Interaction of N-(aryl)picolinamides with iridium. N-H and C-H bond activations

Article abstract of DOI:10.1016/j.jorganchem.2008.07.027

Reaction of N-(4-R-phenyl)picolinamide (R = OCH₃, CH₃, H, Cl and NO₂) with [Ir(PPh₃)₃Cl] in refluxing ethanol in the presence of a base (NEt₃) affords two yellow complexes (1-R and 2-R). The 1-R complexes contain an amide ligand coordinated to the metal center as a monoanionic bidentate N,N donor along with two triphenylphosphines, a chloride and a hydride. The 2-R complexes contain an amide ligand coordinated to the metal center as a monoanionic bidentate N,N donor along with two triphenylphosphines and two hydrides. Similar reaction of N-(naphthyl)picolinamide with [Ir(PPh₃)₃Cl] affords two organometallic complexes, 3 and 4. In complex 3 the amide ligand is coordinated to the metal center, via C-H activation of the naphthyl ring at the 8-position, as a dianionic tridentate N,N,C donor, along with two triphenylphosphines and one chloride. Complex 4 is similar to complex 3, except a hydride is bonded to iridium instead of the chloride. Structures of the 1-OCH₃, 2-Cl and 4 complexes have been determined by X-ray crystallography. All the complexes are diamagnetic, and show characteristic ¹H NMR signals and intense MLCT transitions in the visible region. Cyclic voltammetry on all the complexes shows a Ir^III-Ir^IV oxidation within 0.50-1.16 V vs. SCE and a reduction of the coordinated amide ligand within -1.02 to -1.25 V vs. SCE.

Full text of DOI:10.1016/j.jorganchem.2008.07.027

Journal of Organometallic Chemistry 693 (2008) 3281–3288
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Interaction of N-(aryl)picolinamides with iridium.
N–H and C–H bond activations
Moutusi Dasgupta a, Haregewine Tadesse b, Alexander J. Blake b, Samaresh Bhattacharya a,
*
a Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700 032, India
b School of Chemistry, University of Nottingham, University Park, Nottingham NG72RD, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 May 2008
Received in revised form 18 July 2008
Accepted 22 July 2008
Available online 27 July 2008
Reaction of N-(4-R-phenyl)picolinamide (R = OCH3, CH3, H, Cl and NO2) with [Ir(PPh3)3Cl] in refluxing
ethanol in the presence of a base (NEt3) affords two yellow complexes (1-R and 2-R). The 1-R complexes
contain an amide ligand coordinated to the metal center as a monoanionic bidentate N,N donor along
with two triphenylphosphines, a chloride and a hydride. The 2-R complexes contain an amide ligand
coordinated to the metal center as a monoanionic bidentate N,N donor along with two triphenylphos-
phines and two hydrides. Similar reaction of N-(naphthyl)picolinamide with [Ir(PPh3)3Cl] affords two
organometallic complexes, 3 and 4. In complex 3 the amide ligand is coordinated to the metal center,
via C–H activation of the naphthyl ring at the 8-position, as a dianionic tridentate N,N,C donor, along with
two triphenylphosphines and one chloride. Complex 4 is similar to complex 3, except a hydride is bonded
to iridium instead of the chloride. Structures of the 1-OCH3, 2-Cl and 4 complexes have been determined
by X-ray crystallography. All the complexes are diamagnetic, and show characteristic 1H NMR signals and
intense MLCT transitions in the visible region. Cyclic voltammetry on all the complexes shows a IrIII–IrIV
oxidation within 0.50–1.16 V vs. SCE and a reduction of the coordinated amide ligand within ꢀ1.02 to
ꢀ1.25 V vs. SCE.
Keywords:
N-(Aryl)picolinamides
Iridium
N–H and C–H bond activations
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
the amide linkage plays a key role in the formation and mainte-
nance of protein architectures, which are crucial for their perfor-
The chemistry of iridium has been receiving considerable cur-
rent attention [1], largely because of the interesting properties
exhibited by the complexes of this metal. As properties of the
complexes are dictated primarily by the coordination environ-
ment around the metal center, complexation of iridium by ligands
of selected types has been of significant importance, and the pres-
ent work has originated from our interest in this area [2]. Herein
we have selected a group of amide ligands (L1) derived from
picolinic acid and para-substituted anilines. The selected amide li-
gands (L1) are known to bind to metal ions usually as neutral
N,O-donor (amide form, I) or monoanionic N,N-donor (amidate
form, II) via loss of the amide proton [3]. A third mode of chela-
tion, as monoanionic N,O-donor (III), has also been observed by us
[4]. Chemistry of the amide ligands is of particular interest with
reference to their role in biological processes [5]. For example,
mance in biological systems [6]. Chelation in the amidate mode
(II) is known to stabilize metal ions in their higher oxidation
states, while that in the amide mode (I) is reported to favor rela-
tively lower oxidation states of a metal [3]. Interconversion be-
tween the amide (I) and amidate (II) modes of binding has been
utilized to chemically manipulate redox properties of the metal
center [7]. It may be mentioned here that though the chemistry
of amide complexes of many transition metals has been exten-
sively studied [8], that of iridium appears to have received much
less attention [9]. As the source of iridium the [Ir(PPh3)3Cl] com-
plex has been chosen, because of its demonstrated ability to un-
dergo facile reaction with ligands having dissociable protons
affording stable mixed-ligand hexacoordinated complexes [2].
Reaction of the selected amides (L1) as well as a related ligand,
viz. N-(naphthyl)picolinamide (L2), with [Ir(PPh3)3Cl] has been
found to afford interesting complexes, where the amides (L1 and
L2) are bound to the metal center as in II and IV, respectively.
The chemistry of all these complexes is reported here, with spe-
cial reference to their formation, characterization and, spectral
and electrochemical properties.
* Corresponding author.
E-mail address: samaresh_b@hotmail.com (S. Bhattacharya).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.07.027

3282
M. Dasgupta et al. / Journal of Organometallic Chemistry 693 (2008) 3281–3288
N
N
O
N
N
N
O
M
M
M
H
NH
N
N
O
O
R = OCH3, CH3,
H, Cl, NO2
R
R
R
R
1
I
II
III
L
N
N
M
O
NH
O
N
2
IV
L
sence of any solvent of crystallization in the lattice of 1-OCH3
indicates possible existence of non-covalent interaction(s) be-
tween the individual complex molecules. A closer look at the pack-
ing pattern of the crystal (shown in Fig. 2) reveals that C–Hꢁ ꢁ ꢁO and
2. Results and discussion
2.1. Synthesis and characterization
C–Hꢁ ꢁ ꢁ
p interactions, involving phenyl C–H, are active in the lat-
As delineated in the introduction, the primary objectives of the
present study have been to study the interaction of the selected N-
(4-R-phenyl)picolinamides (L1) with iridium and see how they
bind to the metal center. The five different substituents, with dif-
ferent electron withdrawing properties, have been chosen to study
their influence, if any, on the redox properties of the resulting com-
plexes. Reaction of these amides (L1) with [Ir(PPh3)3Cl] proceeds
smoothly in refluxing ethanol in the presence of triethylamine.
From each of these reactions two yellow complexes (1-R and 2-
R) have been obtained in comparable and decent yields. Prelimin-
ary characterizations (microanalysis, IR, NMR, etc.) on the 1-R
complexes indicate presence of an amide ligand, a chloride, a hy-
dride and two triphenylphosphines in the coordination sphere. In
the 2-R complexes, however, it has been found that along with a
coordinated amide ligand, two triphenylphosphines and two hy-
drides are also present.
In order to find out stereochemistry of these complexes, as well
as to ascertain coordination mode of the amide ligand in them,
structure of a representative member from both the series, viz. 1-
OCH3 and 2-Cl, has been determined by X-ray crystallography.
Structure of the 1-OCH3 complex is shown in Fig. 1 and some rel-
evant bond parameters are listed in Table 2. The structure shows
that the N-(4-methoxyphenyl)picolinamide is coordinated to irid-
ium, via dissociation of the amide proton, as a bidentate N,N-donor
forming a five-membered chelate ring (II). Two triphenylphos-
phines, a chloride and a hydride are also coordinated to the metal
center. The coordinated picolinamide, hydride and chloride have
constituted an equatorial plane with the metal at the center, where
the chloride is trans to the amide nitrogen and the hydride is trans
to the pyridine nitrogen. The two PPh3 ligands have occupied the
remaining two axial positions and hence they are mutually trans.
The Ir–H1, Ir–N1, Ir–P1, Ir–P2 and Ir–Cl1 distances are quite nor-
mal [2], and so are the distances within the chelated amide ligand
[3,8]. However, the Ir–N4 bond is noticeably longer than the Ir–N1
bond. The observed elongation of the Ir–N4 bond is thus attribut-
able to the trans effect of the coordinated hydride (H1). The ab-
tice. Each complex molecule is thus linked with the surrounding
complex molecules through these hydrogen-bonding interactions,
and these extended intermolecular interactions seem to be respon-
sible for holding the crystal together. It may be relevant to note
here that such non-covalent interactions are of significant impor-
tance in crystal engineering and biology [10]. As all the 1-R com-
plexes have been synthesized similarly and they show similar
properties (vide infra), the other four 1-R (R ¼ OCH3) complexes
are assumed to have similar structures as the 1-OCH3 complex.
Structure of the 2-Cl complex (shown in Fig. 3) is found to be
qualitatively similar to that of the 1-OCH3 complex, except that a
second hydride is coordinated to iridium trans to the pyridine ring
in the former instead of the chloride in the latter. While most of the
bond distances (Table 2) are comparable to those observed in the
earlier structure, the Ir–N2 length is significantly longer in this
complex compared to that in the 1-OCH3 complex, which is attrib-
utable, as before, to the trans effect of the second hydride (H2). To
find out the nature of intermolecular interactions in the lattice of
2-Cl, its packing pattern has been examined, which shows that
C–Hꢁ ꢁ ꢁCl, C–Clꢁ ꢁ ꢁ
p
and C–Hꢁ ꢁ ꢁ
p interactions are active (Fig. S1, Sup-
plementary material) and they link the individual complex mole-
cules to form a stable lattice. As all the five complexes in this
series have been synthesized similarly and they show similar prop-
erties (vide infra), the other four (R ¼ Cl) complexes are assumed to
have similar structures as the 2-Cl complex.
Formation of the 1-R and 2-R complexes, containing one and
two coordinated hydrides, respectively, from the same synthetic
reaction has been quite interesting. While the exact sequences be-
hind formation of these two types of complexes are not exactly
clear to us, the speculated steps illustrated in Scheme 1 seem
probable. In the initial step the amide ligand (L1) interacts with
the metal center in [Ir(PPh3)3Cl], whereby oxidative insertion of
iridium into the N–H bond of the amide ligand takes place associ-
ated with the simultaneous loss of a PPh3 from the iridium starting
material, and thus complex 1-R is formed. Precedence of such bond

M. Dasgupta et al. / Journal of Organometallic Chemistry 693 (2008) 3281–3288
3283
Table 2
Selected bond lengths (Å) and bond angels (°) for the 1-OCH3, 2-Cl and 4 complexes
Complex 1-OCH3
Bond distances (Å)
Ir(1)–Cl(1)
Ir(1)–P(1)
Ir(1)–P(2)
Ir(1)–N(1)
Ir(1)–N(4)
2.3990(17)
2.3331(14)
2.3117(13)
2.077(4)
C(2)–O(2)
C(2)–N(1)
C(9)–N(1)
1.246(6)
1.363(6)
1.436(6)
2.144(5)
Bond angles (°)
P(1)–Ir(1)–P(2)
N(1)–Ir(1)–Cl(1)
172.51(9)
174.65(11)
N(1)–Ir(1)–N(4)
78.26(16)
Complex 2-Cl
Bond distances (Å)
Ir(1)–P(1)
Ir(1)–P(2)
Ir(1)–N(2)
2.2890(8)
2.3046(8)
2.179(3)
2.136(2)
C3–O1
C3–N2
C10–N2
1.246(4)
1.338(4)
1.423(4)
Ir(1)–N5
Bond angles (°)
P(1)–Ir(1)–P(2)
165.72(3)
N(2)–Ir(1)–N(5)
76.78(9)
Complex 4
Bond distances (Å)
Ir(1)–P(1)
Ir(1)–P(2)
Ir(1)–C(10)
Ir(1)–N(1)
Fig. 1. View of the 1-OCH3 complex.
2.3124(11)
2.3010(11)
2.041(4)
2.068(3)
2.135(3)
C(3)–O(1)
C(3)–N(1)
C(18)–N(1)
1.241(4)
1.348(5)
1.409(5)
Table 1
Crystallographic data for 1-OCH3, 2-Cl and 4 complexes
Ir(1)–N(5)
Bond angles (°)
P(1)–Ir(1)–P(2)
C(10)–Ir(1)–N(5)
Complex 1-OCH3
49H42N2O2ClP2Ir
980.44
Orthorhombic
Pna21
Complex 2-Cl
Complex 4
172.72(4)
159.84(12)
C(10)–Ir(1)–N(1)
N(1)–Ir(1)–N(5)
81.54(12)
78.33(11)
Empirical formula
Fw
Crystal system
Space group
a (Å)
C
C48H40N2OClP2Ir
950.41
Monoclinic
P21/c
C52H41N2OP2Ir
964.01
Monoclinic
Cc
11.3149(8)
21.7520(15)
17.5661(12)
90
35.583(11)
9.790(3)
11.552(4)
90
12.1755(7)
24.2409(15)
14.3847(9)
90
b (Å)
c (Å)
a
(°)
b (°)
90
90
4042(2)
4
110.432(2)
90
3978.5(4)
4
108.4980(10)
90
4100.0(5)
4
c
(°)
V (Å3)
Z
k (Å)
F (000)
0.71073
1956
0.71073
1888
0.71073
1924
Crystal size (mm)
T (K)
0.02 ꢂ 0.05 ꢂ 1.50
0.09 ꢂ 0.16 ꢂ 0.54
0.05 ꢂ 0.12 ꢂ 0.22
150
150
150
l
R1
wR2
(mmꢀ1
)
3.508
3.544
0.0273
0.0686
1.03
3.377
0.0238
0.0530
1.08
a
0.0323
0.0647
1.08
b
GOFc
P
P
a
R1
=
||Fo| ꢀ |Fc||/ |Fo|.
P
P
b
c
2
wR2 = [ [w(Fo ꢀ Fc2)2]/ [w(Fo2)2]]1/2
.
P
GOF = [ [w(Fo ꢀ Fc2)2]/(M ꢀ N)]1/2, where M is the number of reflections and N
2
is the number of parameters refined.
activation by iridium(I) forming hydride complex of irdium(III) has
been observed before [2]. In the next step the Ir–Cl bond in the 1-R
complex reacts with ethanol in the presence of NEt3, whereby it is
converted into an Ir–H bond and thus complex 2-R is produced.
Such conversion of M–Cl into M–H in alcoholic medium is well
documented in the literature [11]. Evidence for this transformation
comes from the fact that ethanolic solution of any 1-R complex is
observed to slowly convert into the corresponding 2-R complex in
the presence of NEt3 [12]. Therefore, source of the hydride in 1-R
complex is the N–H fragment of the amide ligand and that of the
second hydride in 2-R complex is the solvent (ethanol). It is rele-
vant to mention in this context that when reaction of the amide li-
gands (L1) with [Ir(PPh3)3Cl] is carried out in a nonalcoholic solvent
(viz. toluene) in the presence of triethylamine, only the 1-R com-
plexes are obtained, which also supports our conclusion stated
above regarding origin of hydride(s) in the 1-R and 2-R complexes.
Fig. 2. C–Hꢁ ꢁ ꢁO (top) and C–Hꢁ ꢁ ꢁ
p (bottom) interactions in the lattice of the 1-OCH3
complex.
Disposition of the Ir–H bond, which is trans to the pyridine
nitrogen, with respect to the pendent phenyl ring in both the 1-R
and 2-R complexes points to the possibility of a C–H activation
of the aryl ring. However, such an activation could not take place
in the 1-R or 2-R complexes, because in them the phenyl ring is
not close enough to the metal center and had the phenyl ring
undergone an orthometallation, it would have resulted in the

3284
M. Dasgupta et al. / Journal of Organometallic Chemistry 693 (2008) 3281–3288
formation of a sterically unstable four-membered metallacycle. In
order to induce a facile C–H activation the aryl fragment needs
to be so chosen that one of its C–H bond should come in close prox-
imity to the Ir–H bond. With this simple strategy in mind, a new
amide ligand, viz. N-(naphthyl)picolinamide (L2); where a naphthyl
ring has replaced the phenyl ring in L1, has been chosen as the tar-
get ligand for the C–H activation. Reaction of N-(naphthyl)picolina-
mide with [Ir(PPh3)3Cl] has been carried out similarly as before,
which has afforded two orange complexes (3 and 4). While yield
of complex 3 is low, that of complex 4 is good. Preliminary charac-
terizations on complex 3 indicate the presence of an amide ligand,
two triphenylphosphines and
a chloride in the coordination
sphere. Composition of complex 4 has been found to be similar
to that of complex 3, except a hydride is coordinated to iridium in-
stead of the chloride. Structural characterization of complex 3 has
not been possible, as its crystals could not be grown. However, it
has been assumed to have a similar (hydride replaced by chloride)
structure as complex 4. Structure of complex 4 has been deter-
mined by X-ray crystallography. The structure (shown in Fig. 4)
shows that the N-(naphthyl)picolinamide is indeed coordinated
to iridium in the expected N,N,C-fashion (IV). A hydride is coordi-
nated trans to the amide nitrogen, and two triphenylphosphines
are occupying the axial positions. The Ir–C distance is normal [2],
and the other bond distances are comparable to those in the previ-
ous complexes (Table 2). An examination of the packing pattern of
Fig. 3. View of the 2-Cl complex.
complex 4 shows that C–Hꢁ ꢁ ꢁO and C–Hꢁ ꢁ ꢁ
p interactions are active
(Fig. S2, Supplementary material) in the lattice.
The speculated sequences behind formation of the two organo-
iridium complexes (3 and 4) from the same synthetic reaction are
shown in Scheme 2. As shown earlier in Scheme 1, the N–H frag-
ment of the amide ligand (L2) is believed to add oxidatively to
the metal center in [Ir(PPh3)3Cl] generating a hydride intermediate
(A), which is gradually converted into a dihydride derivative (B).
These intermediates then undergo cyclometallation via elimina-
tion of molecular hydrogen affording the organoiridium complexes
(3 and 4) [13]. It is relevant to mention here that when similar
reaction of the amide ligand (L2) with [Ir(PPh3)3Cl] is carried out
in toluene, complex 3 is obtained as the sole product. This demon-
strates that, as described before, in the absence of alcoholic solvent
the dihydride intermediate B is not formed and thus complex 4 is
not obtained.
2.2. Spectral studies
Magnetic susceptibility measurements show that all the iridium
complexes are diamagnetic, which corresponds to the +3 oxidation
state of iridium (low-spin d6, S = 0) in these complexes. 1H NMR
spectra of the 1-R complexes show broad signals within 7.09–
7.61 ppm for the coordinated PPh3 ligands. The hydride signal is
Fig. 4. View of complex 4.
P
P
N
N
N
N
N
Cl
H
H
[IrI(PPh3)3Cl]
III
Ir
III
Ir
H
ethanol, NEt3
N
O
O
H
O
_
PPh3
P
P
R
R
R
L1
1-R
2-R
Scheme 1. Probable steps for the formation of the 1-R and 2-R complexes.

M. Dasgupta et al. / Journal of Organometallic Chemistry 693 (2008) 3281–3288
3285
P
P
N
N
N
N
N
N
Cl
H
H
[IrI(PPh3)3Cl]
ethanol, NEt3
III
III
Ir
Ir
H
O
O
O
H
_
PPh3
P
P
L2
A
B
P
H2
H2
P
N
N
Cl
H
III
Ir
III
Ir
N
N
O
O
P
P
3
4
Scheme 2. Probable steps for the formation of complexes 3 and 4.
near ꢀ19.3 and ꢀ20.4 ppm. Besides small shifts in the signal posi-
tions, rest of the 1H NMR spectrum of each 2-R complex is qualita-
tively similar to that of the corresponding 1-R complex. 1H NMR
spectrum of complex 3 is mostly similar to that of the 1-H com-
plex. Besides the hydride signal at ꢀ14.48 ppm, 1H NMR spectrum
of complex 4 is similar to that of complex 3.
Table 3
Electronic spectral and cyclic voltammetric data of the complexes
Compound Electronic spectral dataa kmax, nm (
Mꢀ1 cmꢀ1
e,
Cyclic voltammetric
)
datab E/V vs. SCE
1-OCH3
1-CH3
1-H
1-Cl
1-NO2
413 (3500), 278c (17650), 239 (35100)
416 (3300), 270c (17200), 232 (34900)
414 (3300), 276c (18300), 238 (33300)
414 (3000), 272c (18900), 236 (33000)
437 (9400), 298c (11800), 270 (17400), 234
(35900)
0.78d(75)e, ꢀ1.12f
0.78d(76)e, ꢀ1.18f
1.01d(93)e, ꢀ1.12f
1.03d(87)e, ꢀ1.08f
1.16d(98)e, ꢀ1.17f
Infrared spectrum each 1-R complex shows many bands of dif-
ferent intensities in the 400–4000 cmꢀ1 region. No attempt has
been made to assign each individual band to a specific vibration.
However, comparison with the spectra of the corresponding unco-
ordinated ligands shows that the N–H stretch, observed near
3140 cmꢀ1 in the uncoordinated ligand, is absent in the complexes.
The amide C@O stretch, observed at around 1680 cmꢀ1 in the
uncoordinated ligand, is also found to be shifted to around
1579 cmꢀ1 in the 1-R complexes. A strong band displayed near
2181 cmꢀ1 by the 1-R complexes is due to the coordinated hydride.
Three strong bands have been observed near 517, 692 and
746 cmꢀ1 in 1-R complexes indicating the presence of coordinated
PPh3 ligands [2]. Several sharp bands (e.g. near 823, 1093, 1178,
1245, 1346, 1434 and 1502 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 amide ligand. In
the infrared spectra of the 2-R complexes two distinct Ir–H
stretches are observed near 2148 and 2120 cmꢀ1, and the rest of
the spectra are similar to those of the 1-R complexes. Infrared
spectra of complexes 3 and 4 are also similar to those of the earlier
2-OCH3
2-CH3
2-H
417 (4400), 359c (5500), 333 (8100), 267c
(24600), 243 (35300)
0.74g, ꢀ1.13f
0.68g, ꢀ1.02f
0.80g, ꢀ1.12f
0.78g, ꢀ1.25f
0.93g, ꢀ1.16f
0.50g, ꢀ1.23f
0.51g, ꢀ1.22f
414 (3900), 359c (5000), 335 (7900), 274c
(19400), 243 (33100)
407 (3300), 350c (4700), 333 (6500), 265c
(1900), 243 (28800)
2-Cl
2-NO2
3
413 (4100), 346c (6400), 329 (8400), 265c
(22800), 242 (32400)
434 (17700), 339c (10100), 325(13200),
270c (22600), 244 (36800)
478 (3400), 393c (5200), 349 (9300), 283
(13300), 296c (12000), 239 (35900)
489 (2900), 391c (5300), 346 (9200), 302
(10900), 285c (12200), 242 (34100)
4
a
In dichloromethane.
Solvent, 1:9 dichloromethane–acetonitrile; supporting electrolyte, TBAP; scan
b
rate, 50 mV sꢀ1
.
c
Shoulder.
d
E1/2 = 0.5(Epa + Epc), where Epa and Epc are anodic and cathodic peak potentials,
respectively.
complexes, where only complex
4 shows a Ir–H stretch at
e
f
2082 cmꢀ1
.
D
Ep = Epa ꢀ Epc
.
Epc value.
Epa value.
The 1-R and 2-R complexes are soluble in acetone, dichloro-
methane, chloroform, acetonitrile, etc., producing bright yellow
solutions. Complexes 3 and 4 also have similar solubility and pro-
duce orange solutions. Electronic spectra of all the complexes have
been recorded in dichloromethane solution. Spectral data are pre-
sented in Table 3. All the complexes show several intense absorp-
tions in the visible and ultraviolet regions. The absorptions in the
ultraviolet region are attributable to transitions within the ligand
orbitals, while those in the visible region are probably due to
charge-transfer transitions. To have a better insight into the nature
g
observed around ꢀ17.1 ppm. Signals for the methoxy and methyl
groups in the 1-OCH3 and 1-CH3 complexes are observed, respec-
tively, at 3.63 and 2.08 ppm. Most of the expected aromatic proton
signals from the coordinated amide 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. In the 1H NMR spec-
tra of the 2-R complexes two distinct hydride signals are observed

3286
M. Dasgupta et al. / Journal of Organometallic Chemistry 693 (2008) 3281–3288
Fig. 5. Partial molecular orbital diagram of the 1-H complex.
of absorptions in the visible region, qualitative EHMO calculations
have been performed [14] on computer-generated models of the
complexes. Partial MO diagrams for four selected complexes are
shown in Fig. 5 and Figs. S3–S5 (Supplementary material) and the
composition of selected molecular orbitals is given in Table S1
(Supplementary material). In the 1-R complexes, the highest occu-
pied molecular orbital (HOMO) and the next two filled orbitals
(HOMO-1 and HOMO-2) have major (ꢃ55%) contribution from
the iridium d-orbitals. The lowest unoccupied molecular orbital
(LUMO) is delocalized primarily on the amide ligand, and is con-
centrated mostly on the pyridyl ring and the amide fragment
[15]. The next couple of vacant orbitals (e.g. LUMO + 1 and
LUMO + 2) are also concentrated on the amide ligand. The intense
absorption displayed by the 1-R complexes near 410 nm may
therefore be assigned to a charge-transfer transition taking place
complexes. Hence the lowest energy absorption in the 2-R, 3 and 4
complexes are assignable to the HOMO-to-LUMO charge-transfer
transition.
2.3. Electrochemical properties
Electrochemical properties of the iridium complexes have been
studied by cyclic voltammetry in 1:9 dichloromethane–acetonitrile
solution (0.1 M TBAP) [16]. Voltammetric data are given in Table 3
and a selected voltammogram is shown in Fig. 6.
Each complex shows an oxidative response on the positive side
of SCE and a reductive response on the negative side. In the 1-R
complexes the oxidative response is quasi-reversible in nature,
and in view of composition of the HOMO in all these complexes,
this is assigned to IrIII–IrIV oxidation. One-electron nature of this
oxidation has been confirmed by comparing its current height
(ipa) with that of standard ferrocene–ferrocenium couple under
identical experimental conditions. In the other complexes (2-R, 3
and 4) the IrIII–IrIV oxidation appears as an irreversible response.
The reductive response is irreversible in nature in all the com-
plexes, and shows non-stoichiometric current (ipc). Based on com-
position of the LUMO, the reduction is assigned to reduction of the
coordinated amide ligand. Potential of the redox responses in the
1-R and 2-R complexes does not show any systematic variation
with the nature of the substituent R.
from the filled iridium d-orbital (HOMO) to the vacant
of the amide ligand (LUMO). In the 2-R, 3 and 4 complexes, nature
of the frontier molecular orbitals is very similar to that of the 1-R
p
*-orbital
3. Conclusions
The present study shows that upon reaction with [Ir(PPh3)3Cl]
the N-(aryl)picolinamides (L1) can readily bind to iridium as mono-
anionic N,N-donors, whereby iridium oxidatively inserts into the
amide N–H bond and thus generates an Ir–H bond. This study also
demonstrates that the Ir–H bond can be utilized for inducing C–H
activation of selective amides, such as N-(naphthyl)picolinamide.
4. Experimental
Iridium trichloride was purchased from Arora Matthey, Kolkata,
India. Triphenylphosphine, the para-substituted anilines and 2-
picolinic acid were purchased from S.D. Fine-Chem Limited, India.
Fig. 6. Cyclic voltammogram of the 1-CH3 complex in 1:9 dichloromethane–
acetonitrile solution (0.1 M TBAP) at a scan rate of 50 mV sꢀ1
.

M. Dasgupta et al. / Journal of Organometallic Chemistry 693 (2008) 3281–3288
3287
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. The N-(aryl)picolinamides were
prepared by condensing picolinic acid with para-substituted ani-
4.50; N, 3.01%. 1H NMR: ꢀ19.42 (d of t, hydride, Jd = 6.6 Hz,
Jt = 6.6 Hz); ꢀ20.20 (d of t, hydride, Jd = 6.6 Hz, Jt = 6.9 Hz); 3.68
(OCH3); 6.24 (d, 2H, J = 8.9 Hz); 6.32 (t, H, J = 6.4 Hz); 6.85 (d, 2H,
J = 8.9 Hz); 7.12–7.46 (2PPh3); 7.54 (d, H, J = 5.0 Hz); 7.73 (2H)*.
Complex 2-CH3: Yield: (33 mg, 36%); Anal. Calc. for C49H43N2OP2Ir:
C, 63.27; H, 4.62; N, 3.01. Found: C, 63.11; H, 4.68; N, 3.05%. 1H
NMR: ꢀ19.44 (d of t, hydride, Jd = 6.6 Hz, Jt = 6.9 Hz); ꢀ20.20 (d
of t, hydride, Jd = 6.6 Hz, Jt = 6.6 Hz); 2.21 (CH3); 6.34 (t, H,
J = 6.4 Hz); 6.48 (d, 2H, J = 8.1 Hz); 6.77 (d, 2H, J = 8.1 Hz); 7.13–
7.53 (2PPh3); 7.57 (d, H, J = 4.9 Hz); 7.71 (2H)*. Complex 2-H: Yield:
(35 mg, 39%); Anal. Calc. for C48H41N2OP2Ir: C, 62.39; H, 4.47; N,
3.05. Found: C, 63.55; H, 4.34; N, 3.07%. 1H NMR: ꢀ19.39 (d of t,
hydride Jd = 6.9 Hz, Jt = 6.6 Hz); ꢀ20.25 (d of t, hydride, Jd = 6.6 Hz,
Jt = 6.6 Hz); 6.35 (t, H, J = 6.2 Hz); 6.67 (2H)*; 6.94 (d, 2H,
J = 5.5 Hz); 7.12–7.4 (2PPh3); 7.56 (d, H, J = 5.1 Hz); 7.74 (2H)*.
Complex 2-Cl: Yield: (35 mg, 37%); Anal. Calc. for C48H40N2OClP2Ir:
C, 60.64; H, 4.21; N, 2.94. Found: C, 60.61; H, 4.25; N, 2.95%. 1H
NMR: ꢀ19.46 (d of t, hydride, Jd = 6.6 Hz, Jt = 6.6 Hz); ꢀ20.34 (d
of t, hydride, Jd = 6.9 Hz, Jt = 6.9 Hz); 6.35 (t, H, J = 6.3 Hz); 6.60
(d, 2H, J = 8.8 Hz); 6.88 (d, 2H, J = 8.8 Hz); 7.12–7.46 (2PPh3);
7.57 (d, H, J = 5.2 Hz); 7.73 (2H)*. Complex 2-NO2: Yield: (34 mg,
36%); Anal. Calc. for C48H40N3O3P2Ir: C, 59.98; H, 4.16; N, 4.37.
Found: C, 59.93; H, 4.22; N, 4.33%. 1H NMR: ꢀ19.39 (d of t, hydride,
Jd = 6.9 Hz, Jt = 6.6 Hz); ꢀ20.56 (d of t, hydride, Jd = 6.9 Hz,
Jt = 6.9 Hz); 6.43 (t, H, J = 6.3 Hz); 7.12–7.44 (2PPh3); 7.54 (3H)*;
7.63 (2H)*; 7.77 (2H)*.
lines or
a-naphthylamine [17] Purification of dichloromethane
and acetonitrile, and preparation of tetrabutylammonium perchlo-
rate (TBAP) for electrochemical work were performed as reported
in the literature [18]. Microanalyses (C, H, N) were performed using
a Heraeus Carlo Erba 1108 elemental analyzer. IR spectra were ob-
tained on a Perkin–Elmer 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 solu-
tion 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 ana-
lyzer. 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 elec-
trochemical experiments were performed under a dinitrogen
atmosphere. All electrochemical data were collected at 298 K and
are uncorrected for junction potentials.
4.1. Synthesis of complexes
Complexes 1-OCH3 and 2-OCH3: N-(4-methoxyphenyl)picolina-
mide (L1, R = OCH3) (23 mg, 0.10 mmol) was dissolved in warm
ethanol (40 mL) and to it was added triethylamine (10 mg,
0.10 mmol), followed by [Ir(PPh3)3Cl] (100 mg, 0.10 mmol). The
mixture was then refluxed for 24 h to yield a yellow solution.
The solvent was evaporated and the solid mass, thus obtained,
was subjected to purification by thin layer chromatography on a
silica plate. With 1:5 acetonitrile/benzene as the eluant, two yel-
low bands separated, which were extracted with acetonitrile.
Evaporation of these acetonitrile extracts gave 1-OCH3 and 2-
OCH3 as yellow crystalline solids.
Complexes 3 and 4: To a solution of N-(1-naphthyl)picolina-
mide (L2) (25 mg, 0.10 mmol) in warm ethanol (50 mL), triethyl-
amine (10 mg, 0.10 mmol) was added followed by [Ir(PPh3)3Cl]
(100 mg, 0.10 mmol). The mixture was refluxed for 24 h, whereby
an orange solution was obtained. Evaporation of this solution affor-
ded an orange solid, which was subjected to purification by thin
layer chromatography on a silica plate. With 1:5 acetonitrile–ben-
zene two orange bands separated, a thin light-orange band fol-
lowed by
a major deep orange band, both of which were
The other 1-R and 2-R (R ¼ OCH3) complexes were prepared by
following similar procedure as above using the respective amide li-
gands (L1, R ¼ OCH3).
extracted with acetonitrile. Evaporation of these extracts, respec-
tively, gave complexes 3 and 4 as crystalline orange solids.
Complex 3: Yield: (5 mg, 5%); Anal. Calc. for C52H40N2OP2ClIr: C,
62.57; H, 4.01; N, 2.80. Found: C, 62.73; H, 4.08; N, 2.79%. 1H NMR:
6.57 (t, H, J = 7.7 Hz); 6.78 (2H)*; 6.90 (d, H, J = 4.7 Hz); 6.93–7.12
(2PPh3); 7.55 (d, H, J = 6.5 Hz); 7.70 (t, H, J = 7.5 Hz); 7.91 (d, H,
J = 7.8 Hz); 8.10 (d, H, J = 8.2 Hz). Complex 4: Yield: (68 mg, 70%);
Anal. Calc. for C52H41N2OP2Ir: C, 64.78; H, 4.25; N, 2.90. Found: C,
64.56; H, 4.28; N, 2.93%. 1H NMR: ꢀ14.48 (t, hydride,
J = 17.8 Hz); 6.37 (2H)*; 6.56 (d, J = 6.6 Hz, H); 6.82 (d, H,
J = 7.9 Hz); 6.82 (2H)*; 6.99–7.34 (2PPh3); 7.35 (d, H, J = 5.2 Hz);
7.42 (t, H, J = 7.7 Hz); 7.83 (d, H, J = 7.8 Hz); 8.27 (d, H, J = 6.8 Hz).
Complex 1-OCH3: Yield: (34 mg, 35%); Anal. Calc. for
C49H42N2O2ClP2Ir: C, 60.01; H, 4.28; N, 2.85. Found: C, 60.13; H,
4.32; N, 2.83%. 1H NMR [19]: ꢀ17.07 (t, hydride, J = 14.1 Hz);
3.63 (OCH3); 6.08 (d, 2H, J = 9.0 Hz); 6.67 (t, H, J = 6.2 Hz); 6.86
(d, 2H, J = 9.0 Hz); 7.13–7.61 (2PPh3); 7.66 (t, H, J = 5.9 Hz); 7.84
(d, H, J = 7.7 Hz); 8.13 (d, H, J = 5.1 Hz). Complex 1-CH3: Yield:
(34 mg, 36%); Anal. Calc. for C49H42N2OClP2Ir: C, 61.01; H, 4.35;
N, 2.90. Found: C, 60.59; H, 4.34; N, 2.93%. 1H NMR: ꢀ17.10 (t, hy-
dride, J = 14.1 Hz); 2.08 (CH3); 6.31 (d, 2H, J = 8.3 Hz); 6.67 (t, 1H,
J = 6.3 Hz); 6.75 (d, 2H, J = 8.3 Hz); 7.12–7.56 (2PPh3); 7.54 (t, H,
J = 7.6 Hz); 7.83 (d, H, J = 7.8 Hz); 8.11 (d, H, J = 5.1 Hz). Complex
1-H: Yield: (30 mg, 32%); Anal. Calc. for C48H40N2OClP2Ir: C,
60.64; H, 4.21; N, 2.94. Found: C, 60.34; H, 4.20; N, 2.97%. 1H
NMR: ꢀ17.04 (t, hydride, J = 14.0 Hz); 6.52 (t, 1H, J = 7.1 Hz); 6.69
(t, 2H, J = 6.3 Hz); 6.96 (d, H, J = 7.1 Hz); 7.10–7.44 (2PPh3); 7.55
(t, H, J = 7.7 Hz); 7.87 (d, H, J = 7.7 Hz); 8.13 (d, H, J = 5.1 Hz). Com-
plex 1-Cl: Yield: (33 mg, 34%); Anal. Calc. for C48H39N2OCl2P2Ir: C,
58.52; H, 3.96; N, 2.84. Found: C, 58.67; H, 3.96; N, 2.81%. 1H NMR:
ꢀ17.10 (t, hydride, J = 14.1 Hz); 6.45 (d, 2H, J = 8.8 Hz); 6.70 (t, 1H,
J = 6.3 Hz); 6.91 (d, 2H, J = 8.8 Hz); 7.14–7.54 (2PPh3); 7.56 (t, H,
J = 7.6 Hz); 7.85 (d, H, J = 7.7 Hz); 8.16 (d, H, J = 5.0 Hz). Complex
1-NO2: Yield: (34 mg, 35%); Anal. Calc. for C48H39N3O3ClP2Ir: C,
57.90; H, 3.92; N, 4.22. Found: C, 57.69; H, 3.90; N, 4.25%. 1H
NMR: ꢀ17.03 (t, hydride, J = 14.1 Hz); 6.81 (t, H, J = 6.5 Hz); 6.94
(4H)*; 7.09–7.47 (2PPh3); 7.62 (t, H, J = 7.5 Hz); 7.90 (d, H,
J = 7.6 Hz); 8.28 (d, H, J = 5.2 Hz).
4.2. X-ray crystallography
Single crystals of 1-OCH3 were obtained by slow evaporation of
a solution of the complex in 1:1 dichloromethane-acetonitrile, and
those of 2-Cl and 3 were obtained similarly from acetonitrile solu-
tions of the complexes. Selected crystal data and data collection
parameters are given in Table 1. Data were collected on a Bruker
SMART 1000 CCD diffractometer using graphite monochromated
Mo Ka radiation (k = 0.71073 Å). X-ray data reduction, structure
solution and refinement were done using SHELXS-97 and SHELXL-97
programs [20]. The structures were solved by the direct methods.
Acknowledgements
The authors thank the referees for their constructive com-
ments, which have been helpful in preparing the revised manu-
script. Financial assistance received from the Department of
Science and Technology [Grant No. SR/S1/IC-15/2004] is gratefully
Complex 2-OCH3: Yield: (36 mg, 38%); Anal. Calc. for
C49H43N2O2P2Ir: C, 62.20; H, 4.54; N, 2.96. Found: C, 62.31; H,

3288
M. Dasgupta et al. / Journal of Organometallic Chemistry 693 (2008) 3281–3288
[4] M. Dasgupta, S. Bhattacharya, unpublished results.
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1187;
acknowledged. Thanks are also due to the University Grants Com-
mission, New Delhi [Grant No. F31-124/2005(SR)], for financial
help.
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Appendix A. Supplementary material
CCDC 672144, 672145 and 672146 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data
associated with this article can be found, in the online version, at
doi:10.1016/j.jorganchem.2008.07.027.
(i) M.H. Chou, D.J. Szalda, C. Creutz, N. Sutin, Inorg. Chem. 33 (1994) 1674;
(j) S.M. Redmore, C.D.F. Rickard, S.J. Webb, L.J. Wright, Inorg. Chem. 36 (1997)
4743.
[6] B. Albert, D. Bray, J. Lewis, M. Raff, K. Roberts, J.D. Watson, Molecular Biology of
the Cell, third ed., MacMillan, New York, 1994 (Chapter 3).
[7] T. Kojima, K. Hayashi, Y. Matsuda, Inorg. Chem. 43 (2004) 6793.
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(b) H.E. Bryndza, W. Tam, Chem. Rev. 88 (1988) 1163;
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