Cyclometalated Ir(III) complexes containing N-aryl picolinamide ancillary ligands

Article abstract of DOI:10.1016/j.jorganchem.2011.04.011

The reaction of the cyclometalated chloro-bridged iridium(III) dimer, [(ppy)₂ Ir(μ-Cl)]₂ (ppy - 2-phenyl pyridine) with N-aryl picolinamides (LH, LH-NO₂, LH-CH₃, LH-l, LH-F) resulted in the formation of neutral

Full text of DOI:10.1016/j.jorganchem.2011.04.011

Journal of Organometallic Chemistry 696 (2011) 2711e2719
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Cyclometalated Ir(III) complexes containing N-aryl picolinamide ancillary ligands
,
a
b
b
Vadapalli Chandrasekhar a , Bani Mahanti , Priyanka Bandipalli , Kotamarthi Bhanuprakash ,
*
Nisanth N. Nair a
a Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208 016, India
b Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad 500 607, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of the cyclometalated chloro-bridged iridium(III) dimer, [(ppy)2 Ir(m-Cl)]2 (ppy e 2-phenyl
Received 19 February 2011
Received in revised form
6 April 2011
pyridine) with N-aryl picolinamides (LH, LHeNO2, LHeCH3, LHel, LHeF) resulted in the formation of
neutral heteroleptic complexes [Ir(ppy)2L] (1), [Ir(ppy)2LeNO2](2), [Ir(ppy)2LeCH3](3), [Ir(ppy)2LeCl](4)
and [Ir(ppy)2LeF] (5). These complexes contain a six-coordinate iridium with a 2C, 4N coordination
environment. The N-aryl picolinamide ligands are deprotonated during complexation and the resulting
amidates bind to iridium in a chelating manner (N, N). Optical spectroscopic studies revealed that the
Accepted 11 April 2011
Keywords:
complexes 1e5 exhibited intense
p/
p* absorptions in the ultraviolet region. In addition low energy
Cyclometalated
N-Aryl picolinamide
Absorption
Emission
Lifetime
transitions due to 1MLCT, 1LLCT and 3MLCT are also seen. The emission spectra of 1e5, upon excitation at
450 nm, show a single emission with a lmax around 513 nm. The lifetimes of this emission are in between
7.4 and 9.6
functional theory (DFT) calculations on 1 and 3, the three highest occupied orbitals are composed of
ligand orbitals mixed with Ir-d orbitals while the three lowest unoccupied orbitals are mostly
ms while the quantum yields are quite high and range from 0.2 to 0.5. Based on density
Time dependent DFT
p
p
orbitals of the ligands. From the time dependent DFT calculations it is revealed that the lowest energy
electronic singlet and triplet excitations are a mixture of MLCT and LLCT.
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
make steady progress in the understanding of the photophysical
processes in cyclometalated Ir(III) complexes [8].
In recent years there has been considerable research activity that
has been directed towards organometallic cyclometalated Ir(III)
complexes [1]. This interest stems from both practical and academic
viewpoints. Due to a strong spin-orbit coupling cyclometalated
Ir(III) complexes exhibit phosphorescence with moderate to high
quantum yields and because of this such complexes have been
viewed as potential phosphorescent dopants in organic light-
emitting diode (OLED) devices [2]. Other applications of these
complexes are varied and include light-emitting electrochemical
cells [3], electrogenerated chemiluminescence [4], photoinduced
hydrogen production [5], photoelectrochemical solar cells [6] as
well as phosphorescent probes/markers in biological systems [7].
From an academic point of view there has been considerable interest
in unraveling and understanding the precise mechanism of emis-
sion of these complexes including the details of the various excited
state energy levels that are involved in this process. Theoretical
studies, particularly those based on density functional theory (DFT)
have been particularly useful in conjunction with experiments to
In spite of significant progress, considerable amount of work still
remains to be done before one can settle down on the ideal families
of cyclometalated Ir(III) complexes that can be used in various
practical devices. One of the ways of modulating the properties of
the complexes vis-à-vis their molecular structures is by the choice
of an appropriate cyclometalating (C^N) ligand and an ancillary
ligand leading to both homoleptic [Ir(C^N)3] [9] or heteroleptic
[Ir(C^N)2L]nþ (n ¼ 0, 1) complexes [10]. While the cyclometalating
ligand of choice mostly has been a 2-phenyl pyridine (ppy) based
ligand, ancillary ligands have been varied from acetylacetonate to
tetrazole. However, in spite of this attention to the ancillary ligand,
surprisingly there have been very few attempts to utilize amide
ligands [11]. One of these reports the generation of the amide linkage
on the cyclometalated Ir(III) complex, in situ, by a photochemical
singlet oxygen-induced process [11a]. On the other hand amide
ligands have been extensively used in transition metal chemistry
[12] and are known to stabilize higher oxidation states as well as
impart stability to the complexes. For example, recently, amide
ligand complexes of Pd(II) have been reported. These complexes
were shown to be especially stable in solution and serve as excellent
catalysts in CeC bond formation reactions [13]. In view of this we
have explored the possibility of incorporating amide ligands (LH,
* Corresponding author. Tel.: þ91 512 259 7259; fax: þ91 521 259 0007x7436.
E-mail address: vc@iitk.ac.in (V. Chandrasekhar).
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.04.011

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V. Chandrasekhar et al. / Journal of Organometallic Chemistry 696 (2011) 2711e2719
LHeNO2, LHeCH3, LHeCl, LHeF) to form neutral heteroleptic
complexes [Ir(ppy)2L] (1), [Ir(ppy)2LeNO2](2), [Ir(ppy)2LeCH3](3),
[Ir(ppy)2LeCl](4) and [Ir(ppy)2Le] (5) where L and substituted-L are
chelating (N, N) amidate ligands while the cyclometalating ligand is
the phenyl pyridine ligand (Scheme 1). Synthesis, structural studies,
photophysical studies, theory and electrochemistry of these
complexes are reported herein.
2. Results and discussion
2.1. Synthesis
The heteroleptic Ir(III) complexes 1e5 containing two cyclo-
metalating 2-phenyl pyridine ligands and one deprotonated N-aryl
picolinamide ligand were prepared by a sequential two step
synthetic procedure (Scheme 1). The cyclometalated chloro-bridged
iridium(III) dimer, [(ppy)2 Ir(m-Cl)]2 was prepared by using the
Nonoyama protocol; subsequent reaction of this iridium(III) dimer
with sodium methoxide followed by reaction with N-aryl picolina-
mides afforded the neutral, monomeric, heteroleptic complexes 1e5
in good yields (see Experimental Section). The N-aryl picolinamide
ligands undergo deprotonation and the resultant monomeric
ligands bind to the Ir(III) center by a N, N chelating mode (see below
for molecular structures). Infrared spectroscopic studies support the
deprotonation at the ligands as revealed by the disappearance of the
yNeH in the complexes (in the ligands the yNeH is seen between
3300 cmꢀ1 and 3340 cmꢀ1). The amide C]O stretch, observed
between 1670 and 1687 cmꢀ1 in the uncoordinated ligands, is also
found to be shifted to around 1600 cmꢀ1 in all the complexes indi-
cating the influence of coordination on the ligand. ESI-MS spectra of
all the five complexes reveals the presence of the molecular ionpeak
[M þ H]þ; in addition in all the cases peaks due to [2M þ H]þ are also
seen.
Thermogravimetric analyses were performed on all the
complexes to ascertain their thermal stability. It is known that
simple Ir(III) complexes such as Ir(acac)3 suffer thermal degradation
at fairly low temperatures (w243 ꢁC) [8a]. In the present instance all
the compounds remain intact up to approximately 300 ꢁC. Varied
amounts of degradation are found after this stage, ranging from 5%
at 323 ꢁC for complex 5 to 20% at 335 ꢁC for 1 (see Supporting
Information). At 700 ꢁC the char yields of compounds 1e5 are
respectively 21.0%, 61.4%, 50.6%, 42.0 and 60.0% indicating high
thermal stability of these complexes. A representative TGA curve is
shown in Fig. 1 (see Supporting Information for the TGA curves of
other complexes).
Fig. 1. Thermogravimetric curve of 3.
2.2. Molecular structures of 1, 3, 4 and 5
The molecular structures of these heteroleptic complexes as
ascertained by ESI-MS and analytical data (see Experimental Section)
were confirmed by an X-ray crystallographic study of 1, 3, 4 and 5.
While the molecular structure of 2 could be ascertained from its
crystallographic study the poor quality of the X-ray data precludes
any discussion on its bond parameters.
Single crystals of 1, 3, 4 and 5 were obtained from their dichloro-
methane solutions by layering them with a few drops of methanol.
Compounds 1, 3 and 4 crystallize in monoclinic space groups while
compound 5 crystallizes in a triclinic space group. The asymmetric
unit of compound 5 contains two independent molecules. The crys-
tallographic data for these compounds are summarized in Table 1.
ORTEP diagrams of 1, 3, 4 and 5 are given in Fig. 2. Selected bond
parameter data for compounds 1 and 3 are summarized in Tables 2
and 3 while for compounds 4 and 5 is given in supporting infor-
mation (Tables S1 and S2). All the four compounds are mono-
nuclear, neutral, cyclometalated Ir(III) complexes. The metal center
in each complex is bound to three chelating ligands, two phenyl
pyridine ligands (C^N) and one picolinamide ligand (N^N). Thus,
the iridium center in each complex has a 2C, 4N coordination
environment. The coordination geometry around the iridium
center is distorted octahedral with the nitrogen atoms of the phenyl
pyridine ligands occupying trans positions while the two carbon
atoms of these ligands preferring the cis orientation. This ligand
geometry is similar to that found in the precursor [(ppy)2Ir(
m-Cl)]2
suggesting that the formation of the amide complexes involves
a simple replacement of the bridging chlorine atoms without
causing disturbance to the coordination of the cyclometalating
2-phenyl pyridine unit. A similar behavior is observed in other
cyclometalated Ir(III) complexes containing ancillary ligands.
In general picolinamides are known to bind to transition metal
ions in two forms (a) an amide form, where the ligand is neutral and
is a N, O donor and (b) an amidate form where the ligand is
deprotonated and functions as a N, N donor (Chart 1). In the current
instance in all the complexes the coordination mode of the picolina-
mide corresponds to the amidate form. As described above, this mode
of coordination has also been confirmed by infrared spectroscopy.
Interestingly prior tothiswork, in theliterature, onlythreeprecedents
of amide coordination to cyclometalated Ir(III) complexes are known.
In all cases amidate form of coordination has been observed [11].
An inspection of the bond parameters reveals that the IreN
distances involving the chelating amide ligand (N^N) are slightly
Scheme 1. Synthesis of iridium complexes.

V. Chandrasekhar et al. / Journal of Organometallic Chemistry 696 (2011) 2711e2719
2713
Table 1
X-ray crystallographic data for compounds 1, 3, 4 and 5.
Parameters
1
3
4
5
Empirical formula
Formula weight
Temperature (K)
Wavelength (Mo ka)
Crystal system
Space group
a/Å
C35H29IrN4O2
729.82
153(2)
0.71073 Å
Monoclinic
P21/c
8.6865(10
11.5764(13)
27.756(3)
90
C35H27IrN4O
711.81
100(2)
0.71073 Å
Monoclinic
P21/n
16.265(4)
10.204(3)
17.973(5)
90
C34H24ClIrN4O
732.22
100(2)
0.71073 Å
Monoclinic
P21/n
16.247(4)
10.150(3)
18.080(4)
90
C72H64F2Ir2N8O6
1559.71
100(2)
0.71073 Å
Triclinic
P1
13.536(4)
13.611(4)
17.926(5)
91.721(4)
108.355(4)
91.392(4)
3131.1(14)
2
b/Å
c/Å
a
b
g
(deg)
(deg)
(deg)
92.136(2)
90
2789.2(5)
4
113.273(4)
90
2740.2(12)
4
113.696(4)
90
2730.3(12)
4
Volume/Å3
Z
Calculated density (g/cc)
Absorption coefficient (mmꢀ1
F (000)
1.738
4.828
1440
1.725
4.909
1400
1.781
5.024
1432
1.654
4.313
1544
)
q
range for data collection (deg)
2.29e26.00
15236
2.17e26.00
14894
2.18e26.00
14748
2.14e25.00
16207
Reflections collected
Independent reflections
Parameters
5454 [R(int) ¼ 0.0374]
5347 [R(int) ¼ 0.0495]
5320 [R(int) ¼ 0.0555]
10860 [R(int) ¼ 0.0323]
381
371
370
817
Goodness-of-fit on F2
Final R indices
1.046
0.0321
1.050
0.0366
1.016
0.0383
1.048
0.0497
R indices (all data)
Largest diff. peak and hole
0.0414
0.0482
0.0472
0.0712
2.262 and ꢀ0.655 (e. Åꢀ3
)
3.213 and ꢀ1.123 (e. Åꢀ3
)
3.131 and ꢀ1.526 (e. Åꢀ3
)
2.660 and ꢀ1.487 (e. Åꢀ3
)
longer [2.112(7)-2.179(7) Å] than those involving the chelating
2-phenyl pyridine (C^N) ligand [2.031(6)-2.050(7) Å]; the strong
trans influence of the metalated carbon atom of the 2-phenyl
pyridine group presumably is responsible for the longer IreN
distances involving the amide ligand. In each of the complexes the
CeO distance is significantly shorter than the CeN distance, except
in one of the molecules present in complex 5 where the difference
between these two bond types is not very large [CeO, 1.280(10);
CeN, 1.320(11)]. Among the bond angles involving the trans
substituents the NeIreN bond angles are the most linear while the
CeIreN bond angles deviate more from the ideal values. Overall the
bond parameters found in the present instance are comparable
with other cyclometalated Ir(III) complexes containing a similar
ligand environment [14].
nature of this emission. These lifetimes may be compared to those
observed in literature [15]. The quantum yields calculated with
reference to quinine sulfate, are quite high and range from 0.2 to 0.5
(Table 4). The shape of the emission spectra in terms of the lack of
vibronic fine structure indicates that the emission occurs from
a charge-transfer excited state [17]. This hypothesis is corroborated
by density functional theory based calculations (see below).
2.4. DFT and TDDFT studies
To understand the ground and excited state properties we have
performed DFT and TDDFT calculations for compounds 1 and 3. The
results for 1 are discussed below while that of 3 are given in the
Supporting Information.
The main optimized geometry parameters of 1 in the ground
state together with X-ray crystal diffraction data are given in
Table S3 of supporting information which shows that the optimized
bond parameters are well consistent with the corresponding
experimental values in case of PBE0 function.
2.3. Photophysical properties
The utility of cyclometalated Ir(III) complexes as phosphores-
cent dopants in OLED devices is mainly due to the strong spin-orbit
coupling exhibited by the heavy metal center which facilitates
access to the triplet excited states by an efficient mixing of singlet
and triplet excited states. As seen previously in complexes of the
type [(ppy)2Ir(acac)] [15], in the present instance also, intense
To gain insight into the electronic structure of complex 1 we
report in Table 5 the important molecular orbital composition and
in Fig. 5 the isodensity surface plots of some selected molecular
orbitals. The calculations show that HOMO is centered on Ir atom
and the ancillary ligand whereas LUMO is a p* orbital centered
mainly on the cyclometalating ligand (phenyl pyridine). H e 1 is
centered on Ir atom and cyclometalating ligand whereas L þ 1 is
centered mainly on the ancillary ligand. H e 2 is composed of Ir
atom together with contributions from both cyclometallating as
well as ancillary ligand, however L þ 2 is composed of cyclo-
metallating ligand only. In short 17e43% of the three highest
HOMO’s are Ir-d orbitals whereas LUMO and next two unoccupied
orbitals are mostly p* orbitals of ligands with negligibly small
involvement of Ir orbitals.
TDDFT calculations were employed to understand the nature of
the excited states and examine the vertical excitation energies.
TDDFT calculation in the framework of the PCM model for
dichloromethane solvent gives a better agreement with expe-
rimental one. After testing different DFT functional we found
B3LYP/LANL2DZ/6-31G* gave satisfactory description of UVeVis
absorption due to the 2-phenyl pyridine ligands (
p/
p*) is seen.
The
p/
p* absorptions due to the ancillary ligands overlap with
those of the 2-phenyl pyridine ligands(see Supporting Information
for absorption spectra of ancillary ligands). These absorption bands
are in the ultraviolet region with high extinction coefficients (Fig. 3,
Table 4). The low energy bands which are due to 1MLCT, 1LLCT,
3MLCT (see also the results from DFT calculations in the following
section) resemble those that have been observed previously for
[(ppy)2Ir(acac)], [Ir(ppy)2(LeN, N)] (PF6), [Ir(ppy)2
(h
2-XZY)]
[15,16]. Interestingly, the variation of the ancillary ligand in
complexes 1e5 does not lead much variation in the lmax values
although the extinction coefficient does change (Fig. 3). The emis-
sion spectra of 1e5 (excitation wavelength, 450 nm) shows a single
emission with a lmax around 513 nm (see Fig. 4 for a representative
emission spectrum). The lifetimes of this emission fall in the
microsecond region (7.4e9.6
ms) suggesting the phosphorescent

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V. Chandrasekhar et al. / Journal of Organometallic Chemistry 696 (2011) 2711e2719
Table 3
Selected bond parameters of 3.
Ir(1)eN(1)
Ir(1)eN(3)
Ir(1)eC(1)
N(4)eC(28)
N(3)eIr(1)eN(4)
C(1)eIr(1)eN(1)
C(12)eIr(1)eN(1)
N(2)eIr(1)eN(1)
C(1)er(1)eN(4)
2.038(4)
2.148(4)
2.005(5)
1.347(7)
76.53(17)
80.6(2)
95.0(2)
174.31(17)
171.00(18)
Ir(1)eN(2)
Ir(1)eN(4)
Ir(1)eC(12)
C(28)eO(1)
C(12)eIr(1)-N(2)
N(2)eIr(1)eN(3)
C(12)eIr(1)eN(4)
C(12)eIr(1)eN(3)
2.032(4)
2.153(4)
2.001(5)
1.247(6)
80.5(2)
99.56(16)
101.48(19)
178.01(18)
From the transition character it is found that the three lowest
energy singlet excitations are predominantly from the HOMO level.
HOMO is composed of d orbitals of Ir and ancillary ligand orbitals.
This suggests that the lowest energy transitions are a mixture of
MLCT and LLCT. To account for the weak band experimentally found
at 481 nm we looked at the singletetriplet excitations (given in
Table 7), the calculated one is obtained at 483 nm for PBE0 and
497 nm for B3LYP. According to these results T1 states can also be
assigned as a mixture of MLCT and LLCT type transitions. Thus DFT
provides evidence that luminescence observed in complex 1 orig-
inates from Ir perturbed ligand-centered transitions.
To theoretically confirm that the substitutions on the phenyl
ring of the N-aryl picolinamide ligand have little effect on the
emission spectra, we have also looked at the molecular orbitals and
electronic excitations of 3, where a methyl substitution is present.
For 3, the calculated excitation energies of singlet and triplet, and
the participating orbitals are quantitatively and qualitatively the
same as that of 1 (see Supporting Information), which is in accor-
dance with the experimental observation. Overall our computa-
tional investigation basically confirms the similarity of the optical
properties of all the complexes 1e5.
2.5. Electrochemical studies
Cyclic voltammetric studies were performed to study the elec-
trochemical properties of these complexes in dry CH2Cl2 using
1 mM concentration of the complexes and 0.1 M tetrabuty-
lammonium hexaflorophosphate as the supporting electrolyte.
These results are summarized in Table 8. All the redox events have
been found to be quasi-reversible. Representative cyclic voltam-
mograms for complexes 2 and 5 are shown in Fig. 6.
Fig. 2. (a) ORTEP diagrams of 1 and 3 with thermal ellipsoids at 50% probability limit.
H atoms and solvent molecules are omitted for clarity, (b): ORTEP diagram of 4 with
thermal ellipsoids at 50% probability limit. H atoms are omitted for clarity, (c): ORTEP
diagram of 5 with thermal ellipsoids at 50% probability limit. H atoms and solvent
molecules are omitted for clarity. (One asymmetric unit contains two molecules in 5).
Two consecutive oxidation processes are observed for 1, 3, 4 and
5, however, only a single oxidation peak was observed for 2 in this
range. The first oxidation peak is attributed to the metal center with
a substantial contribution from the ligands. DFT calculations (see
above) also indicate that HOMO’s consist of a mixture of metal (d)
absorption determined experimentally (Tables 6 and 7 shows first
10 singlet and 5 triplet transitions using PBE0 and B3LYP func-
tional). For the two methods tested we were successful in repro-
ducing the main spectral features and their relative intensities with
minor deviations from one another. (Fig. S8 of supporting infor-
mation shows simulated UVeVis spectra at the B3LYP/LANL2DZ/
6-31G* level).
and ligand
(p) orbitals. The second oxidation observed for
compounds 1, 3, 4, 5 is not metal-centered, as further oxidation to
Table 2
Selected bond parameters of 1.
Ir(1)eN(1)
Ir(1)eN(3)
2.048(4)
2.141(4)
Ir(1)eN(2)
Ir(1)eN(4)
2.046(4)
2.147(4)
Ir(1)eC(1)
2.016(5)
Ir(1)eC(12)
2.006(5)
N(4)eC(28)
1.328(6)
C(28)eO(1)
1.261(5)
N(3)eIr(1)eN(4)
C(1)eIr(1)eN(1)
C(12)eIr(1)eN(1)
N(2)eIr(1)eN(1)
C(1)eIr(1)eN(4)
75.58(14)
81.03(17)
95.09(17)
175.00(15)
172.60(16)
C(12)eIr(1)eN(2)
N(2)eIr(1)eN(3)
C(12)eIr(1)eN(4)
C(12)eIr(1)eN(3)
80.13(17)
97.87(15)
98.64(16)
174.08(16)
Chat 1. Coordination modes of N-aryl picolinamide ligands: (a) amide form; N, O
donor (b) amidate form, N, N donor.

V. Chandrasekhar et al. / Journal of Organometallic Chemistry 696 (2011) 2711e2719
2715
Fig. 4. Emission spectrum of 3.
Fig. 3. Absorption spectra of 1e5.
the first of these occurs on a mixture of iridium-ligand orbitals. The
second oxidation is ancillary ligand-centered. For 2, however, only
a single oxidation is seen, presumably because the nitro substituent
on the amide ligand makes the ligand more resistant to oxidation.
On, the other hand 2 undergoes a nitro-group centered reduction
process.
an Ir(IV) species is unlikely. Based on cyclic voltammetric studies of
mer-Ir(ppy)3 [18a] where oxidation of the 2-phenyl pyridine ligand
does not occur below 1.65 V (vs Ag/AgCl), in the current instance
the second oxidation is attributed to be on the N-aryl picolinamide
ligand. This assignment also has literature support [18]. A reduction
event was observed only for complex 2 which occurs on the coor-
dinated amide ligand, more specifically due to the formation of
nitro-radical complex [19].
4. Experimental section
4.1. General
3. Conclusions
All procedures involving Ir(III) complexes were carried out
under a nitrogen gas atmosphere using Schlenk techniques.
2-Phenyl pyridine was purchased from Aldrich Chemical Company,
USA and was used as such without further purification. Iridium
chloride was purchased from Arora Matthey, Kolkata, India. Other
chemicals were purchased from S. D. Fine-Chemicals, Mumbai,
India. Solvents were purified by conventional methods and were
freshly distilled under nitrogen atmosphere prior to use according
to standard procedures [20]. Sodium methoxide was freshly
prepared and used in the reactions. Reactions were monitored by
TLC with Merck precoated plates.
In conclusion, we have prepared and characterized neutral
cyclometalated Ir(III) complexes containing N-aryl picolinamide
ligand, [Ir(ppy)2L] (1), [Ir(ppy)2LeNO2] (2) [Ir(ppy)2LeCH3] (3)
[Ir(ppy)2LeCl] (4) and [Ir(ppy)2LeF] (5) where L and substituted-L
are chelating (N, N) amidate ligands while the cyclometalating
ligand is the 2-phenyl pyridine ligand. The use of amide ligands in
cyclometalated Ir(III) complexes is sparse and the current study
reveals the effectiveness of these ligands in the preparation of
heteroleptic organometallic Ir(III) complexes. Complexes 1e5
revealed absorptions as a result of MLCT and LLCT. Further, all of
these complexes emit in the green at w513 nm with moderate to
high quantum yields. DFT and TDDFT studies were carried out on 1
and 3 to throw light on the nature of the frontier orbitals involved
in the absorption and emission process. The three HOMO’s contain
contributions from both ligand and iridium d orbitals while the
three LUMO’s are predominantly ligand based. Calculations also
confirmed the nearly identical optical properties for various
substitutions in N-aryl picolinamide ligand. Lifetime measure-
4.2. Instrumentation
ESI-MS analyses were performed on a Waters Micromass
Quattro Micro triple quadrupole mass spectrometer. Electrospray
ionization (positive ion, full scan mode) was done using acetonitrile
or methanol as the solvent while nitrogen gas used for desolvation.
Capillary voltage was maintained at 2.5e2.7 kV and cone voltage
was kept at approx 30 kV. Thermogravimetric analysis was carried
out on a Perkin Elmer Pyris 6 Thermogravimetric analyzer at
ments indicate that the emissions are in the range of 7.4e9.6 ms
supporting the phosphorescent nature of the emission. Electro-
chemical studies indicate two quasi-reversible oxidation events;
Table 4
Photophysical properties of 1e5 at 25 ꢁC.
Complex
Absorptiona lmax (nm); 10ꢀ4
e
(Mꢀ1 cmꢀ1
)
Emissiona lmax (nm)
s
(m
s)
Fb
1
2
3
4
5
266(5.5), 337(sh, 1.2), 368(sh, 0.9), 446(sh, 0.31), 481(sh, 0.08)
265(6.9), 334(sh, 2.4), 392(sh, 1.8), 446(sh, 0.79), 479(sh, 0.28)
267(5.8), 339(sh, 1.43), 375(sh, 1.11), 448(sh, 0.39), 480(sh, 0.15)
259(12.2), 336(sh, 2.49), 363(sh, 1.69), 447(sh, 0.43), 480(sh, 0.13)
252(5.3), 338(sh, 1.1), 366(sh, 0.8), 444(sh, 0.2)
514
513
512
512
514
9.59
7.36
9.9
9.96
9.01
0.51
0.53
0.26
0.49
0.23
a
Absorption and emission spectral data of all the complexes were recorded in dichloromethane solution at room temperature (absorption in 10ꢀ5 M solution; emission in
10ꢀ4 M solution, lexc ¼ 450 nm); “sh” denotes a shoulder.
b
Quantum yields are measured with respect to quinine sulfate in 1.0 N sulfuric acid solution whose emission yield was V ¼ 0.54 ꢂ 0.2 (lexc ¼ 350 nm).

2716
V. Chandrasekhar et al. / Journal of Organometallic Chemistry 696 (2011) 2711e2719
Table 5
Molecular Orbital composition (%) in the ground state at the PBE0/LANL2DZ/6-31G* level for 1.
Orbital
Energy (eV)
Ir
Ir(d)
Cyclometallating
ligand (LC)
Ancillary
ligand (LA)
Main bond nature
LUMO þ 3
LUMO þ 2
LUMO þ 1
LUMO
ꢀ0.960
ꢀ1.181
ꢀ1.315
ꢀ1.420
ꢀ5.018
ꢀ5.430
ꢀ6.005
ꢀ6.106
3.5
5.9
4.8
0.3
4.2
3.1
66.3
90.7
7.8
88.7
7.7
47.7
23.8
73.2
30.2
3.4
87.4
7.1
74.8
8.8
55.9
21.3
L
C (54.6%) þ LA (30.2%)
LC (90.7%)
LA (87.4%)
LC (88.7%)
Ir-d (16.6%) þ LA (74.8%)
Ir-d (42.7%) þ LC (47.7%)
Ir-d (19.1%) þ LC (23.8%) þ LA (55.9%)
LC (73.2%) þ LA (21.3%)
4.2
2.4
HOMO
17.5
43.6
20.3
5.5
16.6
42.7
19.1
4.8
HOMO e 1
HOMO e 2
HOMO e 3
a heating rate of 10 ꢁC/min under argon. Melting points were
recorded using a JSGW melting point apparatus. None of the
complexes melted up to 200 ꢁC. 1H NMR spectra were obtained on
a JEOL-DELTA 2500 model spectrometer using CDCl3 as the solvent
and TMS as the reference. NMR data are reported in ppm. Elemental
analyses were carried out using a Thermoquest CE instruments
CHNS-O, EA/110 model. The analyses of compounds were carried
out on fully dried samples. IR spectra were recorded as KBr pellets
from 4000 to 400 cmꢀ1 on a Bruker FT-IR Vector 22 model.
UVevisible spectra were recorded on a Perkin Elmer-Lambda 20
UVevisible spectrometer in 1 ꢃ 10ꢀ5 M dichloromethane solution.
The steady state emission spectra were measured using a Fluorolog
dichloromethane solution. The excited state lifetimes were
measured with the same system under microsecond Xenon flash
lamp operating conditions. The best fit was assessed based on the
parameter
yields were measured with reference to quinine sulfate in 1.0 N
sulfuric acid solution with an emission yield of
¼ 0.54 ꢂ 0.2
lexc ¼ 350 nm) [21]. The quantum yields were then calculated
using the following expression:
c
2, which was close to 1.0 for all the samples. Quantum
F
(
À
Á
FU
¼
FRðAU=ARÞ nU2 =nR2
In the above equation, the subscripts U and R denote sample and
reference respectively,
F is the fluorescence quantum yield, A is the
FL3-21 (Horiba Jobin Yvon) spectrofluorimeter using 1 ꢃ 10ꢀ4
M
integrated area under the corrected fluorescence spectra, n is the
refractive index of the solvent.
Cyclic voltammetric studies were performed on a BAS Epsilon
electrochemical workstation using a platinum working electrode,
an Ag/AgCl reference electrode (3 M NaCl), and a platinum-wire
counter electrode. All measurements were performed in 0.1 M
[n-Bu4N][PF6] in dry dichloromethane using 1 mM concentrations
of the complexes. The ferrocene/ferrocenium couple was taken as
internal standard; E1/2 ¼ þ0.51(70) V versus Ag/AgCl under the
same experimental conditions.
4.3. X-ray structural analysis
Important crystallographic data for compounds 1, 3, 4 and 5 are
given in Table 1. Single crystals of 1, 3, 4 and 5 suitable for X-ray
crystallography were obtained by slow evaporation of their
dichloromethane solutions that have been layered with a few drops
of methanol. X-ray data were collected on a CCD Bruker SMART
APEX diffractometer at 100(2) K using a graphite monochromated
Mo Ka radiation (Ka ¼ 0.71073 Å). No decomposition of the crystal
occurred during data collection. The program SMART [22] was used
for collecting frames of data, indexing reflection, and determining
lattice parameters, SAINT for integration of the intensity of
Table 6
Singlet excited states of 1 using PBE0/LANL2DZ/6-31G* and B3LYP/LANL2DZ/6-31G*
using PCM solvation model.
States
PBE0/LANL2DZ/6-31G*
B3LYP/LANL2DZ/6-31G*
l
(nm)
f
l
(nm)
f
S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
401.35
400.12
386.58
372.81
370.98
354.36
342.28
324.48
321.82
318.84
0.0226
0.0324
0.0013
0.0288
0.0448
0.0345
0.0241
0.0100
0.0073
0.0081
424.16
422.86
407.39
393.22
391.54
372.38
363.60
343.20
341.06
336.04
0.0244
0.0151
0.0013
0.0362
0.0294
0.0284
0.0251
0.0081
0.0065
0.0134
Fig. 5. Relevant molecular orbitals of 1 obtained from DFT calculations drawn at an
isovalue of ꢂ0.05 (red/green surfaces): (a) HOMO (b) HOMO-1 (c) HOMO-2 (d) LUMO
(e) LUMOþ1 (f) LUMOþ2. Color codes of atoms: blue (N), black (C), white (H), red (O),
brown (Ir). “(For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article.)”

V. Chandrasekhar et al. / Journal of Organometallic Chemistry 696 (2011) 2711e2719
2717
Table 7
Triplet excited states of 1 using PBE0/LANL2DZ/6-31G* and B3LYP/LANL2DZ/6-31G*
using PCM solvation model.
States
PBE0/LANL2DZ/6-31G*
l
(nm)
B3LYP/LANL2DZ/6-31G* l (nm)
T1
T2
T3
T4
T5
483.80
457.02
451.70
398.85
391.72
497.41
458.96
450.36
413.30
403.84
reflections and scaling, SADABS [23] for absorption correction, and
SHELXTL [24] for space group and structure determination and
least-squares refinements on F2. All non-hydrogen atoms were
refined with anisotropic displacement parameters. Hydrogen
atoms were fixed in geometrically calculated positions using
a riding model and were refined isotropically. For complex 5, one of
the methanol molecules was found to be disordered. The carbon
atom (C71) of the methanol molecule was disordered over two
positions (C71A and C71B) with occupancies 0.70 and 0.30. The
oxygen atom (O5) of the same methanol molecule was disordered
over two positions (O5A and O5B) with occupancies 0.55 and 0.45.
4.4. Computational details
Fig. 6. a): Cyclic voltammogram of 2, b): Cyclic voltammogram of 5.
The ground state geometry of 1 was optimized by two different
density functional theory (DFT) methods: Becke’s LYP(B3LYP)
exchange correlation functional [25] and gradient corrected
correlation functional PBE0 [26]. PBE0 provides the best match
with the geometrical X-ray data. Full geometry optimization
without symmetry constraints were carried out in the gas phase for
the singlet ground state (S0) of 1. The optimized geometry was
confirmed to be potential energy minima by vibrational frequency
calculation at the same level of theory as no imaginary frequencies
were found. In the calculation, the quasi-relativistic pseudo-
potentials of Ir atoms proposed by Hay and Wadt [27] with 17
valence electrons were used, and a double zeta quality basis set
LANL2DZ was adopted. 6-31G* basis set was used on all the
nonmetal atoms.
At the optimized geometry TDDFT calculations were performed
by two different methods: PBE0/LANL2DZ/6-31G*, B3LYP/
LANL2DZ/6-31G* in dichloromethane solution by means of PCM
solvation model [28] as implemented in the Gaussian 03 program
package. 60 singletesinglet excitations and the lowest 10 single-
tetriplet excitations at the S0 optimized geometry allowed us to
simulate a large (up to 250 nm) portion of the absorption spectrum
and gain insight into the nature of the transitions giving rise to the
low energy spectral region. The oscillator strengths of single-
tetriplet transitions are set to zero due to neglect of spin-orbit
coupling in the TDDFT calculations, so that these transitions do
not contribute to the overall spectral profile.
All computations were performed using Gaussian 03 package
[29]. Software Gaussum was used to calculate the fractional
contributions of various groups to each molecular orbital and also
for electronic spectrum simulation with fwhm ¼ 0.4eV [30]. Similar
calculations were performed for 3.
4.5. Synthesis
The chloro-bridged precursor complex [(ppy)2Ir(m-Cl)]2 was
synthesized according to Nonoyama route [31], by refluxing IrCl3
.nH2O with 2.5 equiv of 2-phenyl pyridine (cyclometalating ligand)
in a 3:1 mixture of 2-ethoxyethanol and water. All the ligands (LH,
LHeNO2, LHeCH3, LHeCl and LHeF) were synthesized following
reported methods [32] by the condensation of picolinic acid with
substituted anilines. The complexes 1e5 were prepared using an
identical synthetic procedure. The synthesis of 1 is outlined below
in detail.
4.5.1. [(ppy)2Ir(L)] (1)
In a 100 mL round bottomed flask [(ppy)2 Ir(m-Cl)]2 (0.10 g,
0.093 mmol) was taken along with 30 mL of dichloromethane. To
this was added solid sodium methoxide (0.025 g, 0.46 mmol) and
the reaction mixture was allowed to stir at room temperature for
about 1 h. At this point the solution was orange in color. To this
solution was added at once LH (0.046 g, 0.23 mmol) and the reac-
tion mixture was continued to stir at room temperature for 12 h.
The progress of the reaction was monitored by TLC by following the
Table 8
Half-wave redox potentials (vs Ag/AgCl) of 1e5 at 25 ꢁC.
Complex E11/2, ox/Va (ΔEp/mV)b E21/2, ox/Va (ΔEp/mV)b E11/2, red/Va (ΔEp/mV)b
disappearance of [(ppy)2Ir(m-Cl)]2.(eluant: ethylacetate : petroleum
1
0.71 (57)
1.01 (91)
0.97 (65)
0.68 (52)
0.67 (68)
0.93 (79)
ether (60e80 ꢁC) 30:70 (V:V)). The reaction mixture was then
filtered and the solvent removed in vacuo to afford a yellow oil. To
this was added diethyl ether (10 mL) and triturated affording
a yellow solid. This was filtered and washed with cold methanol.
The residue was recrystallized from dichloromethane by layering
the solution with a few drops of methanol to give compound 1 as
yellow crystals. (0.081 g, Yield: 62%).
2c
3
ꢀ1.26 (204)
1.22 (81)
0.93 (87)
0.92 (79)
4
5
a
Recorded in dichloromethane solution containing 0.001M iridium complexes,
E1/2 ¼ 1/2(EpaþEpc), where Epa and Epc are anodic and cathodic peak potentials,
respectively.
b
ΔEp ¼ EpaꢀEpc
.
c
Characterization data: Anal.Calcd. for C34H25IrN4O: C, 58.52; H,
The second oxidation is not observed for this complex. However, only for this
complex a reduction wave is observed.
3.61; N, 8.03. Found: C, 58.44; H, 3.66; N, 8.08. IR(KBr, lmax/cmꢀ1):

2718
V. Chandrasekhar et al. / Journal of Organometallic Chemistry 696 (2011) 2711e2719
3031m, 2923m, 2853w, 1740w, 1607s, 1579vs, 1554s, 1476s, 1437w,
1415m, 1358m, 1304w, 1288w, 1266m, 1225w, 1156m, 1028m,
944w, 811w, 793w, 760vs, 740s, 730s, 691m, 629w, 590w, 505w. 1H
3031w, 2924m, 2853w, 1609s, 1579vs, 1500s, 1476s, 1438w, 1416m,
1372m, 1305w, 1290w, 1267w, 1216m, 1150w, 1091w, 1062w,
1030w, 1010w, 943w, 821m, 784w, 759vs, 732s, 693w, 629w, 541w,
NMR (500 MHz, CDCl3)
d: 6.05 (d, J ¼ 7.3 Hz,1H), 6.42e6.47 (m, 2H),
527w, 476w. 1H NMR (500 MHz, CDCl3)
d: 6.05 (d, J ¼ 7.6 Hz, 1H),
6.56 (t, J ¼ 7.3 Hz, 1H), 6.63e6.67 (m, 2H), 6.72e6.75 (m, 2H),
6.79e6.85 (m, 1H), 6.91e6.94 (m, 2H), 7.10 (t, J ¼ 6. 7 Hz, 1H),
7.18e7.25 (m, 3H), 7.62e7.74 (m, 6H), 7.82e7.87 (m, 2H), 8.37
(d, J ¼ 7.7 Hz,1H), 9.05 (d, J ¼ 6Hz,1H). ESI-MS (100% Acetonoitrile):
m/z ¼ 699.16, [M þ H]þ.
6.40e6.44(m, 5H), 6.58 (t, J ¼ 7.6 Hz, 1H), 6.67 (t, J ¼ 7.6 Hz, 1H),
6.79e6.83 (m, 2H), 7.09 (t, J ¼ 7.2 Hz, 1H), 7.18e7.25 (m, 3H),
7.62e7.73 (m, 6H), 7.86 (d, J ¼ 6.5 Hz, 2H), 8.35 (d, J ¼ 8.0 Hz, 1H),
8.97 (d, J ¼ 5.3 Hz, 1H). ESI-MS (100% methanol): m/z ¼ 717.16,
[M þ H]þ.
4.5.2. [(ppy)2Ir(LeNO2)] (2)
Acknowledgement
Quantities: [(ppy)2 Ir(m-Cl)]2 (0.10 g, 0.093 mmol), sodium
methoxide (0.025 g, 0.46 mmol), LHeNO2 (0.056 g, 0.23 mmol).
Yield of 2:0.094 g, Yield: 68%.
We thank the Department of Science and Technology (DST),
India for financial support. B.M. and P.B. are thankful to CSIR,
New Delhi, India for Fellowship. We also thank Dr. Pratik Sen (IIT
Kanpur) for valuable suggestions and R. Suriya Narayanan for TGA
studies.
Characterization data: Anal.Calcd. for C34H24IrN5O3: C, 54.98; H,
3.26; N, 9.43. Found; C, 54.55; H, 3.31; N, 9.27. IR(KBr, lmax/cmꢀ1):
3060w, 2924s, 2854m, 1689w, 1598s, 1495vs, 1415w, 1328vs,
1290m, 1245vs, 1174w, 1121m, 1110m, 1039m, 932w, 841w, 754vs,
712m, 691m, 629w, 438w, 413w. 1H NMR (500 MHz, CDCl3)
d: 6.08
Appendix A. Supplementary material
(d, J ¼ 7.2 Hz, 1H), 6.38 (d, J ¼ 7.6 Hz, 1H), 6.63e6.70 (m, 5H), 6.81
(t, J ¼ 7.2 Hz, 1H), 6.95 (t, J ¼ 6.9 Hz, 2H), 7.11e7.13 (m, 2H), 7.20
(d, J ¼ 7.6 Hz, 1H), 7.58e7.67 (m, 5H), 7.75 (m, 2H), 7.88
(d, J ¼ 8.0 Hz, 2H), 8.37 (d, J ¼ 7.6 Hz, 1H), 8.90 (d, J ¼ 5.35 Hz, 1H).
ESI-MS (100% Methanol): m/z ¼ 744.15, [M þ H]þ.
CCDC numbers 784721 – 784724 (1, 3, 4, and 5) contains the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre www.ccdc.cam.ac.uk/data_request/cif.
4.5.3. [(ppy)2Ir(LeCH3)] (3)
Appendix. Supplementary data
Quantities used: [(ppy)2 Ir(m-Cl)]2 (0.10 g, 0.093 mmol), sodium
methoxide (0.025 g, 0.46 mmol), LeCH3 (0.049 g, 0.23 mmol). Yield
of 3: 0.078g, 59%.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2011.04.011.
Characterization data: Anal.Calcd for C35H27IrN4O: C, 59.06; H,
3.82; N, 7.87. Found; C, 58.72; H, 3.86; N, 7.57. IR(KBr, lmax/cmꢀ1):
3037m, 2993w, 2918w, 1608s, 1580vs, 1558s, 1504m, 1474s, 1436w,
1413s, 1359s, 1314w, 1302w, 1291w, 1266m, 1255w, 1224w, 1158m,
1123w, 1110w, 1090w, 1061w, 1049w, 1029m, 1015w, 945w, 923w,
907w, 806w, 794w, 761vs, 748m, 735s, 712w, 695w, 683w, 668w,
629w, 543w, 524w, 496w, 472w, 422w. 1H NMR (500 MHz, CDCl3)
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4: 0.091 g, 67%.
Characterization data: Anal.Calcd. for C34H24 ClIrN4O: C, 55.77;
H, 3.30; N, 7.65. Found; C, 55.53; H, 3.30; N, 7.42. IR(KBr, lmax
/
cmꢀ1): 3038w, 2998w, 2923w, 1872w, 1609s, 1581vs, 1560s, 1478s,
1437s, 1415s, 1358s, 1292w, 1266w, 1257w, 1225w, 1160w, 1050w,
1029m, 1010w, 947w, 923w, 907w, 816w, 794w, 761vs, 737s, 704w,
692w, 669w, 629w, 517w, 475w, 424w. 1H NMR (500 MHz, CDCl3)
d:
6.05 (d, J ¼ 7.65 Hz, 1H), 6.41e6.44 (m, 3H), 6.60 (t, J ¼ 7.25 Hz, 1H),
6.67e6.70 (m, 2H), 6.80e6.83 (m, 2H), 6.92e6.95 (m, 2H), 7.08e7.11
(m, 1H), 7.19e7.22 (m, 1H), 7.25e7.26 (m, 2H), 7.63e7.66 (m, 3H),
7.70e7.73 (m, 2H), 7.83e7.86 (m, 2H), 8.35 (d, J ¼ 7.6 Hz, 1H), 8.95
(d, J ¼ 5.7 Hz,1H). ESI-MS (100% methanol): m/z ¼ 733.15, [M þ H]þ.
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of 5: 0.084 g, 63%.
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V. Chandrasekhar et al. / Journal of Organometallic Chemistry 696 (2011) 2711e2719
2719
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