Iridium mediated N-H and C-H bond activation of N-(aryl)pyrrole-2- aldimines. Synthesis, structure and, spectral and electrochemical properties

Article abstract of DOI:10.1016/j.jorganchem.2012.04.023

Reaction of N-(4′-R-phenyl)pyrrole-2-aldimines (L-R, R = OCH ₃, CH₃, H, Cl and NO₂) with Ir(PPh ₃)₃Cl in refluxing ethanol affords a group of yellow complexes (1-R), in which an imine-ligand (L-R) is coordinated to the metal center as a mono-anionic bidentate NN-donor along with two triphenylphosphines, a chloride and a hydride. The hydride is trans to the coordinated pyrrole-nitrogen, while the chloride is trans to the imine-nitrogen, and the two triphenylphosphines are mutually trans. Structure of the 1-OCH₃ complex has been determined by X-ray crystallography. Similar reaction of N-(naphthyl)pyrrole-2-aldimine (L-nap) with Ir(PPh₃)₃Cl affords an organometallic complex 2, where the imine-ligand is coordinated to the metal center, via C-H activation of the naphthyl ring at the 8-position, as a di-anionic tridentate NNC-donor, along with two triphenylphosphines and a hydride. Structure of this complex 2 has also been determined by X-ray crystallography. All the complexes are diamagnetic, and show characteristic ¹H NMR signals and intense transitions in the visible region. Cyclic voltammetry on all the complexes shows two irreversible oxidations within 0.97-1.46 V vs. SCE and a reduction within -0.83 to -1.40 V vs. SCE.

Full text of DOI:10.1016/j.jorganchem.2012.04.023

Journal of Organometallic Chemistry 713 (2012) 72e79
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Journal of Organometallic Chemistry
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Iridium mediated NeH and CeH bond activation of N-(aryl)pyrrole-2-aldimines.
Synthesis, structure and, spectral and electrochemical properties
*
Piyali Paul, Samaresh Bhattacharya
Department of Chemistry, Inorganic Chemistry Section, 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
Reaction of N-(40-R-phenyl)pyrrole-2-aldimines (L-R, R ¼ OCH3, CH3, H, Cl and NO2) with Ir(PPh3)3Cl in
refluxing ethanol affords a group of yellow complexes (1-R), in which an imine-ligand (L-R) is coordi-
nated to the metal center as a mono-anionic bidentate NN-donor along with two triphenylphosphines,
a chloride and a hydride. The hydride is trans to the coordinated pyrrole-nitrogen, while the chloride is
trans to the imine-nitrogen, and the two triphenylphosphines are mutually trans. Structure of the 1-OCH3
complex has been determined by X-ray crystallography. Similar reaction of N-(naphthyl)pyrrole-2-
aldimine (L-nap) with Ir(PPh3)3Cl affords an organometallic complex 2, where the imine-ligand is
coordinated to the metal center, via CeH activation of the naphthyl ring at the 8-position, as a di-anionic
tridentate NNC-donor, along with two triphenylphosphines and a hydride. Structure of this complex 2
has also been determined by X-ray crystallography. All the complexes are diamagnetic, and show
characteristic 1H NMR signals and intense transitions in the visible region. Cyclic voltammetry on all the
complexes shows two irreversible oxidations within 0.97e1.46 V vs. SCE and a reduction within ꢀ0.83 to
ꢀ1.40 V vs. SCE.
Article history:
Received 27 February 2012
Received in revised form
9 April 2012
Accepted 19 April 2012
Keywords:
N-(aryl)pyrrole-2-aldimine
Iridium
NeH and CeH bond activations
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
N
N
N
N
H
M
The chemistry of iridium has been receiving considerable
current attention [1e6], largely because of the interesting proper-
ties exhibited by the complexes of this metal. As properties of the
complexes are dictated primarily by the coordination environment
around the metal center, complexation of iridium by ligands of
selected types has been of significant importance, and the present
work has originated from our interest in this area [7e15]. Herein we
have selected a group of imine-ligands (L-R) derived from pyrrole-
2-aldehyde and para-substituted anilines. The selected ligands are
known to bind to metal ions, via loss of the NeH proton, as mono-
anionic NN-donors forming stable five-membered chelate ring (I)
[16e22]. Coordination chemistry of the selected imine-ligands,
containing the pyrrole fragment, is of particular interest with
reference to its relevance in biological processes [23e30]. It may be
mentioned here that while coordination chemistry of the N-(aryl)
pyrrole-2-aldimines with few transition and non-transition metal
ions has received some attention [16e22], that with iridium
appears to have remained completely unexplored. As the source of
iridium the Ir(PPh3)3Cl complex has been chosen, because of its
demonstrated ability to undergo facile oxidative insertion reaction
H
H
R = OCH3, CH3,
H, Cl, NO2
R
R
I
L-R
with ligands having acidic fragments affording stable mixed-ligand
hexacoordinated complexes [7e15]. The primary objective of the
present study has been to examine whether the iridium (I) center in
Ir(PPh3)3Cl can oxidatively insert into the NeH bond of the selected
imine-ligands to afford hydrido-complexes of iridium, and utilize
the IreH fragment, if formed, to bring about CeH bond activation of
suitably designed ligands. Reaction of the selected imine-ligands (L-
R) with Ir(PPh3)3Cl has indeed been found to afford a family of
interesting hydrido-complexes of iridium. The chemistry of all these
complexes is reported here, with special reference to their forma-
tion, characterization and utilization for CeH bond activations.
* Corresponding author. Tel./fax: þ91 33 24146223.
E-mail address: samaresh_b@hotmail.com (S. Bhattacharya).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2012.04.023

P. Paul, S. Bhattacharya / Journal of Organometallic Chemistry 713 (2012) 72e79
73
Table 1
2. Results and discussion
Selected bond lengths (Å) and bond angels (ꢁ) for complexes 1-OCH3 and 2.
2.1. Synthesis and characterization
1-OCH3
Bond lengths (Å)
Ir(1)-H
1.420(4)
2.152(2)
2.068(2)
2.3299(7)
2.3364(7)
2.3849(7)
N(1)eC(1)
C(1)eC(2)
C(2)eC(3)
C(3)eC(4)
C(4)eN(1)
C(4)eC(5)
C(5)eN(2)
N(2)eC(6)
1.333(4)
1.391(6)
1.365(6)
1.402(5)
1.371(4)
1.393(5)
1.313(4)
1.440(4)
As delineated in the introduction, the primary objective of the
present study has been to study the interaction of the selected N-
(40-R-phenyl)pyrrole-2-aldimines (L-R) with iridium and see how
they bind to the metal center. The five different substituents, with
different electron withdrawing properties, have been chosen to
study their influence, if any, on the redox properties of the resulting
complexes. Reactions of the selected imine-ligands (L-R) with
Ir(PPh3)3Cl proceed smoothly in refluxing ethanol to afford a group
of yellow complexes (1-R) in decent yields. Preliminary character-
izations (microanalysis, IR, NMR, etc.) on the 1-R complexes indi-
cate presence of an imine-ligand, a chloride, a hydride and two
triphenylphosphines in the coordination sphere. In order to find
out stereochemistry of these complexes, as well as to ascertain
coordination mode of the imine-ligands in them, structure of
a representative member from the series, viz. 1-OCH3, has been
determined by X-ray crystallography. The structure is shown in
Fig. 1 and some relevant bond parameters are listed in Table 1. The
structure shows that the N-(40-methoxyphenyl)pyrrole-2-aldimine
is coordinated to iridium, via dissociation of the NeH proton, as
a bidentate NN-donor forming a five-membered chelate ring (I).
Two triphenylphosphines, a chloride and a hydride are also coor-
dinated to the metal center. The coordinated pyrrole-2-aldimine,
hydride and chloride have constituted an equatorial plane with the
metal at the center, where the chloride is trans to the imine-
nitrogen and the hydride is trans to the pyrrole-nitrogen. The two
triphenylphosphines have occupied the remaining two axial posi-
tions, and hence they are mutually trans. The IreH, IreN, IreP and
IreCl distances are quite normal [7e15], and so are the distances
within the chelated imine-ligand [16e22]. However, the Ir-N1 bond
is noticeably longer than the IreN2 bond, and the observed elon-
gation of the IreN1 bond is attributable to the trans effect of the
coordinated hydride [31e33]. The absence of any solvent of crys-
tallization in the crystal lattice of 1-OCH3 indicates possible exis-
tence of non-covalent interaction(s) between the individual
Ir(1)eN(1)
Ir(1)eN(2)
Ir(1)eP(1)
Ir(1)eP(2)
Ir(1)eCl(1)
Bond angles (ꢁ)
P(1)eIr(1)eP(2)
N(1)eIr(1)eH
172.66(3)
171.50(15)
N(2)eIr(1)eCl(1)
N(1)eIr(1)eN(2)
178.07(6)
78.57(9)
2
Bond lengths (Å)
Ir(1)eH(11)
Ir(1)eC(12)
Ir(1)eN(1)
Ir(1)eN(2)
Ir(1)eP(1)
Ir(1)eP(2)
1.67(7)
2.067(5)
2.138(5)
2.072(5)
2.3182(15)
2.3051(16)
N(1)eC(1)
C(1)eC(2)
C(2)eC(3)
C(3)eC(4)
C(4)eN(1)
C(4)eC(5)
N(2)eC(5)
N(2)eC(6)
1.341(9)
1.411(13)
1.381(13)
1.401(10)
1.383(9)
1.395(9)
1.308(8)
1.403(8)
Bond angles (ꢁ)
P(1)eIr(1)eP(2)
N(2)eIr(1)eH(11)
N(1)eIr(1)eC(12)
170.04(6)
175.4(18)
158.1(2)
N(1)eIr(1)eN(2)
N(2)eIr(1)eC(12)
77.40(18)
80.8(2)
complex molecules. A closer look at the packing pattern in the
crystal lattice (shown in Fig. 2) reveals that CeHeCl and p—p
interactions are active in the lattice. Each complex molecule is thus
linked with the surrounding complex molecules through these
non-covalent interactions, and these extended intermolecular
interactions seem to be responsible for forming a stable crystal
structure. As all the 1-R complexes have been synthesized similarly
and they show similar properties (vide infra), the other four 1-R
complexes (with R s OCH3) are assumed to have similar struc-
tures as the 1-OCH3 complex.
Formation of the 1-R complexes containing a coordinated
hydride, from rather simple reaction between the N-(40-R-phenyl)
pyrrole-2-aldimines (L-R) and Ir(PPh3)3Cl, has been quite inter-
esting. Some speculated sequences behind formation of these
complexes, that seem probable, are illustrated in Scheme 1. In the
initial step the imine-nitrogen of the L-R ligand seems to bind to
the metal center in Ir(PPh3)3Cl, via displacing.a PPh3, to generate an
intermediate A-R. This brings the NeH fragment of the coordinated
imine-ligand in close proximity to the metal center and this
eventually leads to oxidative insertion of iridium into the NeH
bond generating an hydrido-iridium(III) species B-R, where the
hydride is trans to the imine-nitrogen. Intermediate B-R seems to
undergo rapid isomerization to afford the final 1-R complex. In
order to assess the feasibility of the speculated isomerization, we
have optimized structures of intermediate B-R for a particular case,
viz. R ¼ OCH3, and the corresponding isomer 1-OCH3 by using DFT
[34], and computed the energy difference (DG) between them. The
optimized structures are shown in Fig. S1 (Supplementary
material). The 1-OCH3 complex has indeed been found to be
more stable than the corresponding intermediate B-OCH3 by
63 kcal/mol, which indicates that the final isomerization step in
Scheme 1 is thermodynamically feasible.
In the 1-R complexes, disposition of the IreH bond with respect
to the pendent phenyl ring of the imine-ligand points to the
possibility of a CeH activation of the aryl ring. However, such
activation could not take place in the 1-R complexes, because in
them the phenyl ring is not close enough to the IreH fragment. It is
Fig. 1. Molecular structure of 1-OCH3 (20% probability ellipsoids). Hydrogen atoms are
omitted for clarity.

74
P. Paul, S. Bhattacharya / Journal of Organometallic Chemistry 713 (2012) 72e79
Fig. 2. CeHeCl (top) and p—p (bottom) interactions in the lattice of 1-OCH3 complex.
thus hinted that, in order to induce a CeH activation of a similar
imine-ligand, its aryl fragment needs to be modified in such
a fashion that one of its CeH bonds is forced to come in close
proximity to the IreH bond, and undergoes activation leading to
the formation of a cyclometallated species via elimination of
molecular hydrogen (Scheme 2). With this simple strategy in
mind, a slightly modified imine-ligand, viz. N-(naphthyl)pyrrole-
2-aldimine (L-nap), where a naphthyl ring has replaced the
phenyl ring in L-R, has been chosen for the targeted CeH activa-
tion. Reaction of N-(naphthyl)pyrrole-2-aldimine with Ir(PPh3)3Cl
has been carried out similarly as before, which has afforded
a yellow complex 2. Preliminary characterizations on complex 2
indicate the presence of an imine-ligand, two triphenylphos-
phines and a hydride in the coordination sphere. Structure of
complex 2 has also been determined by X-ray crystallography. The
structure (Fig. 3) shows that the N-(naphthyl)pyrrole-2-aldimine
is indeed coordinated to iridium in the expected NNC-fashion
(II). A hydride is coordinated trans to the imine-nitrogen, and
two triphenylphosphines are occupying the axial positions. The
IreC distance is normal [7e15], and the other bond distances
(Table 1) are comparable to those observed in 1-OCH3 complex. An
examination of the packing pattern of complex 2 shows that
and CeH— interactions are active in the lattice (Fig. S2;
Supplementary material). There also exists CeH— interaction
p—p
p
p
within the phenyl rings of the PPh3 ligands belonging to neigh-
boring molecules.
N
N
N
N
H
It is relevant to note here that while formulating the strategy for
inducing cyclometallation (Scheme 2), it was anticipated that the
iridium-bound hydride in the intermediate will activate a proximal
CeH bond and thereby it will be eliminated via formation of
molecular hydrogen. But actually it is found that the iridium-bound
chloride is lost during formation of the organoiridium complex 2,
while the hydride is retained. The probable sequences behind this
difference between the expectation and the reality are presented in
M
II
H
H
L-nap

P. Paul, S. Bhattacharya / Journal of Organometallic Chemistry 713 (2012) 72e79
75
PPh3
N
N
N
H
IrI(PPh3)3Cl
ethanol
H
IrI
Cl
H
H
N
-
PPh3
PPh3
R
R
L-R
A-R
oxidative
insertion
PPh3
PPh3
N
N
N
N
Cl
H
H
III
Ir
III
Ir
isomerization
Cl
H
H
PPh3
PPh3
B-R
Fig. 3. Molecular structure of complex 2 (20% probability ellipsoids). Hydrogen atoms
are omitted for clarity.
R
R
1 R
Scheme 1. Probable steps behind formation of the 1-R complex.
7.08e7.67 ppm for the coordinated PPh3 ligands. The hydride signal
is observed around ꢀ10.1 ppm as a distinct triplet due to coupling
with two equivalent phosphorus nuclei with a coupling constant of
w19 Hz. Signals for the methoxy and methyl groups in the 1-OCH3
and 1-CH3 complexes are observed respectively at 3.25 and
2.29 ppm. Most of the expected aromatic proton signals, as well as
the azomethine-proton signal, from the coordinated imine-ligand
are clearly observed in the expected region, while a few could not
be detected due to their overlap with other signals in the same
region. 1H NMR spectrum of complex 2 also shows broad signals
within 7.17e7.49 ppm for the coordinated PPh3 ligands and the
hydride signal appear at ꢀ11.11 ppm.
Scheme 3. The NeH fragment of the imine-ligand (L-nap) is
believed to add oxidatively to the metal center in Ir(PPh3)3Cl, as
shown earlier in Scheme 1, generating a hydrido intermediate A.
This intermediate then undergoes rapid cyclometallation via
elimination of HCl affording the organoiridium complex 2. Trans-
formation of intermediate A into its geometric isomer B, which is
also energetically more stable, could not take place probably due to
the kinetically more favorable cyclometallation step starting from
A. As a consequence the envisaged organoiridium complex with
a chloride ligand (shown as complex 2a in Scheme 2) could not be
obtained.
Infrared spectrum of each 1-R complex shows many bands of
different intensities in the 400e4000 cmꢀ1 region. No attempt has
been made to assign each individual band to a specific vibration.
However, comparison with the spectra of the corresponding
uncoordinated ligands shows that the NeH stretch, observed near
3230 cmꢀ1 in the uncoordinated ligand, is absent in the complexes.
A strong band displayed near 2161 cmꢀ1 by the 1-R complexes is
due to the coordinated hydride. Three strong bands have been
observed near 521, 694 and 746 cmꢀ1 in all the 1-R complexes
2.2. Spectral studies
Magnetic susceptibility measurements show that all the iridium
complexes are diamagnetic, which corresponds to the þ3 oxidation
state of iridium (low-spin d6, S ¼ 0) in these complexes. 1H NMR
spectra of the 1-R complexes show broad signals within
P = PPh3
P
P
N
N
N
N
N
N
IrI(PPh3)3Cl
- PPh3
- H2
Cl
Cl
C
H
III
Ir
Ir
P
H
H
H
H
H
P
H
C
C
Scheme 2. Strategy for CeH bond activation utilizing the IreH fragment.

76
P. Paul, S. Bhattacharya / Journal of Organometallic Chemistry 713 (2012) 72e79
P = PPh3
P
P
N
N
N
N
N
N
H
Cl
H
isomerization
IrI(PPh3)3Cl
H
III
III
Ir
Ir
ethanol
- PPh3
H
Cl
H
H
P
P
L-nap
A
B
P
- HCl
- H2
P
N
N
N
N
H
Cl
Ir
P
Ir
P
H
H
2
2a
Scheme 3. Probable steps behind formation of complex 2.
indicating the presence of coordinated PPh3 ligands [7e15]. Several
sharp bands (e.g. near 1571, 1505, 1482, 1435, 1385, 1311, 1245, 1184,
1120, 1092, 1038, 828, 807, 722 and 541 cmꢀ1) are also observed in
the 1-R complexes, which were absent in the Ir(PPh3)3Cl complex,
and hence these are attributed to the coordinated imine-ligand.
Infrared spectrum of complex 2 is found to be qualitatively
similar to that of any 1-R complex.
All the complexes are found to be readily soluble in acetone,
dichloromethane, chloroform, acetonitrile, etc., producing bright
yellow solutions. Electronic spectra of the complexes have been
recorded in dichloromethane solution. Spectral data are pre-
sented in Table 2. Each complex shows several intense absorptions
in the visible and ultraviolet regions. The absorptions in the
ultraviolet region are attributable to transitions within the ligand
orbitals. To have an insight into the nature of absorptions in the
visible region, DFT calculations have been performed on two
representative complexes, viz. complex 1-OCH3 and complex 2
[34]. Compositions of the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO) are
given in Table 3 and contour plots of these molecular orbitals for
complex 1-OCH3 are shown in Fig. 4 and those for complex 2 are
deposited as Fig. S3 (Supplementary material). In the 1-OCH3
complex, the HOMO has major (84%) contribution from the imine-
ligand, with minor contribution from iridium and the coordinated
chloride. The LUMO is also delocalized primarily (91%) on the
imine-ligand, with little contribution coming from two proximal
phenyl rings belonging to two PPh3 ligands [35]. The intense
absorption displayed by the 1-R complexes near 380 nm may
therefore be assigned to a transition taking place from the filled
(HOMO) to the vacant orbital of the imine-ligand (LUMO). In the
complex 2, nature of the frontier molecular orbitals is very similar
to that of the 1-R complexes, and hence the lowest energy
absorption in this complex is assignable to a transition within the
orbitals of the imine-ligand.
Table 2
Table 3
Electronic spectral and cyclic voltammetric data.
Composition of selected molecular orbitals of the complexes.
Complex
Electronic spectral dataa
lmax, nm (ε, Mꢀ1 cmꢀ1
Cyclic voltammetric datab
E/V vs. SCE
% Contribution of fragments
)
Ir
PPh3
Imine-ligand
Hydride
Chloride
1-OCH3
1-CH3
1-H
1-Cl
1-NO2
383 (15,800), 261 (15,000)
379 (18,700), 289 (17,700)
382 (17,400), 274 (18,300)
380 (17,300), 298 (28,000)
416c (10,900), 303 (20,100),
261 (20,500)
1.30d, 0.98d, ꢀ1.40e
1.32d, 0.99d, ꢀ1.34e
1.29d, 0.99d, ꢀ1.38e
1.32d, 0.97d, ꢀ1.36e
1.46d, 1.03d, ꢀ0.83e
1-OCH3
1-CH3
1-H
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
10.9
6.8
10.9
5.0
11.0
4.8
11.0
3.7
0
2.4
0
2.3
0
2.1
0
2.1
0
0
0
0
84.0
90.8
84.1
92.7
83.9
93.1
83.8
94.2
79.8
97.6
94.1
97.5
0
0
0
0
0
0
0
0
0
0
0
0
5.1
0
5.0
0
5.1
0
5.2
0
2
492c (9300), 464 (13,400),
438 (10,900), 274 (20,400)
1.30d, 0.98d, ꢀ1.32e
1-Cl
a
In dichloromethane.
Solvent, acetonitrile; supporting electrolyte, TBHP; scan rate, 50 mV sꢀ1
Shoulder.
Epa value.
Epc value.
1-NO2
2
13.8
2.4
5.9
6.4
0
b
c
.
e
d
e
2.5
e

P. Paul, S. Bhattacharya / Journal of Organometallic Chemistry 713 (2012) 72e79
77
responses in the 1-R complexes does not show any systematic
variation with the nature of the substituent R.
3. Conclusions
The present study shows that upon reaction with Ir(PPh3)3Cl
the N-(aryl)pyrrole-2-aldimines undergo iridium mediated
activation of the NeH bond generating a hydrido-complex of iri-
dium(III) as intermediate where the hydride is trans to the imine-
nitrogen. In case an aryl CeH bond comes in close proximity to the
IreCl fragment in this intermediate, rapid cyclometallation
occurs, which is manifested in the reaction with N-(napthyl)
pyrrole-2-aldimine (L-nap) leading to formation of organoiridium
complex 2. However, in case the IreCl fragment in this interme-
diate does not find any close CeH bond to react with, the inter-
mediate is transformed into a thermodynamically more stable
isomer where the hydride is cis to the imine-nitrogen, and this is
exemplified in the reactions with N-(40-R-phenyl)pyrrole-2-
aldimines (L-R) yielding 1-R complexes.
4. Experimental
4.1. Materials
Iridium trichloride was purchased from Arora Matthey, Kolkata,
India. Pyrrole-2-aldehyde was purchased from Spectrochem, Kol-
kata, India. Triphenylphosphine and the para-substituted anilines
were purchased from S.D. Fine-Chem Limited, Mumbai, India. All
other chemicals and solvents were reagent grade commercial
materials and were used as received. Ir(PPh3)3Cl was prepared by
following a reported method [7]. The N-(aryl)pyrrole-2-aldimines
were prepared by condensing pyrrole-2-aldehyde with para-
substituted anilines or a-naphthylamine in hot ethanol. Tetrabu-
tylammonium hexafluorophosphate (TBHP) was purchased from
Aldrich.
4.2. Physical measurements
Microanalyses (C, H, N) were performed using a Heraeus Carlo
Erba 1108 elemental analyzer. Magnetic susceptibilities were
measured using a Sherwood MK-1 balance. IR spectra were
obtained on a PerkineElmer 783 spectrometer with samples
prepared as KBr pellets. Electronic spectra were recorded on
a JASCO V-570 spectrophotometer. 1H NMR spectra were recorded
in CDCl3 solution with a Bruker Avance DPX 300 NMR spectrometer
using TMS as the internal standard. Electrochemical measurements
were made using a CH Instruments model 600A electrochemical
analyzer. A platinum disc working electrode, a platinum wire
auxiliary electrode and an aqueous saturated calomel reference
electrode (SCE) were used in the cyclic voltammetry experiments.
All electrochemical experiments were performed under a dini-
trogen atmosphere. All electrochemical data were collected at
298 K and are uncorrected for junction potentials. Optimization of
ground-state structures and energy calculations for all the
complexes were carried out by density functional theory (DFT)
method using the Gaussian 03 (B3LYP/SDD-6-31G) package [34].
Fig. 4. Contour plot of the HOMO and LUMO of 1-OCH3 complex.
2.3. Electrochemical properties
Electrochemical properties of the iridium complexes have been
studied by cyclic voltammetry in acetonitrile solution (0.1 M TBHP).
Voltammetric data are given in Table 2 and a selected voltammo-
gram is shown in Fig. S4 (Supplementary material). Each complex
shows two oxidative responses on the positive side of SCE and
a reductive response on the negative side. All the responses are
found to be irreversible in nature. In view of composition of the
HOMO in all these complexes, the first oxidative response is
assigned to oxidation of the coordinated imine-ligand. Similarly,
based on the composition of the LUMO, the reduction is assigned to
reduction of the coordinated imine-ligand. Potential of the redox
4.3. Syntheses of complexes
All the five 1-R complexes, as well as complex 2, were prepared
by following a general procedure. Specific details are given below
for a particular complex.
1-OCH3: N-(40-methoxyphenyl)pyrrole-2-aldimine (23 mg,
0.10 mmol) was dissolved in warm ethanol (40 mL). A stream of
nitrogen was passed through this solution for 5 min, and then

78
P. Paul, S. Bhattacharya / Journal of Organometallic Chemistry 713 (2012) 72e79
Ir(PPh3)3Cl (100 mg, 0.10 mmol) was added to it. The mixture was
then refluxed for 24 h under a nitrogen atmosphere to yield
a yellowish-brown solution. The solvent was then evaporated and
the solid mass, thus obtained, was subjected to purification by thin
layer chromatography on a silica plate. With benzene as the eluant,
a yellow band separated, which was extracted with acetonitrile.
Evaporation of the acetonitrile extract gave 1-OCH3 as a yellow
crystalline solid.
J ¼ 7.9); 7.17e7.49 (2PPh3); 7.72 (d, H, J ¼ 7.4); 7.92 (d, H, J ¼ 7.8). IR:
2161, 1620, 1502, 1460, 1417, 1294, 1244, 1203, 1132, 1092, 1031, 872,
833, 750, 724, 606, 552 and 522 cmꢀ1
.
4.4. X-ray crystallography
Single crystals of 1-OCH3 and 2 were obtained by slow
evaporation of solvent from solution of the respective complexes
in acetonitrile. Selected crystal data and data collection param-
eters are given in Table 4. Data were collected on a Bruker
SMART CCD diffractometer using graphite monochromated
1-OCH3: Yield: (55%); Anal. Calc. for C48H42N2OClP2Ir: C, 60.52;
H, 4.41; N, 2.94. Found: C, 60.53; H, 4.39; N. 2.98%. 1H NMR
[36]: ꢀ10.142 (t, hydride, J ¼ 19.0); 3.25 (OCH3); 6.32 (t, H, J ¼ 5.9);
6.73 (d, 2H, J ¼ 7.0); 6.90 (d, 2H, J ¼ 7.4); 7.15e7.65 (2PPh3); 8.34 (d,
H, J ¼ 8.9); 8.51 (d, H, J ¼ 8.1). IR: 2161, 1571, 1505, 1482, 1435, 1385,
1311, 1245, 1184, 1120, 1092, 1038, 828, 807, 745, 722, 695, 541 and
MoK
a
radiation (
l
¼ 0.71073 Å). X-ray data reduction, structure
solution and refinement were done using SHELXS-97 and
SHELXL-97 programs [37]. The structures were solved by the
direct methods.
521 cmꢀ1
.
1-CH3: Yield: (56%); Anal. Calc. for C48H42N2ClP2Ir: C, 61.56; H,
4.49; N, 2.99. Found: C, 61.59; H, 4.44; N, 2.93. 1H NMR: ꢀ10.151 (t,
hydride, J ¼ 19.5); 2.29 (CH3); 6.34 (t, H, J ¼ 6.1); 6.72 (d, 2H, J ¼ 7.3);
6.93 (d, 2H, J ¼ 7.5); 7.14e7.67 (2PPh3); 8.33 (d, H, J ¼ 9.0); 8.52 (d,
H, J ¼ 8.4). IR: 2161, 1573, 1502, 1485, 1436, 1382, 1309, 1244, 1185,
Acknowledgments
The authors thank the reviewers for their constructive
comments, which have been helpful in preparing the revised
manuscript. Financial assistance received from the Department of
Science and Technology, New Delhi, India [Grant No. SR/S1/IC-29/
2009 and SR/S1/RFIC-01/2009] is gratefully acknowledged. Piyali
Paul thanks the Council of Scientific and Industrial Research, New
Delhi, for her fellowship [Grant No. 09/096(0588)/2009-EMR-I].
1118, 1090, 1035, 827, 809, 746, 724, 694, 545 and 521 cmꢀ1
.
1-H: Yield: (51%); Anal. Calc. for C47H40N2ClP2Ir: C, 61.19; H,
4.34; N, 3.02. Found: C, 60.14; H, 4.30 N, 2.99. 1H NMR: ꢀ10.158 (t,
hydride, J ¼ 19.0); 6.32 (t, H, J ¼ 6.7); 6.52 (d, 2H, J ¼ 7.3); 6.92 (d,
2H, J ¼ 7.5); 7.10e7.64 (2PPh3); 7.55 (t, H, J ¼ 7.7); 8.32 (d, H, J ¼ 9.0);
8.47 (d, H, J ¼ 8.3). IR: 2162, 1569, 1504, 1481, 1433,1384, 1312, 1242,
1187, 1122, 1093, 1037, 824, 810, 745, 725, 695, 543 and 520 cmꢀ1
.
1-Cl: Yield: (53%); Anal. Calc. for C47H39N2Cl2P2Ir: C, 58.98; H,
4.08; N, 2.92. Found: C, 58.97; H, 4.06; N, 2.95%. 1H NMR: ꢀ10.164 (t,
hydride, J ¼ 19.5); 6.36 (t, H, J ¼ 6.2); 6.73 (d, 2H, J ¼ 7.6); 6.93 (d,
2H, J ¼ 7.8), 7.14e7.67 (2PPh3); 8.31 (d, H, J ¼ 8.9); 8.53 (d, H,
J ¼ 8.4). IR: 2161, 1574, 1508, 1480, 1432, 1387, 1310, 1245, 1183, 1118,
Appendix A. Supplementary material
CCDC 866123 and 866124 contain the supplementary crystal-
lographic data for this paper. DFT optimized structures of inter-
mediate B-OCH3 and complex 1-OCH3 (Fig. S1), packing diagram of
complex 2 (Fig. S2), contour plot of the HOMO and LUMO of
complex 2 (Fig. S3), and cyclic volammograms of complex 1-OCH3
(Fig. S4) are available as supplementary material.
1092, 1037, 825, 809, 746, 724, 694, 542 and 521 cmꢀ1
.
1-NO2: Yield: (48%); Anal. Calc. for C47H39N3O2ClP2Ir: C, 58.34;
H, 4.03; N, 4.34. Found: C, 58.36; H, 3.99; N, 4.35%. 1H NMR: ꢀ10.139
(t, hydride, J ¼ 19.1); 6.39 (t, H, J ¼ 6.5); 6.94 (4H)*; 7.08e7.57
(2PPh3); 8.37 (d, H, J ¼ 9.0); 8.58 (d, H, J ¼ 8.4). IR: 2160, 1577,
1548, 1509, 1485, 1434, 1388, 1342, 1315, 1249, 1187, 1125, 1095,
References
1041, 831, 811, 746, 727, 695, 549 and 522 cmꢀ1
.
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1-OCH3
48H42N2OClP2Ir
951.7
monoclinic
P21/c
22.0419(7)
10.3096(3)
18.9448(6)
108.368(1)
4085.7(2)
4
2
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C
C51H41N2P2Ir
894.2
monoclinic
P21/c
18.9958(7)
10.2964(3)
22.3306(8)
110.894(2)
4080.4(2)
4
b (Å)
c (Å)
b
(ꢁ)
V (Å3)
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Z
F (000)
1904
1872
Crystal size (mm)
T (K)
0.17 ꢂ 0.12 ꢂ 0.10
0.17 ꢂ 0.15 ꢂ 0.10
298
298
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0.79
a
R1
b
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P
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P
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b
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c
N is the number of parameters refined.

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