Interaction of 2-(2′,6′-dialkylphenylazo)-4-methylphenols with iridium. C-H activation and migration of alkyl group

Article abstract of DOI:10.1016/j.jorganchem.2010.01.015

Reaction of 2-(2′,6′-diethylphenylazo)-4-methylphenol (L²) with [Ir(PPh₃)₃Cl] afforded two organoiridium complexes 3 and 4 via C-H bond activation of an ethyl group in the arylazo fragment of the L² ligand. In both the complexes the azo ligand binds to iridium as a dianionic tridentate C,N,O-donor. Two triphenylphosphines and a hydride (in the case of complex 3) or chloride (in the case of complex 4) are also coordinated to the metal center. A similar reaction of [Ir(PPh₃)₃Cl] with 2-(2′,6′-diisopropylphenylazo)-4-methylphenol (L³) yielded another organoiridium complex 5, where migration of one iso-propyl group from its original location (say, the 2′ position) to the corresponding third position (say, the 4′ position) took place through C-C bond activation. In this complex the modified azo ligand binds to iridium as a dianionic tridentate C,N,O-donor. Two triphenylphosphines and a hydride are also coordinated to the metal center. The structures of complexes 3 and 4 have been optimized through DFT calculations. The structure of complex 5 has been determined by X-ray crystallography. All the complexes show characteristic ¹H NMR signals and intense transitions in the visible region. Cyclic voltammetry on all the complexes shows an oxidation within 0.66-1.10 V vs SCE, followed by a second oxidation within 1.15-1.33 V vs SCE and a reduction within -0.96 to -1.07 V vs SCE.

Full text of DOI:10.1016/j.jorganchem.2010.01.015

Journal of Organometallic Chemistry 695 (2010) 1111–1118
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Interaction of 2-(20,60-dialkylphenylazo)-4-methylphenols with iridium. C–H
activation and migration of alkyl group
Suparna Baksi a, Dipravath Kumar Seth 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 NG7 2RD, UK
a r t i c l e i n f o
a b s t r a c t
Reaction of 2-(20,60-diethylphenylazo)-4-methylphenol (L2) with [Ir(PPh3)3Cl] afforded two organoiridi-
um complexes 3 and 4 via C–H bond activation of an ethyl group in the arylazo fragment of the L2 ligand.
In both the complexes the azo ligand binds to iridium as a dianionic tridentate C,N,O-donor. Two triphe-
nylphosphines and a hydride (in the case of complex 3) or chloride (in the case of complex 4) are also
coordinated to the metal center. A similar reaction of [Ir(PPh3)3Cl] with 2-(20,60-diisopropylphenylazo)-
4-methylphenol (L3) yielded another organoiridium complex 5, where migration of one iso-propyl group
from its original location (say, the 20 position) to the corresponding third position (say, the 40 position)
took place through C–C bond activation. In this complex the modified azo ligand binds to iridium as a
dianionic tridentate C,N,O-donor. Two triphenylphosphines and a hydride are also coordinated to the
metal center. The structures of complexes 3 and 4 have been optimized through DFT calculations. The
structure of complex 5 has been determined by X-ray crystallography. All the complexes show character-
istic 1H NMR signals and intense transitions in the visible region. Cyclic voltammetry on all the complexes
shows an oxidation within 0.66–1.10 V vs SCE, followed by a second oxidation within 1.15–1.33 V vs SCE
and a reduction within ꢀ0.96 to ꢀ1.07 V vs SCE.
Article history:
Received 30 October 2009
Received in revised form 1 January 2010
Accepted 7 January 2010
Available online 20 January 2010
Keywords:
2-(20,60-Dialkylphenylazo)-4-
methylphenols
Organoiridium complexes
C–H and C–C bond activation
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
6
OH
O
5
1
Metal mediated chemical transformation of organic substrates
belongs to an area of research, which has been receiving consider-
able attention [1–16]. A key step in such reactions is the activa-
tion of C–H or C–C bond in the organic substrates, which is
usually achieved through their interaction with soft metal centers.
The metal–carbon bonded species, so generated, serve as active
intermediates towards formation of the final products. Metal
promoted C–H and C–C bond activation of organic molecules
has thus been of significant current interest [17–35]. We have
been active, for quite some time, in the synthesis of organometallic
complexes via platinum-group metal mediated C–H bond activa-
tion of organic ligands in general [36–53] and of 2-(arylazo)-4-
methylphenols (I) in particular [44–49], and the present work has
emerged from our continued interest in this area. In this study irid-
ium has been selected as the platinum-group metal for promoting
M
II
2
4
H3C
N
N
H3C
N
N
3
'
2
'
'
1
3
'
4
'
6
R
R
'
5
I
OH
L
R
L1
L2
L3
CH3
H3C
N
N
R
C2H5
CH(CH3)2
R
* Corresponding author.
E-mail address: samaresh_b@hotmail.com (S. Bhattacharya).
III
0022-328X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.01.015

1112
S. Baksi et al. / Journal of Organometallic Chemistry 695 (2010) 1111–1118
C–H or C–C bond activation and [Ir(PPh3)3Cl] has been utilized as
the iridium starting material. Upon reaction with [Ir(PPh3)3Cl], the
2-(arylazo)-4-methylphenols (I) were observed to undergo C–H
activation at one ortho-position (say, the 20 position) of the phenyl
ring in the arylazo fragment and bind to the metal center as
tridentate C,N,O-donors (II) affording cyclometalated complexes
[46]. This has led us to explore the reactivity of some modified
2-(arylazo)phenols (III), in which both the ortho-positions of
the phenyl ring in the arylazo fragment are strategically
blocked by alkyl groups with the intention of inducing metal-
mediated C–C bond activation at one of the ortho-positions. Ini-
tially, reaction of 2-(20,60-dimethylphenylazo)-4-methylphenol
(L1) was carried out with [Ir(PPh3)3Cl], where one of the two
ortho methyl groups was found to undergo C–H activation to
afford organoiridium complexes (1 and 2) [52]. Although an
ortho methyl group in ligand L1 was observed to undergo inter-
esting elimination [53], migration [51] and oxidative migration
[50], such a metal–carbon bond formation via methyl C–H bond
O
O
Ir
Ir
H
H3C
N
N
C
CH3 H3C
N
N
H
H
H3C
H3C
CH3
CH3
C
C
H
C
H
CH3
IV
V
fragment. Mass spectral and microanalytical data are found to be
in excellent agreement with this composition of complex 3. Pre-
liminary characterizations on complex 4 show that it is very
similar to complex 3 in composition and stereochemistry, the
only difference being that a chloride is coordinated to iridium
instead of a hydride. Structural characterization of complexes 3
and 4 by X-ray crystallography was not possible since single
crystals of these species could not be grown. However, struc-
tures of both the complexes were geometrically optimized
through DFT calculations [63,64]. The optimized structure of
complex 3 is shown in Fig. 1 and some selected bond parame-
ters are given in Table 1. The optimized structure of complex
4 and some selected bond parameters have been deposited as
P
Ir
P
P
Ir
P
H
Cl
O
O
H3C
N
N
CH2
H3C
N
N
CH2
P = PPh3
H3C
H3C
1
2
activation of ligand L1 was not experienced. It is also noteworthy
that methyl C–H activation is less common than phenyl C–H acti-
vation [54–62]. This interesting reactivity of L1 has prompted us
to scrutinize reactions of two related 2-(20,60-dialkylphenylazo)-
4-methylphenols (L2 and L3) with iridium. The main objective
of the present study has been to see whether these two new li-
gands, which bear larger alkyl groups (ethyl and iso-propyl) at
the 20 and 60 positions, undergo any iridium-mediated C–H or
C–C bond activation. Reactions of L2 and L3 with [Ir(PPh3)3Cl]
have been observed to afford interesting organoiridium com-
plexes via C–H (in case of ligand L2) and C–C (in case of ligand
L3) bond activations. An account of the chemistry of these
organoiridium complexes is presented herein, with special refer-
ence to their formation, structure and spectral and electrochem-
ical properties.
2. Results and discussion
2.1. Synthesis and characterization
As delineated in the introduction, the main objective of the
present study was to see whether ligands similar to L1 contain-
ing alkyl groups at the 20 and 60 positions which are larger than
a methyl group (e.g. ethyl in L2 and iso-propyl in L3), can under-
go similar reaction with [Ir(PPh3)3Cl] causing C–H or C–C bond
activation of one of the two ortho alkyl groups. Reaction of 2-
(20,60-diethylphenylazo)-4-methylphenol (L2) with [Ir(PPh3)3Cl]
in refluxing toluene in the presence of triethylamine was carried
out first, which afforded a pink complex 3 and a red complex 4.
Preliminary characterization of complex 3 indicated the presence
of two triphenylphosphines, a hydride and a de-protonated L2 li-
gand in it. The 1H NMR spectrum of this complex (vide infra)
further suggested that the ligand L2 is coordinated to the me-
tal center as a tridentate C,N,O-donor (IV) via loss of one phe-
nolic proton and a proton from one ethyl group of the arylazo
Fig. 1. (a) Optimized structure of complex 3 (phenyl rings of the phosphine ligands
have been added replacing hydrogens), and (b) view of the equatorial plane with
the axial triphenylphosphine ligands removed for clarity.

S. Baksi et al. / Journal of Organometallic Chemistry 695 (2010) 1111–1118
1113
Table 1
Selected bond lengths (Å) and bond angles (°) for complex 3.
Bond lengths (Å)
Ir(1)–P(2)
Ir(1)–P(3)
Ir(1)–O(4)
Ir(1)–N(6)
Ir(1)–C(16)
Ir(1)–H(48)
N(5)–N(6)
O(4)–C(21)
2.3707
2.3892
2.2038
2.0795
2.1083
1.6142
1.2721
1.3133
Bond angles (°)
P(3)–Ir(1)–P(2)
H(48)–Ir(1)–N(6)
O(4)–Ir(1)–C(16)
O(4)–Ir(1)–N(6)
N(6)–Ir(1)–C(16)
167.79
178.28
173.56
79.81
92.68
Supplementary material (Fig. S1 and Table S1). The computed
bond parameters are found to be comparable with those ob-
served in related complexes for which X-ray crystallographic
structure determinations are available [52]. Therefore it can be
inferred that, as observed earlier during reaction of L1 with
[Ir(PPh3)3Cl] [52], a similar C–H activation of an ethyl group in
L2 took place during formation of complexes 3 and 4.
Reaction of 2-(20,60-diisopropylphenylazo)-4-methylphenol (L3)
with [Ir(PPh3)3Cl] was carried out in an analogous manner. How-
ever, in contrast to the earlier reaction this afforded a blue complex
5. Preliminary characterization of complex 5 indicated the pres-
ence of two triphenylphosphines, a hydride and a de-protonated
L3 ligand. However, unlike complexes 1, 2, 3 and 4, no indication
of C–H activation of any iso-propyl group of ligand L3 during for-
mation of complex 5 was obtained. The 1H NMR spectrum of com-
plex 5 shows clear signals for two iso-propyl groups and three
singlets (1H each) in the aromatic region at 6.01, 6.38 and
6.66 ppm. As only one singlet could be expected from the para-cre-
sol fragment of the azo-ligand, the appearance of three singlets
suggested that some unexpected reaction must have taken place
in this azo-ligand during formation of complex 5. To ascertain
the stereochemistry of complex 5, as well as the status of ligand
L3 within it, its structure has been determined by X-ray crystallog-
raphy. The structure determination reveals (Fig. 2) that an interest-
ing transformation of ligand L3 has indeed taken place during
formation of complex 5, whereby an iso-propyl group has migrated
from its original location (say, the 20 position) and is inserted (via
loss of a proton) into the C–H bond at the corresponding 40 posi-
tion, thus generating an iso-propyl group at the 40 position, and
the modified ligand is bound to iridium as a C,N,O-donor (V). Irid-
ium therefore occupies a HCNOP2 coordination sphere, which is
significantly distorted from ideal octahedral geometry, as reflected
in the bond parameters around the metal center. The coordinated
tridentate ligand L3 and the hydride define the equatorial plane
around the iridium center, and the two triphenylphosphines are
disposed mutually trans in the two axial positions. The Ir–P dis-
tances are quite normal and so are the Ir–C, Ir–N, Ir–O, C–O and
N–N bond lengths within the C,N,O-chelated fragment (Table 2)
[46,52]. The mass spectrum of complex 5, which shows the
[MꢀH]+ peak at 1 0 1 1, and the observed microanalytical data
are also in excellent agreement with this composition.
Fig. 2. (a) Crystal structure of complex 5, and (b) view of the equatorial plane with
the axial triphenylphosphine ligands removed for clarity. Except for the coordinated
hydride, all hydrogen atoms are omitted for clarity. The hydride H atom could not
be located but it is added in a calculated position.
Table 2
Selected bond lengths (Å) and bond angles (°) for complex 5.
Bond lengths (Å)
Ir(1)–P(1)
Ir(1)–P(2)
Ir(1)–O(8)
Ir(1)–N(5)
Ir(1)–C(2)
Ir(1)–H(1)
N(4)–N(5)
O(8)–C(7)
2.3052(9)
2.3066(9)
2.168(2)
2.023(2)
2.039(3)
1.60a
1.281(3)
1.309(4)
The exact mechanisms behind formation of complexes 3, 4
(via C–H bond activation of ligand L2) and 5 (via C–C bond acti-
vation of L3), are not completely clear. However, the sequences
shown in Schemes 1 and 2 seem probable. During the formation
of complexes 3 and 4 (Scheme 1), ligand L2 initially binds to the
metal center in [Ir(PPh3)3Cl] via oxidative insertion of iridium
into the phenolic O–H bond and a triphenylphosphine gets dis-
sociated simultaneously, producing two geometric isomers (A
Bond angles (°)
P(1)–Ir(1)–P(2)
O(8)–Ir(1)–C(2)
N(5)–Ir(1)–H(1)
N(5)–Ir(1)–C(2)
O(8)–Ir(1)–N(5)
163.16(3)
157.44(10)
180a
77.76(11)
79.69(9)
a
Values used to calculate the position of this atom.

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S. Baksi et al. / Journal of Organometallic Chemistry 695 (2010) 1111–1118
OH
H
H
H3C
N
N
C CH3
H
H
C
CH3
[Ir(PPh3)3Cl]
-PPh3
P = PPh3
P
P
H
Cl
O
O
Ir
P
Ir
P
Cl
H
H3C
N
N
H3C
N
N
H
H
C
H
C H
CH3
CH3
H
H
H
C
H C
CH3
CH3
(A)
(B)
-HCl
-H2
P
P
H
Cl
O
O
Ir
P
Ir
P
H
CH3
H
H3C
N
N
C
H3C
N
N
C CH3
H
H
H
C
H
C
CH3
CH3
3
4
Scheme 1. Proposed scheme for the formation of complexes 3 and 4.
and B) of a hydride intermediate which are believed to remain
in equilibrium. In intermediate A the coordinated hydride is cis
to the phenolate oxygen and the coordinated chloride is trans,
while in the other intermediate B the relative disposition of
the coordinated hydride and chloride is reversed. Structures of
both the intermediates have been geometrically optimized
through DFT calculations and the optimized structures along
with their respective energies are shown in Fig. S2 (Supplemen-
tary material). Both the intermediates are found to have compa-
rable energies, which is in accordance with the proposed
equilibrium existing between these two species. In the opti-
mized structure of intermediate A, the 20,60-diethylphenyl frag-
ment is almost orthogonal to the para-cresol fragment. In this
structure, a methylene hydrogen atom is only 1.527 Å away
from the metal-bound chloride subtending a C–Hꢁ ꢁ ꢁCl angle of
105.59°, indicating weak C–Hꢁ ꢁ ꢁCl hydrogen bonding interaction.
During the permissible rotation of the 20,60-diethylphenyl frag-
ment around the C15–N5 bond and that of the ethyl group
around the C7–C16 bond, the methylene C–H bond can come
closer to the Ir–Cl bond and would eventually collide with it.
This closeness facilitates formation of the iridium–carbon bond
in the final step via elimination of HCl affording complex 3. In
the optimized structure of intermediate B, the distance between
the metal-bound hydride and the closest methylene-hydrogen is
found to be 1.080 Å subtending a C–Hꢁ ꢁ ꢁH angle of 174.54°, indi-
cating effective C–Hꢁ ꢁ ꢁH hydrogen bonding interaction (Fig. S2).
The rotational flexibility in the 20,60-diethylphenyl fragment
and the resulting proximity between the methylene C–H and
Ir–H bonds lead to cyclometalation via elimination of molecular
hydrogen [65] yielding complex 4. Isolation of the possible inter-
mediates A and B has not been possible, probably due to their
rapid transformation into the cyclometalated species 3 and 4.

S. Baksi et al. / Journal of Organometallic Chemistry 695 (2010) 1111–1118
1115
OH
H3C
N
N
CH(CH3)2
(CH3)2HC
[Ir(PPh3)3Cl]
-PPh3
P = PPh3
P
P
O
O
Cl
H
III
Ir
Ir
P
H
Cl
H3C
N
N
H3C
N
N
P
H
H
CH3
CH3
C
C
CH3
CH3
(CH3)2HC
(CH3)2HC
(C)
(D)
P
-HCl
P
O
N
O
N
CH3
CH3
H
H
C
CH3
CH3
III
Ir
III
Ir
-
C
H3C
H3C
P
+
O
P
N
N
(CH3)2HC
(CH3)2HC
P
P
O
O
III
Ir
H
H
III
Ir
H
CH3
CH3
H3C
N
P
N
H3C
N
P
N
CH(CH3)2
H
(CH3)2HC
(CH3)2HC
Scheme 2. Proposed steps for the formation of complex 5.
During the formation of complex 5 (Scheme 2), ligand L3 is as-
sumed to react initially with [Ir(PPh3)3Cl] as before, generating
two geometric isomers (C and D) of a hydride intermediate. Struc-
tures of both the intermediates have been geometrically optimized
through DFT calculations and the optimized structures along with
their respective energies are shown in Fig. S3 (Supplementary
material). The intermediate C has significantly lower energy than
intermediate D, indicating that the speculated equilibrium between
these two species favors intermediate C. In the optimized structure
of intermediate C, the central hydrogen of one iso-propyl group is
involved in C–Hꢁ ꢁ ꢁCl hydrogen-bonding interaction with the me-
tal-bound chloride (Clꢁ ꢁ ꢁH distance = 1.527 Å, C–Hꢁ ꢁ ꢁCl an-
gle = 105.59°). However, in the optimized structure of
intermediate D, the Scheme 2 metal-bound hydride is 1.487 Å away
from the central hydrogen of the proximal iso-propyl group, sub-
tending a C–Hꢁ ꢁ ꢁH angle of 104.76°, indicating a poorer hydrogen
bonding interaction compared to that existing in the similar inter-
mediate B in Scheme 1. Rapid elimination of HCl is believed to take
place from intermediate C, leading to formation of a metal–carbon
bond from an ortho-position (say, the 20 position) of the phenyl ring
in the phenylazo fragment, with consequent loss of aromaticity in
that ring and the generation of a formal positive charge at the 30 po-
sition. This fast reaction of C shifts the equilibrium between C and D
more towards intermediate C. Elimination of HCl is also associated
with generation of a carbanion at the central carbon of the de-pro-
tonated iso-propyl group, which then forms a C–C bond with the
ring carbon at the 30 position carrying the positive charge. This is
followed by migration of the C(CH3)2 fragment, which was bound
in an g2-fashion to the 20,30 ring carbons, to the corresponding
30,40 position. In the next step usual rearrangements in the g2
-
bound C(CH3)2 fragment take place, whereby the lost aromaticity
of the aryl ring is restored and complex 5 is obtained.

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S. Baksi et al. / Journal of Organometallic Chemistry 695 (2010) 1111–1118
Table 3
Electronic spectral and cyclic voltammetric data of the complexes.
Complex
Electronic spectral dataa kmax/nm (
e
/Mꢀ1 cmꢀ1
)
Cyclic voltammetric datab E/V vs SCE
Complex 3
Complex 4
Complex 5
544(2600), 382c(2900), 306c(7800), 272c(11700)
537(2400), 388c(3200), 324c(7300), 276c(12800)
636(6300), 394c(6500), 348c(9600), 256c(37100)
1.10d, 1.33d, ꢀ0.96e
1.03d, 1.27d, ꢀ1.07e
0.66d, 1.15d, ꢀ1.03e
a
b
c
In acetonitrile solution.
Solvent, 1:9 dichloromethane–acetonitrile; supporting electrolyte, TBAP; scan rate 50 mV sꢀ1; reference electrode, SCE.
Shoulder.
Anodic peak potential (Epa) value.
Cathodic peak potential (Epc) value.
d
e
Table 4
2.2. Spectral studies
Composition of selected molecular orbitals.
Magnetic susceptibility measurements show that complexes 3,
4 and 5 are diamagnetic, consistent with the +3 oxidation state
of iridium (low-spin d6, S = 0) in these complexes. The 1H NMR
spectrum of each complex, recorded in CDCl3 solution, shows
broad signals in the range 7.04–7.48 ppm for the coordinated
PPh3 ligands. The methyl signal from the phenol fragment is ob-
served as a distinct peak near 2.3 ppm. Signals from the ethyl
and metal-bound CHCH3 fragment in complexes 3 and 4, as well
as from iso-propyl groups in complex 5, appear in the expected re-
gions. Among the expected aromatic proton signals for the coordi-
nated azo-ligands, most were clearly observed while a few could
not be detected due to their overlap with the PPh3 signals. Infrared
spectra of complexes 3, 4 and 5 show many bands of different
intensities in the 400–4000 cmꢀ1 region. Attempt to assign each
band to a specific vibration has not been made. However, sharp
bands displayed near 746, 694 and 520 cmꢀ1 by all the three com-
plexes are attributable to the coordinated PPh3 ligands. Strong
bands are also observed near 1483, 1434, 1245, 1188 and
1095 cmꢀ1 in all these complexes, which are absent in the infrared
spectrum of [Ir(PPh3)3Cl], and hence these are assignable to the
coordinated azo-ligands. The 1H NMR and IR spectral data of the
complexes are therefore consistent with their compositions.
The complexes 3, 4 and 5 are found to be soluble in dichlo-
romethane, chloroform, acetonitrile, acetone, etc., producing in-
tense pink, red and blue solutions, respectively. Electronic
spectra of these complexes have been recorded in acetonitrile
solution. Each complex shows an intense absorption in the visi-
ble region and a few very strong absorptions in the ultraviolet
region (Table 3). The absorptions in the ultraviolet region are
assignable to transitions within the ligand orbitals. To under-
stand the origin of the absorption in the visible region, elec-
tronic structures of the complexes have been probed with the
help of DFT calculations [63,64] and the composition of selected
molecular orbitals is given in Table 4. Electron distributions in
the highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) for complex 3 are shown
in Fig. 3 and the same for complexes 4 and 5 are shown in Figs.
S4 and S5 (Supplementary material). In complexes 3 and 4, both
the HOMO and LUMO are distributed almost entirely over the
azo-ligand. Hence the absorption in the visible region (near
540 nm) is assignable to a transition within the filled (HOMO)
and vacant (LUMO) orbitals of the azo-ligand. In complex 5,
however, the HOMO is mostly (>78%) concentrated on the metal
center, while the LUMO is primarily (>83%) delocalized over the
coordinated azo-ligand. Hence the absorption at 636 nm is
assignable to charge-transfer transition from iridium to the
azo-ligand. It is interesting to note, particularly between the
two hydride complexes 3 and 5, that the nature of the HOMO
depends on whether metallation occurs at an alkyl carbon (as
in 3) or at a phenyl carbon (as in 5).
Complex
Contributing fragments
% Contribution of fragments to
HOMO
LUMO
Complex 3
Ir
1.14
0.09
1.19
0.09
Triphenylphosphine
Azo-ligand
98.77
98.72
Complex 4
Complex 5
Ir
2.67
0
2.96
0.1
Triphenyl phosphine
Azo-ligand
97.039
96.85
Ir
78.1
3.27
18.53
10.78
5.39
83.63
Triphenyl phosphine
Azo-ligand
Fig. 3. Partial molecular orbital diagram of complex 3.

S. Baksi et al. / Journal of Organometallic Chemistry 695 (2010) 1111–1118
1117
2.3. Electrochemical properties
plexes were carried out by density functional theory (DFT) method
using the GAUSSIAN 03 (B3LYP/SDD-6-31G) package [63].
Electrochemical properties of the complexes have been studied
by cyclic voltammetry in 1:9 dichloromethane–acetonitrile solution
(0.1 M TBAP) [66]. Voltammetric data are given in Table 3. Com-
plexes 3 and 4 show two oxidative responses on the positive side
of SCE and a reductive response on the negative side. In view of the
composition of the HOMO in these two complexes, the first oxidative
response is assigned to oxidation of the coordinated azo-ligand. Sim-
ilarly based on the composition of the LUMO, the reductive response
is assigned to reduction of the coordinated azo-ligand. The second
oxidative response is tentatively assigned to oxidation of the coordi-
nated azo-ligand. Complex 5 also shows two oxidative responses on
thepositiveside of SCEand a reductive response on thenegativeside.
However, as the HOMO in this complex is largely an iridium-based
orbital, the first oxidative response is assigned to Ir(III)–Ir(IV) oxida-
tion. The other two redox responses are assigned, as before, to oxida-
tion and reduction of the coordinated azo-ligand. All the redox
responses for all three complexes are irreversible.
4.1. Synthesis of complexes
Complexes 3 and 4. A solution of 2-(20,60-diethylphenylazo)-4-
methylphenol (26 mg, 0.10 mmol) in toluene (40 mL) was purged
with a stream dinitrogen for 5 min. To this solution were added tri-
ethylamine (10 mg, 0.10 mmol) and [Ir(PPh3)3Cl] (100 mg,
0.10 mmol) successively. The resulting mixture was then heated
at reflux under a dinitrogen atmosphere for 24 h to yield an orange
solution. The solvent was then evaporated to give a solid mass,
which was subjected to purification by thin layer chromatography
on a silica plate. With benzene as the eluant, a pink band followed
by a red band separated, and these were extracted with acetoni-
trile. Concentration of these extracts gave complexes 3 and 4 as
crystalline pink and red solids, respectively. Complex 3: Yield:
22%. Anal. Calc. for C53H49N2OP2Ir: C, 64.69; H, 4.98; N, 2.85. Found:
C, 64.54; H, 4.78; N, 2.78%. Mass: 983, [MꢀH]+; 955,
[Mꢀ(H+C2H4)]+. 1H NMR [69]: ꢀ21.34 (t, hydride, J = 12.6); 1.26
(t, 3H, J = 7.5); 1.75 (d, 3H, J = 4.2); 2.40 (CH3); 2.74 (q, 2H,
J = 7.5); 3.52 (q, 1H, J = 7.2); 7.10–7.34*(2H+2PPh3); 7.46 (t, 1H,
J = 7.2); 7.64 (d, 1H, J = 8.2); 7.66 (d, 1H, J = 8.3). Complex 4: Yield:
30%. Anal. Calc. for C53H48N2OP2ClIr: C, 62.49; H, 4.72; N, 2.75.
Found: C, 62.14; H, 4.63; N, 2.63%. Mass: 983, [MꢀCl]+; 955,
[Mꢀ(Cl+C2H4)]+. 1H NMR: 1.20 (t, 3H, J = 7.5); 1.98 (d, 3H,
J = 4.2); 2.39 (CH3); 2.71 (q, 2H, J = 7.5); 4.19 (q, 1H, J = 7.0); 6.96
(d, 1H, J = 8.4); 7.04–7.46*(1H+2PPh3); 7.49 (t, 1H, J = 7.1); 7.55
(d, 1H, J = 6.3); 7.63 (d, 1H, J = 6.3).
Complex 5. This complex was prepared by following the same
procedure as above but using 2-(20,60-diisopropylphenylazo)-4-
methylphenol instead of 2-(20,60-diethylphenylazo)-4-methylphe-
nol. Complex 5 was obtained as a crystalline blue solid. Yield:
40%. Anal. Calc. for C55H53N2OP2Ir: C, 65.27; H, 5.24; N, 2.77. Found:
C, 65.35; H, 5.26; N, 2.73%. Mass: 1011, [MꢀH]+. 1H NMR: ꢀ17.06
(t, hydride, J = 15.3); 0.68 (d, 6H, J = 6.6); 0.76 (d, 6H, J = 6.8);
2.21 (CH3); 3.26 (m, 1H, J = 6.9); 3.41 (m, 1H, J = 7.2); 6.01 (s,
1H); 6.38 (s, 1H); 6.66 (s, 1H); 6.47 (d, 1H, J = 8.6); 6.94 (d, 1H,
J = 8.6); 7.05–7.48*(2PPh3).
3. Conclusions
Along with our earlier observations [52], the present study shows
that blocking both the 20,60 positions of the 2-(arylazo)phenol (I) by
alkyl groups can successfully induce metal-mediated C–H or C–C
bond activation of one alkyl (R) group at one of these two positions.
This has been manifested in the reaction of 2-(20,60-dialkylpheny-
lazo)-4-methylphenols (III) with [Ir(PPh3)3Cl]. In the case of
R = methyl and ethyl, C–H activation of the alkyl group takes place,
whereas when R = iso-propyl, migration of an iso-propyl group oc-
curs via C–C activation. These studies thus demonstrate that the nat-
ure of bond activation of the alkyl group in ligands of type III and the
organoiridium complexes produced from these depend primarily on
the size of the alkyl group attached to the alpha-carbon.
4. Experimental
Iridium trichloride was obtained from Arora Matthey, Kolkata,
India. All other chemicals and solvents were reagent grade com-
mercial materials and were used as received. [Ir(PPh3)3Cl] was syn-
thesized by following a reported procedure [46]. The 2-(20,60-
diethylphenylazo)-4-methylphenol and 2-(20,60-diisopropylpheny-
lazo)-4-methylphenol were prepared following reported proce-
dures [53]. Purification of dichloromethane and acetonitrile, and
preparation of tetrabutylammonium perchlorate (TBAP) for elec-
trochemical work were performed following reported procedures
[67,68]. Microanalyses (C, H, N) were performed using a Heraeus
Carlo Erba 1108 elemental analyzer. Mass spectra were recorded
with a Micromass LCT electrospray (Qtof Micro YA263) mass spec-
trometer by electrospray ionization method. IR spectra were ob-
tained on a Shimadzu FTIR-8300 spectrometer with samples
prepared as KBr pellets. Electronic spectra were recorded on a JAS-
CO V-570 spectrophotometer. Magnetic susceptibilities were mea-
sured using a PAR 155 vibrating sample magnetometer fitted with a
Walker scientific L75FBAL magnet. 1H NMR spectra were recorded
in CDCl3 solution on 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 aux-
iliary electrode and an aqueous saturated calomel reference elec-
trode (SCE) were used in a three-electrode configuration. All
electrochemical experiments were performed under a dinitrogen
atmosphere. All electrochemical data were collected at 298 K and
are uncorrected for junction potentials. Optimization of ground-
state structures and energy calculations of the organoiridium com-
4.2. X-ray structure determination
Single crystals of complex 5 were grown by slow evaporation of
the solvent from an acetonitrile solution of the complex. Selected
Table 5
Crystallographic data for complex 5.
Empirical formula
Formula weight
Space group
a (Å)
b (Å)
c (Å)
C55H53N2OP2Ir
1012.14
Triclinic, P1
12.0985(8)
13.7457(9)
14.8119(10)
86.583(2)
67.888(2)
82.637(2)
2263.1(3)
2
a
(°)
b (°)
c
(°)
V (Å3)
Z
k (Å)
Crystal size (mm)
T (K)
0.71073
0.11 ꢂ 0.19 ꢂ 0.20
150
l
R1
wR2
(mmꢀ1
)
3.063
0.0285
0.0642
1.03
a
b
Goodness-of-fit (GOF)c
a
b
c
R1
wR2 = [
GOF = [
=
R
||Fo| ꢀ |Fc||/
R|Fo|.
2
R
[w(Fo ꢀ Fc2)2]/
R .
[w(Fo2)2]]1/2
2
R
[w(Fo ꢀ Fc2)2]/(M ꢀ N)]1/2, where M is the number of reflections and N
is the number of parameters refined.

1118
S. Baksi et al. / Journal of Organometallic Chemistry 695 (2010) 1111–1118
[30] E. Carmona, M. Paneque, L.L. Santos, V. Salazar, Coord. Chem. Rev. 249 (2005)
1729.
[31] M. Lersch, M. Tilset, Chem. Rev. 105 (2005) 2471.
crystal data and data collection parameters are given in Table 5.
Data were collected on a Bruker SMART1000 CCD diffractometer
using graphite monochromated Mo Ka radiation. Structure solu-
[32] W. Leis, H.A. Mayer, W.C. Kaska, Coord. Chem. Rev. 252 (2008) 1787.
[33] Y.J. Park, J.W. Park, C.H. Jun, Acc. Chem. Res. 41 (2008) 222.
[34] J.C. Lewis, R.G. Bergman, J.A. Ellman, Acc. Chem. Res. 41 (2008) 1013.
[35] O. Daugulis, H.Q. Do, D. Shabashov, Acc. Chem. Res. 42 (2009) 1074.
[36] F. Basuli, S.M. Peng, S. Bhattacharya, Inorg. Chem. 40 (2001) 1126.
[37] I. Pal, S. Dutta, F. Basuli, S. Goverdhan, S.M. Peng, G.H. Lee, S. Bhattacharya,
Inorg. Chem. 42 (2003) 4338.
[38] P. Gupta, R.J. Butcher, S. Bhattacharya, Inorg. Chem. 42 (2003) 5405.
[39] R. Acharyya, S. Dutta, F. Basuli, S.-M. Peng, G.-H. Lee, L.R. Falvello, S.
Bhattacharya, Inorg. Chem. 45 (2006) 1252.
[40] S. Basu, S. Dutta, M.G.B. Drew, S. Bhattacharya, J. Organomet. Chem. 691 (2006)
3581.
[41] S. Baksi, R. Acharyya, S. Dutta, A.J. Blake, M.G.B. Drew, S. Bhattacharya, J.
Organomet. Chem. 692 (2007) 1025.
[42] S. Nag, R.J. Butcher, S. Bhattacharya, Eur. J. Inorg. Chem. (2007) 1251.
[43] C. GuhaRoy, S.S. Sen, S. Dutta, G. Mostafa, S. Bhattacharya, Polyhedron 26
(2007) 3876.
[44] S. Dutta, S.M. Peng, S. Bhattacharya, J. Chem. Soc., Dalton Trans. (2000) 4623.
[45] K. Majumder, S.M. Peng, S. Bhattacharya, J. Chem. Soc., Dalton Trans. (2001)
284.
tion by direct methods and refinement by least squares were car-
ried out using the SHELXS97 and SHELXL97 programs, respectively
[70].
Acknowledgements
The authors thank the reviewers 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, New Delhi [Grant No. SR/S1/IC-15/2004]
is gratefully acknowledged. Dipravath Kumar Seth thanks the
Council of Scientific and Industrial Research, New Delhi, for his
fellowship [Grant No. 9/096(0511)/2006-EMR I]. HT and AJB thank
the EPSRC (UK) for the award of
diffractometer.
a single-crystal X-ray
[46] R. Acharyya, F. Basuli, R.-Z. Wang, T.C.W. Mak, S. Bhattacharya, Inorg. Chem. 43
(2004) 704.
[47] P. Gupta, S. Dutta, F. Basuli, S.-M. Peng, G.-H. Lee, S. Bhattacharya, Inorg. Chem.
45 (2006) 460.
Appendix A. Supplementary material
[48] S. Halder, R. Acharyya, S.M. Peng, G.H. Lee, M.G.B. Drew, S. Bhattacharya, Inorg.
Chem. 45 (2006) 9654.
CCDC 706553 contains 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.
2010.01.015.
[49] S. Halder, M.G.B. Drew, S. Bhattacharya, Organometallics 25 (2006) 5969.
[50] R. Acharyya, S.M. Peng, G.H. Lee, S. Bhattacharya, Inorg. Chem. 42 (2003) 7378.
[51] S. Nag, P. Gupta, R.J. Butcher, S. Bhattacharya, Inorg. Chem. 43 (2004) 4814.
[52] R. Acharyya, F. Basuli, S.M. Peng, G.H. Lee, R.Z. Wang, T.C.W. Mak, S.
Bhattacharya, J. Organomet. Chem. 690 (2005) 3908.
[53] S. Baksi, R. Acharyya, F. Basuli, S.M. Peng, G.H. Lee, M. Nethaji, S. Bhattacharya,
Organometallics 26 (2007) 6596.
[54] M. Akhter, K.A. Azam, S.M. Azad, S.E. Kabir, K.M. Abdul Malik, R. Mann,
Polyhedron 22 (2003) 355.
[55] S.P. Tunik, I.A. Balova, M.E. Borovitov, E. Norlander, M. Haukka, T.A. Pakknen, J.
Chem. Soc., Dalton Trans. (2002) 827.
[56] R.L. Zuckerman, S.W. Krska, R.G. Bergman, J. Organomet. Chem. 608 (2000)
172.
[57] A. Zucca, M.A. Cinellu, M.V. Pinna, S. Stoccoro, G. Minghetti, M. Manassero, M.
Sansoni, Organometallics 19 (2000) 4295.
References
[1] B.M. Trost, T.R. Verhoeven, Comprehensive Organometallic Chemistry, in: E.
Abel, F.G.A. Stone, G. Wilkinson (Eds.), Pergamon Press, Oxford, 1982, p. 8.
[2] J.P. Collman, L.S. Hegedus, J.R. Norton, R.G. Finke, Principles and Applications of
Organotransition Metal Chemistry, Mill Valley, CA, 1987.
[3] L.S. Hegedus, Transition Metals in the Synthesis of Complex Organic Molecules,
Mill Valley, CA, 1994.
[4] E. Abel, F.G.A. Stone, G. Wilkinson (Eds.), Comprehensive Organometallic
Chemistry, Pergamon Press, Oxford, 1995, p. 12.
[5] L.S. Liebeskind (Ed.), Advances in Metal-Organic Chemistry, Jai Press,
Greenwich, CT, 1996.
[6] B. Cornils, W.A. Hermann (Eds.), Applied Homogenous Catalysis with
Organometallic Compounds: A Comprehensive Handbook in Two Volumes,
VCH, Weinheim, 1996.
[7] M. Bellar, C. Bolm (Eds.), Transition Metals for Organic Synthesis 1–2, Wiley-
VCH, Weinheim, 1998.
[8] L.S. Hegedus, Coord. Chem. Rev. 168 (1998) 49.
[9] J. Tsuji, Transition Metal Reagents Catalysts, Wiley-VCH, Weinheim, 2000.
[10] A. de Meijere, Chem. Rev. 100 (2000) 2739.
[11] F. Alonso, I.P. Beletskaya, M. Yus, Chem. Rev. 102 (2002) 4009.
[12] C.M. Che, J.S. Huang, Coord. Chem. Rev. 242 (2003) 97.
[13] B.M. Trost, M.L. Crawley, Chem. Rev. 103 (2003) 2921.
[14] T. Mallat, A. Baiker, Chem. Rev. 104 (2004) 3037.
[15] B.H. Lipshutz, Y. Yamamoto, Chem. Rev. 108 (2008) 2793.
[16] N.A. Foley, J.P. Lee, Z. Ke, T.B. Gunnoe, T.R. Cundari, Acc. Chem. Res. 42 (2009)
585.
[17] C. Jia, T. Kitamura, Y. Fujiwara, Acc. Chem. Res. 34 (2001) 633.
[18] C. Slugovc, I. Padilla-Martnez, S. Sirol, E. Carmona, Coord. Chem. Rev. 213
(2001) 129.
[19] A. Vigalok, D. Milstein, Acc. Chem. Res. 34 (2001) 798.
[20] K. Matsumoto, M. Ochiai, Coord. Chem. Rev. 231 (2002) 229.
[21] V. Ritleng, C. Sirlin, M. Pfeffer, Chem. Rev. 102 (2002) 1731.
[22] C.S. Chin, G. Won, D. Chong, M. Kim, H. Lee, Acc. Chem. Res. 35 (2002) 218.
[23] K. Matsumoto, H. Sugiyama, Acc. Chem. Res. 35 (2002) 915.
[24] C.B. Pamplin, P. Legzdins, Acc. Chem. Res. 36 (2003) 223.
[25] M.E.V. Boom, D. Milstein, Chem. Rev. 103 (2003) 1759.
[26] H.-Y. Jang, M.J. Krische, Acc. Chem. Res. 37 (2004) 653.
[27] S.R. Klei, K.L. Tan, J.T. Golden, C.M. Yung, R.K. Thalji, K.A. Ahrendt, J.A. Ellman,
T.D. Tilley, R.G. Bergman, Activation and Functionalization of C–H Bonds, 2004,
pp. 46–55 (Chapter 2).
[58] J.A.M. Brandts, E. Kruiswijk, J. Boersma, A.L. Spek, G. Van Koten, J. Organomet.
Chem. 585 (1999) 93.
[59] J.R. Torkelson, R. McDonald, M. Cowie, Organometallics 18 (1999) 4134.
[60] P.J. Alaimo, R.G. Bergman, Organometallics 18 (1999) 2707.
[61] H.F. Luecke, R.G. Bergman, J. Am. Chem. Soc. 119 (1997) 11538.
[62] H.F. Luecke, B.A. Arndtsen, P. Burger, R.G. Bergman, J. Am. Chem. Soc. 118
(1996) 2517.
[63] M.J. Frisch, G.W. Tracks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman
Jr., J.A. Montgomery, T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar,
J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalamani, N. Rega, G.A.
Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, I. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox,
H.P. Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E.
Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y.
Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S.
Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K.
Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. baboul, S. Clifford, J.
Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskroz, I. Komaromi, R.L.
Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M.
P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A.
Challacombe,
Pople, Gaussian 03, revision D01, Gaussian Inc., Pittsburgh, PA, 2003.
[64] In optimizing the structures of complexes 3 and 4, the crystal structure of
complex 1 has been used as the starting model and necessary modifications
have been made. Phenyl rings of the PPh3 ligands have been replaced by
hydrogens.
[65] The evolved hydrogen could not be detected.
[66] A little dichloromethane was necessary to take the complex into solution.
Addition of a large excess of acetonitrile was necessary to record the redox
responses in proper shape.
[67] D.T. Sawyer, J.L. Roberts Jr., Experimental Electrochemistry for Chemists,
Wiley, New York, 1974.
[68] M. Walter, L. Ramaley, Anal. Chem. 45 (1973) 165.
[69] Chemical shifts are given in ppm and multiplicity of the signals along with the
associated coupling constants (J in Hz) are given in parentheses. Overlapping
signals are marked with an asterisk.
[28] P. Legzdins, C.B. Pamplin, Activation and Functionalization of C–H Bonds, 2004,
pp. 184–197 (Chapter 11).
[70] G.M. Sheldrick, Acta Crystallogr., Sect. A 64 (2008) 112.
[29] H.A.Y. Mohammad, J.C. Grimm, K. Eichele, H.G. Mack, B. Speiser, F. Novak, W.C.
Kaska, H.A. Mayer, Activation and Functionalization of C–H Bonds, 2004, pp.
234–247 (Chapter 14).