)
1.317(6)
2.089
2.161
2.352
2.362
1.873
1.379
1.319
N(1)–Ir(1)–N(4)
N(1)–Ir(1)–P(1)
N(1)–Ir(1)–P(2)
N(4)–Ir(1)–P(1)
N(4)–Ir(1)–P(2)
C(19)–Ir(1)–P(1)
C(19)–Ir(1)–P(2)
N(1)–Ir(1)–C(19)
N(4)–Ir(1)–C(19)
P(1)–Ir(1)–P(2)
73.75(18)
91.98(13)
93.09(15)
94.32(13)
100.89(13)
89.52(18)
85.87(18)
177.74(16)
104.4(2)
73.74
91.92
93.13
94.39
100.84
89.78
85.72
177.38
104.14
164.74
164.77(6)
for iridium(I) complexes [67–70]. The complex 2c adopts a dis-
torted square pyramidal arrangement around the Ir atom with
CO, two PPh3 and imidazole-N donor centers spanning over the
four coordinating sites in the base of the distorted pyramid,
which is supported by the bond angle data \N(1)–Ir(1)–C(19),
177.74(16)° and \P(1)–Ir(1)–P(2), 164.77(6)°. The apical position
is coordinated by the azo-nitogen atom, N(4). The distortion from
the ideal square pyramid geometry may be due to the acute
\N(1)–Ir(1)–N(4), chelate angle, 73.75(18)°. The ligand is present
in the complex in its usual trans-geometry with respect to the
azo bond and the torsion angle between the azoimidazole moiety
with the coumarin, N(3)–N(4)–C(29)–C(34), is 39.31°. The twisting
may probably minimize the steric interactions between the com-
ponents of the Ir(PPh3)2 moiety and the coumarin ring. The bond
angle between the two PPh3 ligands (\P(1)–Ir(1)–P(2)) of
164.77(6)° deviates from the ideal trans angle value and may cause
distortion to the geometry. The Ir–N(azo) bond length (Ir(1)–N(4),
2.165(5) Å) is longer than the Ir–N(imidazole) (Ir(1)–N(1), 2.078 Å)
bond length which may suggest a stronger interaction between
Ir(I) and imidazole-N than azo-N. This reflects the downfield shift-
ing of the imidazole-H (5-H) in the 1HHH NMR spectra of the com-
plexes compared to their free ligand data (see Section 2). The Ir(1)–
C(19), Ir(I)–P(1) and Ir(1)–P(2) bond distances are 1.856(7),
2.3492(16) and 2.3599(16) Å, respectively and these are in good
agreement with reported data [71]. Similar trends in the metric
parameters are also observed in the theoretically optimized struc-
ture (Table 2).
3.3. Spectroscopic characterization
The infrared spectra of the complexes exhibit a strong sharp
stretching frequency for the CO group in the region 1990–
2008 cmꢁ1, which appears at a higher frequency than that of
m
(CO) for Vaska’s complex (1967 cmꢁ1). This implies that a strong
competition for
p-acceptance is given by the co-ligand (CZ-X) in
the coordination sphere of Ir(I) for the present complexes with
coordinated CO. The lactone-carbonyl stretching shifts to ꢂ10–
15 cmꢁ1 higher frequency (1735–1740 cmꢁ1) compared to the free
ligand data (see Section 2). Other significant peaks of moderate
intensity appear in the 1370–1380 and 1550–1560 cmꢁ1 regions,
and these correspond to m(N@N) and m(C@N) stretching vibrations.
The shifting of the azo stretching frequency to a lower frequency
region compared to the free ligand value supports the effective
back donation dp
(Ir(I)) ? p⁄(azo).
The 1H NMR spectra of the complexes were recorded in CD3CN.
The summarized spectral data are reported in Section 2. The mul-
tiplets observed in the range 7.20–7.55 ppm are due to the protons
of PPh3. 5-H imidazole experiences a maximum downfield shift
(ꢂ0.8 ppm) amongst the other protons and the signal for the
N(1)–H proton is not observed in the complexes, which supports

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P. Datta et al. / Polyhedron 30 (2011) 1516–1523
Table 3
UV–Visa, fluorescenceb and cyclic voltammetricc data.
Complexes Absorptiona
kex (nm)b kem (nm)b Quantum yield (/)b Cyclic voltammetryc
kmax/nm (10ꢁ4 /Mꢁ1 cmꢁ1
e )
Oxidation E1/2 (V)e Reduction E1/2 (V) ( Ep, mV)d
D
2a
2b
2c
642 (0.010), 440 (1.203), 379 (1.045), 265 (3.146) 379
655 (0.012, 441 (1.165), 385 (1.111), 263 (3.223) 385
700 (0.014), 494 (1.355), 362 (0.743), 266 (3.975) 362
479
487
515
0.0025 (0.0012)
0.0033 (0.0021)
0.0055 (0.0024)
0.64
0.60
0.47
ꢁ0.83 (150)
ꢁ0.84 (170)
ꢁ0.86 (180)
a
b
c
In CH3CN solvent.
In CH3CN, reference used anthracene (/R of 0.62) and data in parentheses represent the free ligand quantum yields.
Conditions: solvent CH3CN; supporting electrolyte TBAP (0.1 M); working electrode platinum; reference electrode SCE; scan rate 50 mV sꢁ1
.
d
e
E1/2 = 1/2(Epa + Epc);
D
Ep = |Epa ꢁ Epc|, mV.
Epa (anodic-peak-potential) and Epc (cathodic-peak-potential).
the dissociation of N(1)–H and coordination to the metal center.
The signals of the phenyl protons at the C-4 position (Scheme 1)
of 2c merge with the PPh3 protons. The coumaryl protons suffer
the least perturbation upon coordination to Ir(I).
gion, and the fluorescence quantum yields of the complexes lie
in the range 0.0025–0.0055 and are higher than the free ligand val-
ues (0.0012–0.0024) (Table 3). It may be due to the structural rigid-
ity provided by the chelate complexes to the coumarin azo ligands
which resists vibrational relaxation and the loss of energy followed
by PET. No emission was observed when the complexes were ex-
cited in the MLCT (>440 nm) region.
The electronic spectra of the complexes were recorded in aceto-
nitrile solution at room temperature and the resulting data are
summarized in Table 3. The absorption spectra of the ligands exhi-
bit three transitions in the region 260–460 nm with high molar
It might be expected that excitation into the
p
–
p
⁄ state would
extinction coefficients (
e
, 103–104 Mꢁ1 cmꢁ1). From the analogy
lead to a rapid internal conversion to the lowest energy excited
state, which is presumably the MLCT state, and that emission
should occur from the MLCT state. However, it appears that the
with the absorption spectrum of 1-alkyl-(2-arylazo)imidazole, it
is proposed that the absorption band at 370–374 nm corresponds
to
p–
p⁄ transitions, while the tail >400 nm is due to n–p⁄ transi-
emission in these complexes is coming from a high energy
p–p
⁄
tions. The spectra of the iridium(I) complexes are show four signif-
icant transitions (Fig. 2 and Table 3). The absorption bands are not
sensitive to solvent polarity. The low intense band observed at
state, which ruled out the possibility of competitive energy trans-
fer to the MLCT state. It is probable that the emission occurred
from the liberated free ligand which may be present as an impurity
or produced as a result of photo-dissociation. This possibility is
also ruled out by collecting 1H NMR spectra of the complexes after
irradiation for 5 min in CD3CN solution. The spectra of non-irradi-
ated and irradiated complexes are identical in all aspects. Besides,
the excitation spectra of the complexes are exactly matched with
the absorption spectra.
>640 nm can be ascribed to a MLCT transition, dp
(Ir(I)) ? p⁄(CZ-
X), since it is absent in the case of the free ligands. The bands
appearing in the higher energy region (<400 nm) are assigned as
intraligand charge transfer (ILCT) transitions on comparing with
the free ligand data, while the transition at 440–494 nm is assigned
to an admixture of MLCT and ILCT transitions.
The emission properties of the complexes were studied in ace-
tonitrile solution at room temperature. The ligands are photoactive
and exhibit fluorescence spectra near the 420–450 nm region
3.4. Electrochemistry
when excited at the
p
–
p
⁄ band, ꢂ370 nm. The presence of the pen-
The electrochemical properties of the complexes were exam-
ined by cyclic voltammetry. A summary of the redox potentials is
given in Table 3 and one representative voltammogram is given
in Fig. 3. The nature of the voltammogram does not suffer any
change with scan rate (50–200 mV sꢁ1), which supports the chem-
ical stability of the complexes under the electrode field. The com-
plexes show one irreversible anodic peak potential in the range
0.45–0.65 V, which is quite close to the oxidation of Ir(I) [75,76].
We have attempted to record the EPR spectra of the coulometrical-
ly oxidized species but this was in vain since the oxidation current
dant coumaryl group may cause an emission to free ligands [72–
74], since 2-(arylazo)imidazoles are not emissive under the same
conditions [65]. When the complexes are excited near 360–
380 nm, they exhibit fluorescence spectra in the 480–515 nm re-
Fig. 2. Electronic absorption spectra of [Ir(CZ-4-Ph)(CO)(PPh3)2] (2c) (–––),
[Ir(CZ-4-Me)(CO)(PPh3)2] (2b)
(CO)(PPh3)2] (2c) (–ꢀ–ꢀ–), [Ir(CZ-4-Me)(CO)(PPh3)2] (2b)
magnified absorption spectra at higher wavelength are shown in the inset.
(
)
and fluorescence spectra of [Ir(CZ-4-Ph)-
(
)
in benzene. The
Fig. 3. Cyclic voltammogram of [Ir(CZ-4-Ph)(CO)(PPh3)2] (2c) in acetonitrile using a
Pt-working electrode and SCE reference electrode.

P. Datta et al. / Polyhedron 30 (2011) 1516–1523
1521
increased continuously and the reaction underwent a rapid trans-
formation to some unidentified insoluble products. The complexes
exhibit one quasireversible redox response on the negative side of
SCE and appear at ꢂꢁ0.85 V. The reduction of the coumarinazoim-
idazoles appear at negative potentials and can be assign as an accu-
mulation of negative charge in the azo group.
are due to metal-to-ligand charge transfer transitions, while in case
of 2c a transition from PPh3 to the ligand is also included. The red
shifting and low intensity of these transitions in 2c are consistent
with the calculated data. Theoretically, the highest kmax transitions
of 2a, 2b and 2c are obtained at 673, 687 and 730 nm, respectively.
The observed spectral data 642 (2a), 655 (2b) and 700 nm (2c) cor-
roborate with the calculated information. The observed bands at
440 (2a), 441 (2b) and 494 nm (2c) are calculated as the admixture
of intraligand, metal-to-PPh3 and metal-to-ligand charge transfer
transitions. The spectral transitions of 2c are shifted to longer
wavelengths compared to 2a and 2b, which supports the experi-
mental observation. This is due to the decrease in energy difference
between the HOMO and LUMO/LUMO+n states which is mainly
due to electronic and steric effects of the substituent in the imidaz-
ole ring. Similarly the observed peak at ꢂ370 nm is a mixture of
multiple transitions.
3.5. DFT calculation
The geometric structures of 2a, 2b and 2c in the ground state
(S0) were fully optimized at the B3LYP level. The optimized struc-
tures are depicted in Supplementary Fig. S1, and representative
bond parameters of 2c are listed in Table 2 along with the experi-
mentally determined data. The composition and energy of some of
the MOs are given in Supplementary Table S1. The correlation be-
tween the highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) of the complexes is given in
Fig. 4. DFT calculations reveal that the HOMO of the complexes is
substantially delocalized over the metal (ꢂ60%), ligand (17–26%)
(vide Supplementary Fig. S2) and PPh3 (ꢂ17%). The energies of
HOMO of 2a and 2b are closely spaced (EHOMO, 2a: ꢁ5.05 eV; 2b,
ꢁ4.97 eV) whereas 2c is substantially higher (EHOMO, ꢁ4.05 eV) in
energy. This may be due to the introduction of a sterically crowded
phenyl group in 2c. As a matter of fact, the energy difference be-
tween the HOMO and LUMO in 2c significantly decreases com-
pared to that of 2a and 2b (Fig. 4), indicating a lower kinetic
stability of 2c compared to 2a and 2b [77]. The LUMO is delocalized
over the ligand (CZ-X) orbitals. Other unoccupied orbitals (L+2 to
L+5) are mainly delocalized over the PPh3 orbitals. The HOMOꢁn
(where n = 1, 2 etc.) MOs are mixed and are delocalized over the
metal, ligand and PPh3 orbitals. The energy difference between
the HOMO and HOMOꢁ1 of 2c is much higher (
D
EHOMOꢁ(HOMOꢁ1)
:
ꢂ1.4 eV) compared to 2a and 2b (
D
EHOMOꢁ(HOMOꢁ1): ꢂ0.45 eV).
The TD-DFT results of the three complexes are in good agree-
ment with the experimental results, and selected transitions, oscil-
lator strength and assignments are summarized in Supplementary
Table S2. In the time-dependent calculations, we are considering
singlet ? singlet spin allowed transitions. The low energy elec-
tronic transitions for the complexes (HOMO to LUMO/LUMO+1)
Fig. 5. Correlation diagram between the energy of the HOMO (EHOMO) and the
anodic peak potential.
Fig. 4. Energy correlation diagram (only HOMO and LUMO levels are shown).

1522
P. Datta et al. / Polyhedron 30 (2011) 1516–1523
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The electrochemical behavior of the complexes can be rational-
ized in terms of the DFT calculation. Oxidation involves removal of
an electron from the HOMO and since in the present case the
HOMO is largely delocalized (ꢂ60–65%) over the d
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This work describes the synthesis and structural characteriza-
tion of iridium(I)–carbonyl complexes of coumarinazoimidazoles,
[Ir(CZ-X)(CO)(PPh3)2] (2). The structural characterization shows a
distorted square pyramidal geometry. The complexes exhibit emis-
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Acknowledgement
Financial support from the Council of Scientific and Industrial
Research (CSIR), New Delhi is gratefully acknowledged.
Appendix A. Supplementary data
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