Iridium mediated methyl and phenyl C-H activation of 2-(arylazo)phenols. Synthesis, structure, and spectral and electrochemical properties of some organoiridium complexes

Article abstract of DOI:10.1016/j.jorganchem.2005.05.025

Reaction of 2-(2′,6′-dimethylphenylazo)-4-methylphenol with [Ir(PPh₃)₃Cl] in refluxing ethanol in the presence of a base (NEt₃) affords an organoiridium complex 5, where the 2-(2′,6′-dimethylphenylazo)-4-methylphenol is coordinated to iridium, via C-H activation of a methyl group, as a dianionic tridentate C,N,O-donor. Two triphenylphosphines and a hydride are also coordinated to the metal center. A similar reaction carried out in toluene affords complex 5 along with a similar complex 7, where a chloride is coordinated to iridium instead of the hydride. Reaction of 2-(2′-methylphenylazo)-4-methylphenol with [Ir(PPh₃)₃Cl] in refluxing ethanol in the presence of a base (NEt₃) affords an organoiridium complex 12, where the 2-(2′-methylphenylazo)-4-methylphenol is coordinated to iridium, via C-H activation at the ortho position of the phenyl group in the 2′- methylphenylazo fragment, as a dianionic tridentate C,N,O-donor. Two triphenylphosphines and a hydride are also coordinated to the metal center. A similar reaction carried out in toluene affords a complex 12 along with a similar complex 13, where a chloride is coordinated to iridium instead of the hydride. Structures of complexes 5, 12 and 13 have been determined by X-ray crystallography. In all these complexes, the two triphenylphosphines are trans. All these complexes show intense MLCT transitions in the visible region. Cyclic voltammetry on all the complexes shows an Ir(III)-Ir(IV) oxidation within 0.60-0.73 V vs. SCE, followed by an oxidation of the coordinated 2-(arylazo)phenolate ligand within 1.08-1.39 V vs. SCE. A reduction of the coordinated 2-(arylazo)phenolate ligand is observed within -1.10 to -1.26 V vs. SCE.

Full text of DOI:10.1016/j.jorganchem.2005.05.025

Journal of Organometallic Chemistry 690 (2005) 3908–3917
www.elsevier.com/locate/jorganchem
Iridium mediated methyl and phenyl C–H activation of
2-(arylazo)phenols. Synthesis, structure, and spectral and
electrochemical properties of some organoiridium complexes
a
Rama Acharyya , Falguni Basuli , Shie-Ming Peng , Gene-Hsiang Lee ,
a
b
b
c
Ren-Zhang Wang , Thomas C.W. Mak , Samaresh Bhattacharya
c
a,
*
a
Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700 032, India
b
Department of Chemistry, National Taiwan University, Taipei, Taiwan, ROC
c
Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong
Received 8 March 2005; received in revised form 20 May 2005; accepted 20 May 2005
Available online 27 June 2005
Abstract
Reaction of 2-(20,60-dimethylphenylazo)-4-methylphenol with [Ir(PPh3)3Cl] in refluxing ethanol in the presence of a base (NEt3)
affords an organoiridium complex 5, where the 2-(20,60-dimethylphenylazo)-4-methylphenol is coordinated to iridium, via C–H acti-
vation of a methyl group, as a dianionic tridentate C,N,O-donor. Two triphenylphosphines and a hydride are also coordinated to
the metal center. A similar reaction carried out in toluene affords complex 5 along with a similar complex 7, where a chloride is
coordinated to iridium instead of the hydride. Reaction of 2-(20-methylphenylazo)-4-methylphenol with [Ir(PPh3)3Cl] in refluxing
ethanol in the presence of a base (NEt3) affords an organoiridium complex 12, where the 2-(20-methylphenylazo)-4-methylphenol
is coordinated to iridium, via C–H activation at the ortho position of the phenyl group in the 20-methylphenylazo fragment, as a
dianionic tridentate C,N,O-donor. Two triphenylphosphines and a hydride are also coordinated to the metal center. A similar reac-
tion carried out in toluene affords a complex 12 along with a similar complex 13, where a chloride is coordinated to iridium instead
of the hydride. Structures of complexes 5, 12 and 13 have been determined by X-ray crystallography. In all these complexes, the two
triphenylphosphines are trans. All these complexes show intense MLCT transitions in the visible region. Cyclic voltammetry on all
the complexes shows an Ir(III)–Ir(IV) oxidation within 0.60–0.73 V vs. SCE, followed by an oxidation of the coordinated 2-(aryl-
azo)phenolate ligand within 1.08–1.39 V vs. SCE. A reduction of the coordinated 2-(arylazo)phenolate ligand is observed within
ꢀ1.10 to ꢀ1.26 V vs. SCE.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: C–H activation; 2-(Arylazo)phenols; Organoiridium complexes; Structure; Spectral and electrochemical properties
1. Introduction
[Ir(PPh3)3Cl], and afford organoiridium complexes
where the 2-(arylazo)phenols are coordinated to iridium
as tridentate C,N,O-donors 2 [1]. We have witnessed li-
gand 1 to display the same mode of coordination in sev-
eral other reactions [2]. With an intention of inducing a
different mode of coordination in such ligands, we have
selected two modified 2-(arylazo)phenols for the present
study, viz. 2-(20,60-dimethylphenylazo)-4-methylphenol
(3) where both the ortho positions of the phenyl ring
in arylazo fragment are blocked by methyl groups, and
The present work has originated from a recent study
in our laboratory, where we have observed that the 2-
(arylazo)phenols (1) undergo both phenolic O–H and
phenyl C–H activation upon their reaction with
*
Corresponding author. Tel.: +91 332 414 6223; fax: +91 332 414
6584.
E-mail address: samaresh_b@hotmail.com (S. Bhattacharya).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.05.025

R. Acharyya et al. / Journal of Organometallic Chemistry 690 (2005) 3908–3917
3909
2-(20-methylphenylazo)-4-methylphenol (4) where only
one ortho position is blocked by a methyl group. To
study the consequences of these ligand modifications,
reaction of ligands 3 and 4 has been carried out with
[Ir(PPh3)3Cl]. It may be noted here that unlike its
rhodium analogue, viz. the WilkinsonÕs catalyst
[Rh(PPh3)3Cl], reactivity of this iridium complex has
been much less explored [1,3]. However, we have found
it to be an excellent starting material for the synthesis of
different mixed-ligand complexes of iridium [1,4]. Reac-
tion of ligands 3 and 4 with [Ir(PPh3)3Cl] has been found
to afford interesting organoiridium complexes via C–H
activation. It is relevant to mention here that metal med-
iated C–H activation of organic molecules has been of
considerable current interest, as chemical transforma-
tions of organic molecules often proceed via a C–H acti-
vation step leading to the formation of organometallic
complexes as reactive intermediates, which then undergo
further reactions to yield the desired product [5]. An ac-
count of the chemistry of all the organoiridium com-
plexes, synthesized in this study, is presented here with
special reference to their formation, structure, and spec-
tral and electrochemical properties.
three signals indicates that ligand 3 is probably coordi-
nated to iridium as a tridentate C,N,O-donor 6 via dis-
sociation of the phenolic proton and another proton
from one methyl group in the 20,60-dimethylphenylazo
fragment. All the six aromatic proton signals, expected
from the coordinated ligand in 6, are also clearly ob-
1
served in the expected region. The H NMR spectrum
also shows a distinct triplet at ꢀ15.32 ppm indicating
the presence of a coordinating hydride and broad signals
within 7.17–7.43 ppm, corresponding to 30 hydrogens,
attributable to two triphenylphosphines. The triplet nat-
ure of the hydride signal indicates that the two triphe-
nylphosphines are magnetically equivalent in complex
1
5. The H NMR spectral data thus point to a definite
composition and stereochemistry for complex 5, in
which a doubly deprotonated ligand 3, two triphenyl-
phosphines and a hydride are coordinated to the metal
center and the two triphenylphosphines are mutually
OH
O
Ir
H3C
N
N
H3C
N
N
R = OCH3, CH3,
H, Cl, NO2
R
R
1
2
OH
OH
H3C
N
N
CH3
H3C
N
N
CH3
H3C
3
4
2. Results and discussion
2.1. Synthesis and characterization
Reaction of ligand 3 with [Ir(PPh3)3Cl] proceeds
smoothly in refluxing ethanol in the presence of triethyl-
amine to afford a pink complex 5 in descent yield.
Though ligand 3 contains three methyl groups, 1H
NMR spectrum of complex 5 shows only two methyl
signals of equal intensity at 1.95 and 2.29 ppm. How-
ever, a distinct signal corresponding to two hydrogens
is observed at 3.59 ppm, which is indicative of a signifi-
cantly deshielded CH2 fragment. Appearance of these
Fig. 1. View of (a) the complex 5 and (b) the equatorial plane in
complex 5.

3910
R. Acharyya et al. / Journal of Organometallic Chemistry 690 (2005) 3908–3917
Table 1
tion mode of ligand 3 in it, its structure has been deter-
mined by X-ray crystallography. The structure is shown
in Fig. 1 and relevant bond parameters are listed in
Table 1. The structure shows that all the indications
˚
Selected bond lengths (A) and bond angles (ꢁ) for complexes 5, 12 and 13
Complex 5
Bond lengths
Ir(1)–H(1)
Ir(1)–C(43)
Ir(1)–N(2)
Ir(1)–O(1)
Ir(1)–P(1)
1.45(3)
1
given by the H NMR spectrum are indeed correct. Li-
2.088(3)
2.074(3)
2.156(2)
2.3300(9)
2.3277(8)
1.410(5)
1.510(5)
1.426(5)
1.425(4)
1.427(4)
1.276(4)
1.313(4)
gand 3 is coordinated to iridium as a C,N,O-donor 6,
with bite angles of 80.10(10)ꢁ [O(1)–Ir(1)–N(2)] and
91.97(12)ꢁ [C(43)–Ir(1)–N(2)]. Two triphenylphosphines
and a hydride are also coordinated to the metal center.
Iridium is thus sitting in a HCNOP2 coordination
sphere, which is significantly distorted from ideal octa-
hedral geometry as reflected in the bond parameters
around the metal center. The coordinated ligand 3 and
the hydride are sharing the same equatorial plane with
iridium at the center, and the two triphenylphosphines
have occupied the remaining two axial positions. The
Ir–H and Ir–P distances are quite normal and so are
the Ir–C, Ir–N, Ir–O, C–O and N–N bond lengths with-
in the C,N,O-chelated fragment 6 [1,6]. The observed
C–H activation of a methyl group in ligand 3 has been
quite interesting, and it may be mentioned here that
though ligand 3 has recently been observed to undergo
interesting chemical transformations via migration of
one methyl group [7], such a methyl C–H activation of
ligand 3 has been observed for the first time. It is also
worth noting here that compared to phenyl C–H activa-
tion, methyl C–H activation is relatively less common
[8].
Ir(1)–P(2)
C(37)–C(42)
C(37)–C(43)
C(45)–C(46)
C(46)–N(2)
C(42)–N(1)
N(1)–N(2)
C(45)–O(1)
Bond angles
H(1)–Ir(1)–N(2)
C(43)–Ir(1)–O(1)
P(1)–Ir(1)–P(2)
C(43)–Ir(1)–N(2)
N(2)–Ir(1)–O(1)
176.6(16)
171.56(11)
170.12(3)
91.97(12)
80.10(10)
Complex 12
Bond lengths
Ir–H(1A)
Ir–N(1)
1.47(3)
2.017(3)
2.179(2)
2.3059(8)
2.3107(8)
1.410(4)
1.397(4)
1.276(4)
1.311(4)
Ir–O(1)
Ir–P(1)
Ir–P(2)
C(7)–N(1)
C(6)–N(2)
N(1)–N(2)
C(12)–O(1)
O
Ir
Bond angles
H(1A)–Ir–N(1)
C(1)–Ir–O(1)
P(1)–Ir–P(2)
C(1)–Ir–N(1)
N(1)–Ir–O(1)
174.7(11)
156.83(12)
163.05(3)
77.60(12)
79.32(9)
H3C
N
N
CH2
H3C
Complex 13
Bond lengths
Ir(1)–C(8)
Ir(1)–N(1)
Ir(1)–O(1)
Ir(1)–P(1)
Ir(1)–P(2)
Ir(1)–Cl(1)
C(6)–N(1)
C(7)–N(2)
N(1)–N(2)
C(1)–O(1)
6
2.030 (10)
1.941(8)
2.191(9)
2.349(4)
2.365(4)
2.395(3)
1.345(14)
1.438(14)
1.312(12)
1.312(13)
The presence of a hydride in complex 5 has also been
quite interesting. Two sources of the hydride seem prob-
able, ethanol may serve as a source of the hydride, or
oxidative insertion of iridium(I) into the phenolic O–H
bond may generate a hydride. To check this, reaction
of ligand 3 with [Ir(PPh3)3Cl] has also been carried out
in a nonalcoholic solvent, viz. toluene, in the presence
of triethylamine. A mixture of two complexes, with dis-
tinctly different colors (pink and purple), has been ob-
tained from this reaction. The individual complexes
have been separated by chromatography. The combined
yield of the two complexes has been satisfactory. Char-
acterization on the pink complex has confirmed it to be
the same complex 5, which was obtained as the sole
product from the ethanol reaction. However, the purple
Bond angles
C(8)–Ir(1)–O(1)
N(1)–Ir(1)–Cl(1)
P(1)–Ir(1)–P(2)
C(8)–Ir(1)–N(1)
N(1)–Ir(1)–O(1)
158.4(4)
173.3(3)
172.42(11)
77.9(4)
80.5(4)
trans. The observed microanalytical data of complex 5
are also consistent with this composition. To verify the
stereochemistry of complex 5, as well as the coordina-
1
complex has been found to be different. H NMR spec-
trum of the purple complex does not show any hydride

R. Acharyya et al. / Journal of Organometallic Chemistry 690 (2005) 3908–3917
3911
signal. Besides that, rest of the 1H NMR spectrum of the
OH
purple complex is qualitatively similar to that of com-
plex 5. Infrared spectrum of this purple complex indi-
cates the presence of a coordinated chloride in it (vide
H3C
N
N
CH3
1
infra). The H NMR and IR spectral data thus show
that the purple complex has a similar composition as
complex 5, except that a chloride is coordinated to irid-
ium instead of a hydride, and hence we assume a struc-
ture 7 for this purple complex (henceforth referred to as
complex 7). Structural characterization of complex 7 by
X-ray crystallography has not been possible, as its crys-
tals could not be grown. However, trans disposition of
two triphenylphosphines has been confirmed by the
31P NMR spectrum of this complex, which shows a sin-
gle resonance at 50.20 ppm.
H3C
[Ir(PPh3)3Cl]
- PPh3
P
Ir
P
P = PPh3
P
Ir
P
H
Cl
O
O
Cl
H
H3C
N
N
H3C
N
N
CH3
CH3
P
Cl
O
H3C
H3C
P = PPh3
Ir
P
8
9
H3C
N
N
CH2
(?)
- H2
- HCl
H3C
P
Ir
P
P
Ir
P
7
H
Cl
O
O
H3C
N
N
CH2
H3C
N
N
CH2
Formation of the hydride complex 5 in addition to its
chloride analogue 7 from the reaction carried out in tol-
uene clearly shows that ethanol has not been the source
of hydride in complex 5 in the earlier synthetic reaction.
The exact mechanism of the synthetic reactions is not
completely clear to us. However, the sequences shown
in Scheme 1 seem probable. In the initial step, ligand 3
binds to the metal center in [Ir(PPh3)3Cl] via oxidative
insertion of iridium into the phenolic O–H bond, and
simultaneous dissociation of one triphenylphosphine,
and thus affords two geometric isomers of a reactive
intermediate (complexes 8 and 9). These intermediates
then undergo cyclometalation via elimination of either
HCl (in case of 8) or H2 (in case of 9) affording com-
plexes 5 and 7, respectively. It seems that in the ethanol
reaction only the intermediate 8 is formed, and its isom-
erization to complex 9 could not take place because of
the relatively low boiling point of ethanol. Isolation of
these speculated intermediates has not been possible
probably due to their rapid transformation into the cor-
responding cyclometalated species.
H3C
H3C
5
7
Scheme 1. Probable steps of the synthetic reactions.
position is still unsubstituted. Hence, two types of
C–H activation, viz. methyl C–H activation and phenyl
C–H activation, are in principle possible in this ligand
affording cyclometallated species of type 10 and 11,
respectively. To check the preference of C–H activation,
if any, of ligand 4, its reaction with [Ir(PPh3)3Cl] has
been carried out in refluxing ethanol in the presence of
triethylamine, which has afforded a blue complex 12.
1H NMR spectrum of this complex shows two methyl
signals at 1.94 and 2.31 ppm and thus excludes the pos-
sibility of linkage isomer 10. Structure of this complex
12 has been determined by X-ray crystallography. The
structure (Fig. 2) shows that ligand 4 is coordinated as
shown in 11. Two triphenylphosphines and a hydride
are also coordinated to the metal center. The observed
bond distances (Table 1) compare well with those in
complex 5. To investigate whether a chloride analogue
of complex 12 can be obtained from a non-alcoholic sol-
vent, reaction of ligand 4 with [Ir(PPh3)3Cl] has also
been carried out in refluxing toluene in the presence of
Facile C–H activation of the methyl group in ligand
3, observed on its reaction with [Ir(PPh3)3Cl], encour-
aged us to explore the possibility of such C–H activation
in similar ligands. Thus, we have selected 2-(20-meth-
ylphenylazo)-4-methylphenol (4) as the next ligand. This
particular ligand has been chosen because in it only one
ortho position of the phenyl ring in the arylazo fragment
is blocked by a methyl group, while the other ortho

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R. Acharyya et al. / Journal of Organometallic Chemistry 690 (2005) 3908–3917
plex 12, except that the position of hydride in 12 is occu-
pied by a chloride in 13. The Ir–Cl bond length is
normal [1] and the other bond parameters around irid-
ium in complex 13 are found to be similar to those ob-
served in complex 12. Besides the absence of the
hydride signal, 1H NMR spectrum of complex 13 is very
similar to that of complex 12. The difference in the num-
ber and nature of the product in two different solvents
viz. ethanol and toluene may be rationalized as ex-
plained in Scheme 1.
O
O
Ir
Ir
H3C
N
N
CH2
H3C
N
N
H3C
10
11
Fig. 2. View of the complex 12.
triethylamine. A mixture of two blue complexes has
been obtained from this reaction, which have been sep-
arated by chromatography. The blue complex, which
moves faster through the column, has been found to
be complex 12. The other blue complex, which moves
relatively slowly through the column, has been found
to be the chloride analogue of complex 12. This second
blue complex, henceforth referred to as complex 13, has
also been structurally characterized. The structure (Fig.
3) shows that complex 13 has a similar structure as com-
2.2. Spectral studies
Infrared spectra of all the complexes show many
sharp and strong bands of different intensities within
2200–200 cmꢀ1. Attempt has not been made to assign
each individual band to a specific vibration. However,
the presence of three strong bands near 515, 695 and
750 cmꢀ1 in all the complexes is attributable to the coor-
dinated PPh3 ligands. In complexes 5 and 12, a sharp
band is observed at 2110 and 2050 cmꢀ1, respectively,
which is assigned to the Ir–H stretch. In complexes 7
and 13, the Ir–Cl stretch has been observed at 295 and
293 cmꢀ1, respectively. Electronic spectra of the com-
plexes have been recorded in dichloromethane solution.
Each complex shows several intense absorptions in the
visible and ultraviolet region (Table 2). The absorptions
in the ultraviolet region are believed to be due to transi-
tions within the ligand orbitals, and those in the visible
region are probably due to metal-to-ligand charge-trans-
fer transitions. To have an insight into the nature of
absorptions in the visible region, qualitative EHMO cal-
culations have been performed [9] on computer-gener-
ated models of all the four complexes, where phenyl
rings of the triphenylphosphines have been replaced by
hydrogens. The results are found to be qualitatively sim-
ilar for all the complexes [10]. Compositions of selected
molecular orbitals are given in Table 3 and partial MO
diagram of a selected complex is shown in Fig. 4. Partial
MO diagrams of the other three complexes are deposited
as supporting information (Fig. S1–S3). The highest
occupied molecular orbital (HOMO) and the next two
filled orbitals (HOMO ꢀ 1 and HOMO ꢀ 2) have major
contribution from the iridium dxy, dyz and dzx orbitals
Fig. 3. View of the complex 13.

R. Acharyya et al. / Journal of Organometallic Chemistry 690 (2005) 3908–3917
3913
Table 2
Electronic spectral and cyclic voltammetric data
Compound
Electronic spectral data kmax, nm (e, Mꢀ1 cmꢀ1 a
)
Cyclic voltammetric datab E, V vs. SCE
Complex 5
Complex 7
Complex 12
Complex 13
a
548(13,600), 406(7200)c, 346(17,200), 292(26,400)c, 266(44300)c
606(13,100), 572(12,850), 406(14,600)c, 340(25,800), 268(51,300)
604(6400), 574(6000)c, 412(4000)c, 342(10,100), 278(22,000)c
644(3100), 600(2800), 388(3700), 354(5600)c, 338(5700), 258(20,700)
1.15d, 0.64d, ꢀ1.22e
1.08d, 0.60d, ꢀ1.26e
1.22d, 0.64(67)f, ꢀ1.23e
1.39d, 0.73(60)f, ꢀ1.10e
In dichloromethane.
Solvent, 1:9 dichloromethane-acetonitrile; supporting electrolyte, TBAP; scan rate 50 mVsꢀ1
Shoulder.
.
b
c
d
e
f
Epa value.
Epc value.
E1/2 value (DEp value), where E1/2 = 0.5 (Epa + Epc) and DEp = (Epa ꢀ Epc).
Table 3
Composition of selected molecular orbitals
Compound
Contributing fragments
% Contribution of fragments to
HOMO
HOMO ꢀ 1
HOMO ꢀ 2
LUMO
LUMO + 1
LUMO + 2
Complex 5
Ir
63
33
6
65
27
–
20
75
–
14
86
54
–
–
Ligand
azo
97
–
95
–
Complex 7
Complex 12
Complex 13
Ir
71
24
–
65
27
–
64
33
–
15
79
52
–
–
Ligand
azo
97
–
95
–
Ir
84
7
58
36
–
66
25
–
12
83
53
–
–
Ligand
azo
94
–
96
–
–
Ir
84
5
61
34
–
64
29
–
17
80
47
–
48
43
–
Ligand
azo
97
–
–
Fig. 4. Partial molecular orbital diagram of complex 5.
[10]. These three filled orbitals may therefore be re-
garded as iridium-t2 orbitals. The lowest unoccupied
molecular orbital (LUMO) has P80% contribution
from the 2-(arylazo)phenolate ligands and is concen-
trated mostly (P50%) on the azo fragment. The
LUMO + 1 and LUMO + 2 are localized on other parts

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R. Acharyya et al. / Journal of Organometallic Chemistry 690 (2005) 3908–3917
of the 2-(arylazo)phenolate ligands. The lowest energy
absorption in the visible region is therefore assignable
to an allowed charge-transfer transition from the filled
OH
OH
R1
R2
R1
R2
H
H
H3C
N
N
C
H3C
N
N
C
iridium-t orbital (HOMO) to the vacant p (azo)-orbi-
*
2
tal of the 2-(arylazo)phenolate ligand (LUMO). The
other intense absorptions in the visible region may be as-
signed to charge-transfer transitions from the filled t2
orbitals to the higher energy vacant orbitals.
R1 = H, CH3
R2 = H, CH3
H
R1
C
R2
14
15
2.3. Electrochemical properties
Electrochemical properties of all the complexes have
been studied by cyclic voltammetry in 1:9 dichloro-
methane–acetonitrile solution (0.1 M TBAP) [11].
Voltammetric data are given in Table 2. Each complex
shows two oxidative responses on the positive side of
SCE and a reductive response on the negative side.
In view of the results of the EHMO calculations, the
first oxidative response is assigned to Ir(III)–Ir(IV)
oxidation and the reductive response is assigned to
reduction of the azo function of the coordinated
2-(arylazo)phenolate ligand. The second oxidative
response is tentatively assigned to oxidation of the
coordinated 2-(arylazo)phenolate ligand. In complex
5, the Ir(III)–Ir(IV) oxidation has been found to be
completely irreversible, however, in complex 7 the
same is found to be quasi-reversible. In complexes 12
and 13 the Ir(III)–Ir(IV) oxidation has been found to
be reversible, characterized by a peak-to-peak separa-
tion (Ep) of about 60 mV, which remains unchanged
upon variation of scan rate, and the anodic peak-cur-
rent (ipa) is also found to be equal to the cathodic
peak-current (ipc), as expected for a reversible couple.
The ligand oxidation, as well as the ligand reduction,
have been found to be irreversible in all the complexes.
One-electron nature of the Ir(III)–Ir(IV) oxidation has
been established by comparing its current height (ipa)
with that of standard ferrocene–ferrocenium couple
under identical experimental conditions. The irrevers-
ible redox responses have been found to show non-
stoichiometric currents.
4. Experimental
Iridium trichloride was purchased from Arora
Matthey, Kolkata, India, and 2,6-dimethylaniline was
obtained from Loba, Mumbai, India. 2-Methylaniline
and p-cresol were purchased from S.D., India. The
2-(arylazo)phenols were prepared by coupling respec-
tive diazotized aniline with p-cresol. [Ir(PPh3)3Cl]
was prepared as before [1]. Purification of acetonitrile
and dichloromethane, and preparation of tetrabuty-
lammonium perchlorate (TBAP) for electrochemical
work were performed as reported in the literature
[12]. Microanalyses (C,H,N) were performed using a
Perkin–Elmer 240C elemental analyzer. IR spectra
were obtained on a Perkin–Elmer 1601 spectrometer
with samples prepared as KBr pellets. Electronic spec-
tra were recorded on a JASCO V-570 spectrophotom-
eter. NMR spectra were recorded in CDCl3 solution
using a Bruker drx500 or Bruker Avance dpx300
NMR spectrometer. 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
dinitrogen atmosphere. All electrochemical data were
collected at 298 K and are uncorrected for junction
potentials.
4.1. Synthesis of complexes
3. Conclusions
4.1.1. Complex 5
2-(20,60-Dimethylphenylazo)-4-methylphenol (25 mg,
0.10 mmol) was dissolved in ethanol (50 mL) and the
solution was purged with a stream of dinitrogen for
5 min. To the solution were added triethylamine
(20 mg, 0.20 mmol) and [Ir(PPh3)3Cl] (100 mg,
0.10 mmol) successively. The mixture was refluxed under
a dinitrogen atmosphere for 24 h, when a red solution was
obtained. Evaporation of this solution afforded a red so-
lid, which was purified by thin layer chromatography on a
silica plate with benzene as the eluant. A pink band sepa-
rated, which was extracted with acetonitrile. Upon slow
The present study shows that C–H activation of suit-
able alkyl groups, introduced into the ortho positions of
the phenyl ring in the arylazo fragment of 2-(ary-
lazo)phenols (1) can be easily achieved by reacting such
ligands with [Ir(PPh3)3Cl]. Such studies involving li-
gands of type 14 are currently in progress. The present
study also shows that in ligands of type 15, where two
types of C–H activation is possible, the C–H activation
at the unsubstituted ortho position of the aryl ring is
much more likely compared to C–H activation of the al-
kyl (–CHR1R2) group.

R. Acharyya et al. / Journal of Organometallic Chemistry 690 (2005) 3908–3917
3915
evaporation of the acetonitrile extract complex 5 was ob-
tained as a crystalline pink solid. Yield: 60%.
Anal. Calc. for C51H45N2OP2Ir (5): C, 64.08; H, 4.71;
atmosphere for 12 h, when a blue solution was obtained.
Evaporation of this solution afforded a blue solid, which
was purified by thin layer chromatography on a silica
plate with benzene as the eluant. A blue band separated,
which was extracted with acetonitrile. Slow evaporation
of the eluate gave complex 12 as a crystalline solid.
Yield: 70%.
1
N, 2.93. Found: C, 64.00; H, 4.67; N, 2.95%. H NMR
[13]: ꢀ15.32 (t, hydride, J = 18.6), 1.95 (CH3), 2.29
(CH3), 3.59 (t, CH2, J = 10.2), 6.14 (d, 1H, J = 7.4),
6.36 (d, 1H, J = 8.5), 6.50–6.64 (2H*), 6.67 (s, 1H),
6.85 (d, 1H, J = 7.1), 7.17–7.43 (2PPh3).
Anal. Calc. for C50H43N2OP2Ir (12): C, 63.76; H,
1
4.57; N, 2.98. Found: C, 64.00; H, 4.63; N, 3.01%. H
4.1.2. Complex 7
NMR [13]: ꢀ12.29 (t, hydride, J = 18.0), 1.94 (CH3),
2.31 (CH3), 5.95 (t, 1H, J = 7.3), 6.09 (d, 1H, J = 7.6),
6.25 (d, 1H, J = 8.7), 6.33 (d, 1H, J = 7.2), 6.43 (s,
1H), 6.50 (d, 1H, J = 8.8), 7.12–7.50 (2PPh3).
This complex was prepared by following the same
above procedure using toluene instead of ethanol and
the refluxing time was 48 h. Purification was achieved
by thin layer chromatography on a silica plate with ben-
zene as the eluant. Two bands (pink and purple) sepa-
rated, which were extracted with acetonitrile. Upon
slow evaporation of the pink and purple extracts
complex 5 (Yield: 35%) and 7 (Yield: 20%) were
obtained, respectively, as crystalline solids.
4.1.4. Complex 13
This complex was prepared by following the same
above procedure using toluene instead of ethanol and
the refluxing time was 24 h. Purification was achieved
by thin layer chromatography on a silica plate with ben-
zene as the eluant. Two blue bands separated, which
were extracted with acetonitrile. Slow evaporation of
these extracts gave complex 12 (Yield: 40%) and com-
plex 13 (Yield: 32%) as crystalline solids.
Anal. Calc. for C51H44N2OP2ClIr (7): C, 61.85; H,
1
4.45; N, 2.83. Found: C, 61.53; H, 4.37; N, 2.89%. H
NMR [13]: 1.82 (CH3), 2.29 (CH3), 5.01 (t, CH2,
J = 10.0), 6.12 (d, 1H, J = 7.2), 6.50 (d, 1H, J = 7.5),
6.60 (s, 1H), 6.79 (t, 1H, J = 7.3), 7.00 (d, 1H, J = 7.0),
7.03–7.64 (2PPh3). 31P NMR: 50.20 (s).
Anal. Calc. for C50H42N2OP2ClIr (13): C, 61.51; H,
1
4.31; N, 2.87. Found: C, 61.77; H, 4.37; N, 2.89%. H
NMR [13]: 1.79 (CH3), 2.37 (CH3), 5.87 (s, 1H), 6.26
(d, 1H, J = 8.9), 6.32 (d, 1H, J = 7.6), 6.38–6.50 (2H*),
6.92 (d, 1H, J = 7.4), 7.02–7.59 (2PPh3).
4.1.3. Complex 12
2-(20-Methylphenylazo)-4-methylphenol (25 mg, 0.11
mmol) was dissolved in ethanol (50 mL) and the solu-
tion was purged with a stream of dinitrogen for 5 min.
To the solution were added triethylamine (20 mg,
0.20 mmol) and [Ir(PPh3)3Cl] (100 mg, 0.10 mmol) suc-
cessively. The mixture was refluxed under a dinitrogen
4.2. X-ray structure determination
Single crystals of complexes 5 and 12 were obtained
by slow evaporation of acetonitrile solutions of the
Table 4
Selected crystallographic data for the complexes 5, 12 and 13
Complex 5
Complex 12
Complex 13
Empirical formula
Fw
C
956.03
51H45N2OP2Ir
C50H43N2OP2Ir
942.00
Monoclinic, P21/c
C50H42N2OP2ClIr
976.45
ꢀ
ꢀ
Space group
Unit cell dimensions
Triclinic, P1
Triclinic, P1
˚
a (A)
11.1750(12)
11.3021(12)
18.8872(19)
89.168(2)
87.418(2)
61.819(2)
2100.5(4)
2
17.4737(6)
10.2005(4)
23.1955(9)
90
10.0981(5)
11.8715(6)
17.9205(9)
99.7520(10)
91.8930(10)
95.6850(10)
2104.13(18)
2
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
92.798(1)
90
3
˚
V (A )
4129.4(3)
4
Z
˚
k (A)
Crystal size (mm)
T (K)
0.71073
0.71073
0.35 · 0.35 · 0.10
295(2)
0.71073
0.42 · 0.30 · 0.14
293(2)
3.295
0.50 · 0.30 · 0.20
273(2)
3.353
l (mꢀ1
R1
)
3.351
0.0277
a
0.0296
0.0741
1.012
0.0301
0.0823
0.705
b
WR2
Goodness-of-fitc
0.0633
1.076
a
R1 = RiFo| ꢀ |Fci/R|Fo|.
2
1=2
b
c
wR2 ¼ ½RfwðF 2o ꢀ F c2Þ g=RfwðF 2oÞgꢁ
.
2
GOF ¼ ½RðwðF 2o ꢀ F c2Þ Þ=ðM ꢀ NÞꢁ1=2, where M is the number of reflections and N is the number of parameters refined.

3916
R. Acharyya et al. / Journal of Organometallic Chemistry 690 (2005) 3908–3917
(b) E. Mueller, C. Beissner, Chemiker-Zeitung 96 (1972) 170;
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581.
respective complexes. Crystals of complex 13 were
grown by slow diffusion of hexane into a dichlorometh-
ane solution of the complex. Selected crystal data and
data collection parameters are given in Table 4. Data
were collected on a Siemens Smart CCD diffractometer
using graphite monochromated Mo Ka radiation
[4] R. Acharyya, S. Basu, S.M. Peng, G.H. Lee, S. Bhattacharya,
unpublished results.
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˚
(k = 0.71073 A) by / and x scans. X-ray data reduction,
and structure solution and refinement were done using
SHELXS-97 and SHELXL-97 programs [14]. The structures
were solved by direct methods.
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Acknowledgments
(g) M. Bellar, C. Bolm (Eds.), Transition Metals for Organic
Synthesis, vols. 1–2, Wiley-VCH, Weinheim, 1998;
(h) L.S. Hegedus, Coord. Chem Rev. 168 (1998) 49;
(i) B. Cornils, W.A. Hermann (Eds.), Applied homogeneous
Financial assistance received from the Department of
Science and Technology, New Delhi, India [Grant No.
SR/S1/IC-15/2004] is gratefully acknowledged. The
authors thank the RSIC at Central Drug Research Insti-
tute, Lucknow, India, for the C,H,N analysis data, and
the Department of Chemistry, Indian Institute of Tech-
nology – Kanpur, India, for the X-ray diffraction data of
complex 13. Thanks also to Dr. Surajit Chattopadhyay,
Department of Chemistry, Kalyani University, for his
help with the IR spectral measurements. R.A. thanks
the Council of Scientific and Industrial Research, New
Delhi for her fellowship [Grant No. 9/96(438)2004 –
EMR-I].
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A comprehensive
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Crystallographic data for structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDCs 265561–265563 for complex 5,
12 and 13, respectively. Copies of this information
may be obtained free of charge from The Director,
CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK,
fax (int code): +44 1223 336 033, or email: depos-
it@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk.
Partial molecular orbital diagrams of complex 7 (Fig.
S1), complex 12 (Fig. S2), and complex 13 (Fig. S3)
are available as supporting information. Supplementary
data associated with this article can be found, in the on-
line version at doi:10.1016/j.jorganchem.2005.05.025.
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R. Acharyya et al. / Journal of Organometallic Chemistry 690 (2005) 3908–3917
3917
[10] The HOMO ꢀ 2 in complex 5 has less metal character.
[11] A little dichloromethane was necessary to take the complex into
solution. Addition of large excess of acetonitrile was necessary to
record the redox responses in proper shape.
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are given in parentheses. Overlapping signals are marked with
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