Iridium(III) Complexes Formed by O-H and/or C-H Activation of 2-(Arylazo)phenols

Article abstract of DOI:10.1021/ic034785w

Reaction of 2-(arylazo)phenols with [Ir(PPh₃)₃Cl] in refluxing ethanol in the presence of a base (NEt₃) affords complexes of three different types, viz. [Ir(PPh₃)₂(NO-R)(H)Cl] (R = OCH₃, CH₃, H, Cl and NO₂), [Ir(PPh ₃)₂(NO-R)(H)₂] and [Ir(PPh₃) ₂(CNO-R)(H)]. Structures of the [Ir(PPh₃) ₂(NO-Cl)(H)Cl], [Ir(PPh₃)₂(NO-Cl)(H) ₂] and [Ir(PPh₃)₂(CNO-Cl)(H)] complexes have been determined by X-ray crystallography. In the [Ir(PPh₃) ₂(NO-R)(H)Cl] and [Ir(PPh₃)₂(NO-R)(H) ₂] complexes, the 2-(arylazo)phenolate ligands are coordinated to the metal center as monoanionic bidentate N,O-donors, whereas in the [Ir(PPh₃)₂(CNO-R)(H)] complexes, they are coordinated to iridium as dianionic tridentate C,N,O-donors. In all three products formed in ethanol, the two PPh₃ ligands are trans. Reaction of 2-(arylazo)phenols with [Ir(PPh₃)₃Cl] in refluxing toluene in the presence of NEt₃ affords complexes of two types, viz. [Ir(PPh₃)₂(CNO-R)(H)] and [Ir(PPh₃) ₂(CNO-R)Cl]. Structure of the [Ir(PPh₃) ₂(CNO-Cl)Cl] complex has been determined by X-ray crystallography, and the 2-(arylazo)phenolate ligand is coordinated to the metal center as a dianionic tridentate C,N,O-donor and the two PPh₃ ligands are cis. All of the iridium(III) complexes show intense MLCT transitions in the visible region. Cyclic voltammetry shows an Ir(III)-Ir(IV) oxidation on the positive side of SCE and an Ir(III)-Ir(II) reduction on the negative side for all of the products.

Full text of DOI:10.1021/ic034785w

Inorg. Chem. 2004, 43, 704−711
Iridium(III) Complexes Formed by O−H and/Or C−H Activation of
2-(Arylazo)phenols
Rama Acharyya,1a Falguni Basuli,1a Ren-Zhang Wang,1b Thomas C. W. Mak,1b and
,1a
Samaresh Bhattacharya*
Department of Chemistry, Inorganic Chemistry Section, JadaVpur UniVersity,
Kolkata 700 032, India and Department of Chemistry, The Chinese UniVersity
of Hong Kong, Shatin, New Territories, Hong Kong
Received July 8, 2003
Reaction of 2-(arylazo)phenols with [Ir(PPh3)3Cl] in refluxing ethanol in the presence of a base (NEt3) affords com-
plexes of three different types, viz. [Ir(PPh3)2(NO−R)(H)Cl] (R ) OCH , CH , H, Cl and NO ), [Ir(PPh3)2(NO−R)-
3
3
2
(H)2] and [Ir(PPh3)2(CNO−R)(H)]. Structures of the [Ir(PPh3)2(NO−Cl)(H)Cl], [Ir(PPh3)2(NO−Cl)(H)2] and [Ir(PPh3)2-
(CNO−Cl)(H)] complexes have been determined by X-ray crystallography. In the [Ir(PPh3)2(NO−R)(H)Cl] and
[Ir(PPh ) (NO−R)(H)2] complexes, the 2-(arylazo)phenolate ligands are coordinated to the metal center as monoanionic
3 2
bidentate N,O-donors, whereas in the [Ir(PPh3)2(CNO−R)(H)] complexes, they are coordinated to iridium as dianionic
tridentate C,N,O-donors. In all three products formed in ethanol, the two PPh3 ligands are trans. Reaction of
2-(arylazo)phenols with [Ir(PPh3)3Cl] in refluxing toluene in the presence of NEt3 affords complexes of two types,
viz. [Ir(PPh3)2(CNO−R)(H)] and [Ir(PPh3)2(CNO−R)Cl]. Structure of the [Ir(PPh3)2(CNO−Cl)Cl] complex has been
determined by X-ray crystallography, and the 2-(arylazo)phenolate ligand is coordinated to the metal center as a
dianionic tridentate C,N,O-donor and the two PPh3 ligands are cis. All of the iridium(III) complexes show intense
MLCT transitions in the visible region. Cyclic voltammetry shows an Ir(III)−Ir(IV) oxidation on the positive side of
SCE and an Ir(III)−Ir(II) reduction on the negative side for all of the products.
Introduction
Though 2-(arylazo)phenols (1) usually bind to metal ions
as bidentate N,O-donors forming a six-membered chelate ring
(2),2 they have also been observed to coordinate metal ions
as bidentate N,O-donors forming five-membered chelate ring
(3).3 In some cases, C-H activation from an ortho position
of the pendant phenyl ring has been observed and the 2-(aryl-
azo)phenol derivative ligand serves as a tridentate C,N,O-
donor (4).4 In our recent studies on the interaction of 2-(aryl-
azo)phenols (1), Wilkinson’s catalyst (viz. [Rh(PPh3)3Cl])
was found to successfully induce C-H activation of the
* Author to whom correspondence should be addressed. E-mail:
samaresh_b@hotmail.com.
(1) (a) Jadavpur University. (b) The Chinese University of Hong Kong.
(2) (a) Rath, R. K.; Nethaji, M.; Chakravarty, A. R. J. Organomet. Chem.
2001, 633, 79. (b) Sui, K.; Peng, S. M.; Bhattacharya, S. Polyhedron
1999, 19, 631. (c) Bhawmik, R.; Biswas, H.; Bandyopadhyay, P. J.
Organomet. Chem. 1995, 498, 81. (d) Sinha, C. R.; Bandyopadhyay,
D.; Chakravorty, A. J. Chem. Soc., Chem. Commun. 1988, 468. (e)
Dyachenko, O. A.; Atovmyan, L. O.; Aldosin, S. M. J. Chem. Soc.,
Chem. Commun. 1975, 105. (f) Kalia, K. C. Indian J. Chem. 1970, 8,
1035. (g) Price, R. J. J. Chem. Soc. A 1969, 1296. (h) Jarvis, J. A. J.
Acta Crystallogr. 1961, 14, 961.
2-(arylazo)phenols. Encouraged by this result, the analogous
iridium complex, viz. [Ir(PPh3)3Cl], has been chosen to ex-
plore further C-H activation of the 2-(arylazo)phenols (1).
The array of products formed from such reactions is pre-
sented herein with special reference to their synthesis and
(3) Basuli, F.; Peng, S. M.; Bhattacharya, S. Polyhedron 1998, 18, 391.
704 Inorganic Chemistry, Vol. 43, No. 2, 2004
10.1021/ic034785w CCC: $27.50 © 2004 American Chemical Society
Published on Web 12/13/2003

Iridium(III) Complexes
6.49 (d, 1H, J ) 8.6); 6.57 (t, 1H, J ) 7.4); 7.10 (d, 1H, J ) 7.6);
1.91(CH3); -12.38 (hydride, J ) 19.5).
All the other [Ir(PPh3)2(NO-R)(H)Cl], [Ir(PPh3)2(NO-R)(H)2],
and [Ir(PPh3)2(CNO-R)(H)] complexes were prepared by following
the same above procedure using appropriate 2-(arylazo)phenols (1,
R * H) instead of 2-(phenylazo)-4-methylphenol.
characterization. In this paper, the coordinated 2-(arylazo)-
phenolate ligands are in general referred to as NO-R
while coordinated as in 3, and as CNO-R while coordinated
as in 4.
Experimental Section
Anal. Calcd for [Ir(PPh3)2(NO-OCH3)(H)Cl]: C, 60.38; H,
4.43; N, 2.82. Found: C, 60.07; H, 4.49; N, 2.91. 1H NMR: 7.15-
7.48 (2PPh3); 6.63 (s, 1H); 6.39-6.50 (4H*); 6.78 (d, 2H, J )
8.3); 1.98 (CH3); 3.85 (OCH3); -20.69 (hydride, J ) 12.5).
Anal. Calcd for [Ir(PPh3)2(NO-OCH3)(H)2]: C, 62.55; H, 4.69;
N, 2.92. Found: C, 61.85; H, 4.37; N, 3.07. 1H NMR: 7.13-7.43
(2PPh3); 6.25 (d, 2H, J ) 8.0); 6.59 (d, 1H, J ) 8.6); 6.92 (d,
1H, J ) 8.1); 6.79 (d, 2H, J ) 7.8); 7.15 (s, 1H); 2.11 (CH3);
3.84 (OCH3); -19.23 (hydride, J ) 20.0); -23.19 (hydride,
J ) 15.5). Anal. Calcd for [Ir(PPh3)2(CNO-OCH3)(H)]: C,
Materials. Iridium trichloride was obtained from Arora Matthey,
Kolkata, India. The para-substituted anilines and p-cresol were
purchased from S.D., India. The 2-(arylazo)phenols (1) were
prepared by coupling diazotized para-substituted anilines with
p-cresol, and [Ir(PPh3)3Cl] was prepared by a convenient method
developed in our laboratory (see below). Purification of acetonitrile
and preparation of tetrabutylammonium perchlorate (TBAP) for
electrochemical work were performed as reported in the literature.5
All other chemicals and solvents were reagent grade commercial
materials and were used as received.
1
62.68; H, 4.49; N, 2.92. Found: C, 63.19; H, 4.47; N, 2.85. H
NMR: 7.16-7.48(2PPh3); 5.68 (s, 1H); 6.17 (d, 1H, J ) 8.5);
6.25 (d, 1H, J ) 8.5); 6.37 (s, 1H); 6.47 (d, 1H, J ) 8.5); 7.09
(d, 1H, J ) 8.5); 1.91 (CH3); 3.19 (OCH3); -12.50 (hydride, J )
19.5).
Preparations of Complexes. [Ir(PPh3)3Cl]. Triphenylphosphine
(6 g) was dissolved in hot tert-butyl alcohol (200 mL), and the
solution was purged with a stream of dinitrogen for 10 min. Iridium
trichloride (1 g) was then added to the solution, and it was heated
at reflux for 72 h under a dinitrogen atmosphere. [Ir(PPh3)3Cl]
separated as a yellow crystalline precipitate, which was collected
from the hot solution by filtration, washed thoroughly with ether,
and dried in air. Yield: 94%.
Anal. Calcd for [Ir(PPh3)2(NO-CH3)(H)Cl]: C, 61.37; H, 4.50;
N, 2.86. Found: C, 60.85; H, 4.39; N, 2.87. 1H NMR: 6.92-7.25
(2PPh3); 6.04 (d, 2H, J ) 7.7); 6.44 (s, 1H); 6.50-6.63 (4H*);
1.81 (CH3); 2.19 (CH3); -20.97 (hydride, J ) 12.0). Anal. Calcd
for [Ir(PPh3)2(NO-CH3)(H)2]: C, 63.61; H, 4.77; N, 2.97. Found:
C, 64.01; H, 5.02; N, 2.99. 1H NMR: 7.11-7.45 (2PPh3); 6.21 (d,
2H, J ) 8.1); 6.44 (d, 1H, J ) 8.7); 6.72 (d, 1H, J ) 8.9); 6.76 (d,
2H, J ) 7.7); 7.02 (s, 1H); 2.08 (CH3); 2.39 (CH3); -19.26
(hydride, J ) 19.5); -23.22 (hydride, J ) 15.0). Anal. Calcd for
[Ir(PPh3)2(CNO-CH3)(H)]: C, 63.75; H, 4.57; N, 2.97. Found: C,
[Ir(PPh3)2(NO)H)(H)Cl], [Ir(PPh3)2(NO)H)(H)2] and
[Ir(PPh3)2(CNO)H)(H)]. 2-(Phenylazo)-4-methylphenol (25 mg,
0.12 mmol) was dissolved in ethanol (50 mL) and triethylamine
(24 mg, 0.24 mmol) was added to it. The solution was then purged
with a stream of dinitrogen for 10 min and to it was added
[Ir(PPh3)3Cl] (100 mg, 0.10 mmol). The mixture was refluxed under
a dinitrogen atmosphere for 12 h, whereby a bluish-green solution
was obtained. Evaporation of this solution afforded a dark solid,
which was subjected to purification by thin-layer chromatography
on a silica plate. With benzene as the eluant, three distinct bands
(viz. purple, pink and blue) separated, which were extracted
separately with acetonitrile. Slow evaporation of the purple, pink,
and blue extracts respectively afforded the [Ir(PPh3)2(NO-H)-
(H)Cl] (yield: 5%), [Ir(PPh3)2(NO-H)(H)2] (yield: 25%), and
[Ir(PPh3)2(CNO-H)(H)] (yield: 42%) complexes.
Anal. Calcd for [Ir(PPh3)2(NO-H)(H)Cl]: C, 61.01; H, 4.36;
N, 2.91. Found: C, 61.53; H, 4.37; N, 2.89. 1H NMR:6 7.12-7.43-
(2PPh3); 6.27 (d, 1H, J ) 7.9); 6.81 (s, 1H); 6.84-6.98(3H*); 7.00-
7.11(3H*); 1.99(CH3); -20.76(hydride, J ) 13.5). Anal. Calcd for
[Ir(PPh3)2(NO-H)(H)2]: C, 63.28; H, 4.63; N, 3.01. Found: C,
63.27; H, 4.55; N, 3.09. 1H NMR: 7.15-7.35(2PPh3); 6.23 (d, 2H,
J ) 7.6); 6.44 (d, 1H, J ) 8.5); 6.72 (d, 1H, J ) 8.7); 6.93 (t, 2H,
J ) 7.2); 7.06 (s, 1H); 7.12 (t, 1H, J ) 7.5); 2.08 (CH3); -19.16
(hydride, J ) 19.5); -23.26 (hydride, J ) 15.0). Anal. Calcd for
[Ir(PPh3)2(CNO-H)(H)]: C, 63.42; H, 4.42; N, 3.02. Found: C,
63.88; H, 4.42; N, 3.09. 1H NMR: 7.14-7.56(2PPh3); 6.05 (t, 1H,
J ) 7.3); 6.23 (d, 1H, J ) 8.6); 6.27 (d, 1H, J ) 7.6); 6.38 (s, 1H);
1
63.99; H, 4.51; N, 3.02. H NMR: 7.14-7.50 (2PPh3); 5.88 (s,
1H); 6.25 (d, 1H, J ) 8.6); 6.38 (d, 1H, J ) 7.6); 6.40 (s, 1H);
6.48 (d, 1H, J ) 7.8); 7.09 (d, 1H, J ) 7.9); 1.90 (CH3); 1.74
(CH3); -12.53 (hydride, J ) 19.5).
Anal. Calcd for [Ir(PPh3)2(NO-Cl)(H)Cl]: C, 58.90; H, 4.11;
N, 2.80. Found: C, 58.76; H, 4.08; N, 2.81. 1H NMR: 7.15-7.45-
(2PPh3); 6.24 (d, 1H, J ) 8.5); 6.79 (s, 1H); 6.88 (d, 1H, J ) 8.6);
7.68 (d, 2H, J ) 10.9); 1.98 (CH3); -20.73 (hydride, J ) 12.0).
Anal. Calcd for [Ir(PPh3)2(NO-Cl)(H)2]: C, 61.01; H, 4.36; N,
1
2.91. Found: C, 61.00; H, 4.22; N, 3.04. H NMR: 7.18-7.35
(2PPh3); 6.19 (d, 2H, J ) 8.6); 6.40 (d, 1H, J ) 8.7); 6.70 (d, 1H,
J ) 8.8); 6.86 (d, 2H, J ) 8.6); 7.06 (s, 1H); 2.08 (CH3); -19.15
(hydride, J ) 20.0); -23.12 (hydride, J ) 15.5). Anal. Calcd for
[Ir(PPh3)2(CNO-Cl)(H)]: C, 61.14; H, 4.16; N, 2.91. Found: C,
1
61.59; H, 4.13; N, 2.88. H NMR: 7.15-7.48 (2PPh3); 6.02 (s,
1H); 6.22 (d, 1H, J ) 8.5); 6.41 (s, H); 6.48 (d, 1H, J ) 8.7); 6.54
(d, 1H, J ) 8.2); 7.07 (d, 1H, J ) 8.3); 1.92 (CH3); -12.44
(hydride, J ) 19.5).
Anal. Calcd for [Ir(PPh3)2(NO-NO2)(H)Cl]: C, 58.29; H, 4.06;
N, 4.16. Found: C, 58.47; H, 4.07; N, 4.15. 1H NMR: 7.17-7.47-
(2PPh3); 6.38 (d, 2H, J ) 8.9); 6.76 (s, 1H); 6.87 (d, 1H, J ) 8.1);
6.98 (d, 1H, J ) 9.1); 7.73 (d, 2H, J ) 9.2); 2.00 (CH3); -21.45
(hydride, J ) 12.0). Anal. Calcd for [Ir(PPh3)2(NO-NO2)(H)2]:
C, 60.36; H, 4.31; N, 4.31. Found: C, 60.73; H, 4.37; N, 4.29. 1H
NMR: 7.15-7.40 (2PPh3); 6.24 (d, 2H, J ) 8.6); 6.39 (d, 1H, J )
8.8); 6.74 (d, 1H, J ) 8.7); 7.13 (s, 1H); 7.70 (d, 2H, J ) 9.0);
2.10 (CH3); -19.00 (hydride, J ) 19.5); -23.14 (hydride, J )
15.0). Anal. Calcd for [Ir(PPh3)2(CNO-NO2)(H)]: C, 60.48; H,
4.11; N, 4.32. Found: C, 60.35; H, 4.08; N, 4.29. 1H NMR: 7.14-
7.50 (2PPh3); 6.24 (d, 1H, J ) 8.7); 6.41 (s, 1H); 6.50 (d, 1H, J )
8.7); 6.99 (s, 1H); 7.17 (d, 1H, J ) 8.1); 1.91 (CH3); -12.12
(hydride, J ) 19.5).
(4) (a) Gupta, P.; Butcher, R. J.; Bhattacharya, S. Inorg. Chem. 2003, 42,
5405. (b) Majumder, K.; Peng, S. M.; Bhattacharya, S. J. Chem. Soc.,
Dalton Trans. 2001, 284. (c) Dutta, S.; Peng, S. M.; Bhattacharya, S.
J. Chem. Soc., Dalton Trans. 2000, 4623. (d) Lahiri, G, K.; Bhatta-
charya, S.; Mukherjee, M.; Mukherjee, A, Chakravorty, A. Inorg.
Chem. 1987, 26, 3359.
(5) (a) Sawyer, D. T.; Roberts, J. L., Jr. Experimental Electrochemistry
for Chemists; Wiley: New York, 1974; pp 167-215. (b) Walter, M.;
Ramaley, L. Anal. Chem. 1973, 45, 165.
(6) Chemical shifts are given in ppm and multiplicity of the signals along
with the associated coupling constants(J in Hz) are given in paren-
theses. Overlapping signals are marked with an asterisk.
Inorganic Chemistry, Vol. 43, No. 2, 2004 705

Acharyya et al.
Table 1. Crystallographic Data for the [Ir(PPh3)2(NO-Cl)(H)Cl], [Ir(PPh3)2(NO-Cl)(H)2], [Ir(PPh3)2(CNO-Cl)(H)], and [Ir(PPh3)2(CNO-Cl)Cl]
Complexes
[Ir(PPh3)2(NO-Cl)(H)Cl]
[Ir(PPh3)2(NO-Cl)(H)2] 0.025 CH3CN
[Ir(PPh3)2(CNO-Cl)(H)]
[Ir(PPh3)2(CNO-Cl)Cl]
empirical formula
fw
C49H41N2OP2Cl2Ir
998.88
C49.5H42.75N2.25OP2ClIr
974.70
C49H40N2OP2ClIr
962.42
C49H39N2OP2Cl2Ir
996.86
space group
a, Å
b, Å
orthorhombic, Pbca
19.1548(11)
19.6711(11)
23.2453(14)
90
90
90
8758.7(9)
8
triclinic, P1h
12.1611(6)
13.8131(7)
14.9683(8)
104.7540(10)
98.5050(10)
113.6240(10)
2137.24(19)
2
triclinic, P1h
11.9489(14)
17.659(2)
21.253(3)
80.931(3)
86.527(3)
76.068(3)
4297.0(9)
4
monoclinic, C2/c
25.440(2)
20.054(2)
19.425(2)
90
112.239(3)
90
9172.8(17)
8
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
λ, Å
0.710 73
0.40 × 0.22 × 0.16
293(2)
3.282
0.0398
0.710 73
0.710 73
0.42× 0.30 × 0.14
293(2)
3.282
0.0585
0.710 73
0.37 × 0.23 × 0.09
293(2)
1.567
0.0638
cryst size, mm
T, K
0.45 × 0.24 × 0.20
293(2)
µ, mm-1
R1a
3.301
0.0289
0.0554
0.979
wR2b
0.0765
0.906
0.0923
0.885
0.1666
0.963
GOFc
a R1 ) ∑| Fo| - |Fc|/∑|Fo|. b wR2 ) [∑{w(Fo2 - Fc )2}/∑{w(Fo )}]1/2
.
c GOF ) [∑(w(Fo2 - Fc )2)/(M - N)]1/ 2, where M is the number of reflections
2
2
2
and N is the number of parameters refined.
[Ir(PPh3)2(CNO)H)Cl]. 2-(Phenylazo)-4-methylphenol (25 mg,
0.12 mmol) was dissolved in toluene (50 mL) and triethylamine
(24 mg, 0.24 mmol) was added to it. The solution was then purged
with a stream of dinitrogen for 10 min, and to it was added
[Ir(PPh3)3Cl] (100 mg, 0.10 mmol). The mixture was refluxed un-
der a dinitrogen atmosphere for 24 h, whereby a bluish-green
solution was obtained. Evaporation of this solution afforded a
dark solid, which was purified by thin-layer chromatography on a
silica plate. With benzene as the eluant, two bands (viz. blue and
green) separated, which were extracted separately with acetonitrile.
Upon slow evaporation of the blue and green extracts respectively
afforded the [Ir(PPh3)2(CNO-H)(H)] (yield: 32%) and [Ir(PPh3)2-
(CNO-H)Cl] (yield: 35%) complexes. Anal. Calcd for [Ir(PPh3)2-
(CNO-H)Cl]: C, 61.14; H, 4.16; N, 2.91. Found: C, 61.11; H,
3.86; N, 2.90. 1H NMR: 6.63-7.35 (2PPh3); 6.22 (d, 1H, J ) 8.2);
6.28 (s, 1H); 6.39 (t, 1H, J ) 7.3); 6.46 (d, 1H, J ) 8.7); 6.52-
6.62 (2H*); 1.74 (CH3).
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 disk 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.
X-ray Structure Determinations. Single crystals of the
[Ir(PPh3)2(NO-Cl)(H)Cl], [Ir(PPh3)2(NO-Cl)(H)2], [Ir(PPh3)2-
(CNO-Cl)(H)], and [Ir(PPh3)2(CNO-Cl)Cl] complexes were
obtained by slow evaporation of acetonitrile solutions of the re-
spective complexes. Selected crystal data and data collection param-
eters are given in Table 1. Data on the crystals of the [Ir(PPh3)2-
(NO-Cl)(H)Cl], [Ir(PPh3)2(NO-Cl)(H)2], and [Ir(PPh3)2(CNO-
Cl)(H)] complexes were collected on a Siemens Smart CCD dif-
fractometer using graphite-monochromated Mo KR radiation
(λ ) 0.710 73 Å) by φ and ω scans. Data on the crystal of the
[Ir(PPh3)2(CNO-Cl)Cl] complex were collected similarly on a
Bruker P4 diffractometer. X-ray data reduction and, structure
solution and refinement were done using SHELXS-97 and SHELXL-
97 programs.7 The structure was solved by direct methods.
All of the other [Ir(PPh3)2(CNO-R)Cl] complexes were prepared
by following the same above procedure using appropriate 2-(aryl-
azo)phenols (1, R * H) instead of 2-(phenylazo)-4-methylphenol.
Anal. Calcd for [Ir(PPh3)2(CNO-OCH3)Cl]: C, 60.50; H, 4.24;
N, 2.82. Found: C, 60.19; H, 4.22; N, 2.79. 1H NMR: 6.72-7.47
(2PPh3); 5.55 (s, 1H); 6.08 (d, 1H, J ) 8.0); 6.20 (d, 1H, J ) 8.5);
6.31 (s, 1H); 6.37 (d, 1H, J ) 8.5); 7.02 (d, 1H, J ) 8.5); 1.74
(CH3); 3.22 (OCH3). Anal. Calcd for [Ir(PPh3)2(CNO-CH3)Cl]:
C, 61.49; H, 4.30; N, 2.87. Found: C, 61.73; H, 4.31; N, 2.91. 1H
NMR: 6.79-7.55 (2PPh3); 5.30 (s, 1H); 5.59 (d, 1H, J ) 8.6);
6.19 (d, 1H, J ) 7.5); 6.35 (s, 1H); 6.39 (d, 1H, J ) 7.8); 7.00 (d,
1H, J ) 7.6); 1.74 (CH3); 2.15 (CH3). Anal. Calcd for [Ir(PPh3)2-
(CNO-Cl)Cl]: C, 59.02; H, 3.91; N, 2.81. Found: C, 58.89; H,
3.88; N, 2.87. 1H NMR: 6.83-7.56 (2PPh3); 6.44 (d, 1H, J ) 8.8);
6.49 (s, 1H); 6.84 (d, 1H, J ) 9.1); 6.76 (d, 1H, J ) 7.3); 7.11 (s,
1H); 1.94 (CH3). Anal. Calcd for [Ir(PPh3)2(CNO-NO2)Cl]: C,
Results and Discussion
Five 2-(arylazo)phenols (1) have been used in the present
study, differing in the inductive effect of the substituent R,
to observe their influence, if any, on the redox potentials of
the complexes. Reaction of each 2-(arylazo)phenol with
[Ir(PPh3)3Cl] was first carried out in refluxing ethanol in the
presence of triethylamine, which afforded a mixture of three
complexes with distinctly different colors (viz. purple, pink,
and blue). Structure of one representative member of each
family, viz. the purple, pink, and blue complexes obtained
from the reaction with 2-(4′-chlorophenylazo)-4-methylphe-
nol (1, R ) Cl), has been determined by X-ray crystal-
lography. The structures are shown in Figures 1-3 and some
1
58.41; H, 3.87; N, 4.17. Found: C, 58.83; H, 3.91; N, 4.20. H
NMR: 6.75-7.56 (2PPh3); 6.43 (d, 1H, J ) 8.7); 6.49 (s, 1H);
6.70 (d, 1H, J ) 8.8); 7.61 (d, 1H, J ) 8.6); 8.15 (s, 1H); 1.94
(CH3).
Physical Measurements. Microanalyses (C, H, N) were per-
formed using a Heraeus Carlo Erba 1108 elemental analyzer. IR
spectra were obtained 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
(7) Sheldrick, G. M. SHELXS-97 and SHELXL-97, Fortran programs for
crystal structure solution and refinement, University of Gottingen:
Gottingen, Germany; 1997.
706 Inorganic Chemistry, Vol. 43, No. 2, 2004

Iridium(III) Complexes
Table 2. Selected Bond Distances and Bond Angles for the
[Ir(PPh3)2(NO-Cl)(H)Cl], [Ir(PPh3)2(NO-Cl)(H)2],
[Ir(PPh3)2(CNO-Cl)(H)], and [Ir(PPh3)2(CNO-Cl)Cl] Complexes
[Ir(PPh3)2(NO-Cl)(H)Cl]
Bond Distances (Å)
Ir(1)-H(1)
Ir(1)-N(2)
Ir(1)-O(1)
Ir(1)-P(1)
Ir(1)-P(2)
Ir(1)-Cl(1)
1.42(4)
2.029(5)
2.149(4)
2.3640(17)
2.3295(17)
2.3859(17)
C(43)-O(1)
C(43)-C(44)
C(44)-N(2)
N(1)-N(2)
C(38)-N(1)
1.293(8)
1.409(8)
1.449(7)
1.248(7)
1.399(8)
Bond Angles (deg)
H(1)-Ir(1)-O(1)
N(2)-Ir(1)-Cl(1)
P(1)-Ir(1)-P(2)
168(2)
175.91(15)
174.52(6)
N(2)-Ir(1)-O(1)
H(1)-Ir(1)-Cl(1)
81.87(18)
96(2)
[Ir(PPh3)2(NO-Cl)(H)2]
Bond Distances (Å)
Ir(1)-H(1)
Ir(1)-H(2)
Ir(1)-N(2)
Ir(1)-O(1)
Ir(1)-P(1)
Ir(1)-P(2)
1.36(3)
1.42(3)
2.167(2)
2.166(2)
2.3000(8)
2.2906(2)
C(43)-O(1)
C(43)-C(44)
C(44)-N(2)
C(42)-N(1)
N(1)-N(2)
1.299(4)
1.420(4)
1.404(4)
1.401(5)
1.286(3)
Figure 1. View of the [Ir(PPh3)2(NO-Cl)(H)Cl] complex.
Bond Angles (deg)
H(1)-Ir(1)-N(2)
H(2)-Ir(1)-O(1)
P(1)-Ir(1)-P(2)
174.7(14)
172.9(13)
164.15(3)
N(2)-Ir(1)-O(1)
H(1)-Ir(1)-H(2)
77.68(9)
75.4(19)
[Ir(PPh3)2(CNO-Cl)(H)]
Bond Distances (Å)
Ir(1)-H(1)
Ir(1)-C(37)
Ir(1)-N(2)
Ir(1)-O(1)
Ir(1)-P(1)
Ir(1)-P(2)
1.34(4)
C(37)-C(42)
1.452(12)
1.393(11)
1.275(9)
1.413(11)
1.316(10)
2.011(9)
2.033(7)
2.174(6)
2.309(2)
2.315(2)
C(42)-N(1)
N(1)-N(2)
N(2)-C(43)
C(44)-O(1)
Bond Angles (deg)
H(1)-Ir(1)-N(2)
C(37)-Ir(1)-O(1)
P(1)-Ir(1)-P(2)
168(4)
157(3)
161.86(9)
C(37)-Ir(1)-N(2)
N(2)-Ir(1)-O(1)
77.7(3)
79.3(3)
[Ir(PPh3)2(CNO-Cl)Cl]
Bond Distances (Å)
Ir(1)-C(1)
Ir(1)-N(1)
Ir(1)-O(1)
Ir(1)-P(1)
Ir(1)-P(2)
Ir(1)-Cl(1)
2.044(11)
2.026(8)
2.177(8)
2.351(3)
2.305(3)
2.406(3)
C(7)-O(1)
1.317(13)
1.404(16)
1.392(13)
1.259(11)
1.375(14)
1.452(14)
C(7)-C(8)
C(8)-N(1)
N(1)-N(2)
C(6)-N(2)
C(1)-C(6)
Figure 2. View of the [Ir(PPh3)2(NO-Cl)(H)2] complex.
trans. The Ir-H, Ir-N, Ir-O, Ir-P, and Ir-Cl distances
are all quite normal.8 The structure of the pink complex (Fig-
ure 2) is very similar to that of the [Ir(PPh3)2(NO-Cl)-
(H)Cl] complex, except that a hydride is coordinated to
iridium instead of the chloride, and hence the pink complexes
are generally represented as [Ir(PPh3)2(NO-R)(H)2]. Struc-
ture of the blue complex (Figure 3) shows that the 2-(aryl-
azo)phenol is coordinated to iridium, via loss of two protons,
as a tridentate C,N,O-donor (4). Two triphenylphosphines
and a hydride are also coordinated to iridium. The blue
complexes are therefore formulated in general as [Ir(PPh3)2-
Bond Angles (deg)
C(1)-Ir(1)-O(1)
N(1)-Ir(1)-P(1)
P(2)-Ir(1)-Cl(1)
158.4(3)
166.5(2)
168.88(10)
C(1)-Ir(1)-N(1)
N(1)-Ir(1)-O(1)
P(1)-Ir-P(2)
79.1(4)
79.3(3)
101.56(10)
relevant bond parameters are listed in Table 2. The structure
of the purple complex (Figure 1) shows that the 2-(arylazo)-
phenol is coordinated to iridium, via loss of the phenolic
proton, as a bidentate N,O-donor forming a five-membered
chelate ring (3). Two triphenylphosphines, a hydride, and a
chloride are also coordinated to the metal center. The pur-
ple complexes are therefore formulated in general as
[Ir(PPh3)2(NO-R)(H)Cl]. The coordinated 2-(arylazo)phe-
nolate ligand, hydride, and chloride have constituted one
equatorial plane with the metal at the center, where the chlor-
ide is trans to the azo nitrogen and the hydride is trans to
the phenolate oxygen. The PPh3 ligands have taken up the
remaining two axial positions and hence they are mutually
(8) (a) Canepa, G.; Sola, E.; Martin, M.; Lahoz, F. J.; Ora, L. A.; Werner,
H. Organometallics 2003, 22, 2151. (b) Lo, K. K. W.; Chung, C. K.;
Ng, D. C. M.; Zhu, N. New. J. Chem. 2002, 26, 81. (c) Ortmann, D.
A.; Weberndorfer, B.; Ilg, K.; Laubender, M.; Werner, H. Organo-
metallics 2002, 21, 2369. (d) Torres, F.; Sola, E.; Martin, M.; Ochs,
C.; Picazo, G.; Lopez, J. A.; Lahoz, F. J.; Oro, L. A. Organometallics
2001, 20, 2716. (e) Werner, H.; Heohn, A.; Schulz, M. J. Chem. Soc.,
Dalton Trans. 1991, 777.
Inorganic Chemistry, Vol. 43, No. 2, 2004 707

Acharyya et al.
Scheme 1. Probable Steps of the Reaction in Ethanol
Figure 3. View of the [Ir(PPh3)2(CNO-Cl)(H)] complex.
donor (4), as in the [Ir(PPh3)2(CNO-R)(H)] complexes. Two
triphenylphosphines and a chloride are also coordinated to
the metal center. Unlike in the previous complexes, the PPh3
ligands are cis in this complex. The green complexes are
therefore represented in general as [Ir(PPh3)2(CNO-R)Cl].
Structural features in the Ir(CNO-Cl) fragment of this
[Ir(PPh3)2(CNO-Cl)Cl] complex compare well with those
in the same fragment of the [Ir(PPh3)2(CNO-Cl)(H)] com-
plex. The Ir(1)-P(2) bond, which has occupied an axial
position, is similar in length with the other axial Ir-P bonds
in the previous complexes, while the Ir(1)-P(1) bond,
which is sharing the equatorial plane with the 2-(arylazo)-
phenolate ligand, is much longer. The Ir(1)-Cl(1) distance
is also found to be slightly longer than that observed in
[Ir(PPh3)2(NO-Cl)(H)Cl].
Figure 4. View of the [Ir(PPh3)2(CNO-Cl)Cl] complex.
As all five complexes belonging to each group, viz.
[Ir(PPh3)2(NO-R)(H)Cl], [Ir(PPh3)2(NO-R)(H)2], [Ir(PPh3)2-
(CNO-R)(H)], and [Ir(PPh3)2(CNO-R)Cl], have been syn-
thesized similarly and they show similar properties (vide
infra), the other four members of each group (with R * Cl)
are assumed to have structures similar to those of their
respective structurally characterized (R ) Cl) analogues. The
observed microanalytical data of all the complexes agree well
with their compositions.
(CNO-R)(H)]. The tricoordinated 2-(arylazo)phenolate ligand
is sharing the same equatorial plane with the hydride, and
the PPh3 ligands are trans as before. The Ir-C distance is
normal,8a and the other bond distances are comparable to
those in the previous complexes.
The presence of hydride in all the three types of complexes
was quite interesting. Two sources of the hydrides seem
probable. Oxidative insertion of iridium(I) into the phenolic
O-H bond may generate a hydride, and the solvent (ethanol)
may also serve as a source of the hydride. To check this,
reaction of each 2-(arylazo)phenol (1) with [Ir(PPh3)3Cl] was
also carried out in a nonalcoholic solvent, viz. toluene, in
the presence of triethylamine, which afforded two complexes
with different colors, viz. blue and green. Characterization
on the blue complexes shows that they are the same
[Ir(PPh3)2(CNO-R)(H)] complexes obtained from the etha-
nol reaction, which clearly indicates that ethanol has not been
the only source of hydride. Structure determination of a
representative green complex, viz. the complex obtained from
the reaction with 2-(4′-chlorophenylazo)-4-methylphenol (1,
R ) Cl), shows (Figure 4, Table 2) that the 2-(arylazo)-
phenol is again coordinated to iridium as a tridentate C,N,O-
The synthetic reactions carried out in two different solvents
show that the nature of products obtained depends on the
nature of solvent used. When ethanol is used as the solvent,
complexes of three types, viz. [Ir(PPh3)2(NO-R)(H)Cl],
[Ir(PPh3)2(NO-R)(H)2], and [Ir(PPh3)2(CNO-R)(H)], are
obtained. The mechanism behind the formation of these three
types of complexes is not completely clear to us. However,
the sequences shown in Scheme 1 seem probable. In the
initial step, the 2-(arylazo)phenol 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
PPh3, and thus affords [Ir(PPh3)2(NO-R)(H)Cl]. There is
precedence for such O-H activation by iridium(I) in the
708 Inorganic Chemistry, Vol. 43, No. 2, 2004

Iridium(III) Complexes
literature.9 [Ir(PPh3)2(NO-R)(H)Cl] then reacts with ethanol
in the presence of NEt3, whereby the coordinated chloride
gets displaced by a hydride and thus [Ir(PPh3)2(NO-R)(H)2]
is produced. Reports of such displacement reaction are
available,10 and evidence for this displacement reaction
comes from the fact that purple ethanolic solution of any
[Ir(PPh3)2(NO-R)(H)Cl] complex is observed to convert into
the corresponding pink solution of [Ir(PPh3)2(NO-R)(H)2]
complex in the presence of NEt3. In the final step, C-H
activation at the ortho position of the pendant phenyl ring
of the N,O-coordinated 2-(arylazo)phenolate ligand takes
place via elimination of molecular hydrogen leading to the
formation of [Ir(PPh3)2(CNO-R)(H)]. Similar C-H activa-
tion by iridium(III) is well documented in the literature.11
Such cyclometalation reaction, involving a coordinated
hydride close to a pendant phenyl ring, has recently been
observed by us.4a Evidence for this cyclometalation step also
comes from the fact that pink solutions of the [Ir(PPh3)2-
(NO-R)(H)2] complexes are observed to gradually transform
into blue solutions of the [Ir(PPh3)2(CNO-R)(H)] com-
plexes. The cyclometalated [Ir(PPh3)2(CNO-R)(H)] complex
thus appears to be the ultimate product, and the [Ir(PPh3)2-
(NO-R)(H)Cl] and [Ir(PPh3)2(NO-R)(H)2] complexes are
the intermediates formed under the prevailing reaction
conditions. Source of the hydride in the [Ir(PPh3)2(NO-R)-
(H)Cl] complexes seems to be the phenolic proton of the
2-(arylazo)phenols, while that of the second hydride in the
[Ir(PPh3)2(NO-R)(H)2] complexes appears to be ethanol.
Scheme 2. Probable Steps of the Reaction in Toluene
When reaction of the 2-(arylazo)phenols (1) with
[Ir(PPh3)3Cl] is carried out in toluene, organoiridium com-
plexes of two types, viz. [Ir(PPh3)2(CNO-R)(H)] and
[Ir(PPh3)2(CNO-R)Cl], are obtained and the probable steps
behind their formation are illustrated in Scheme 2. As before,
oxidative addition of 2-(arylazo)phenol to iridium(I) takes
place in the first step, yielding two geometric isomers of
the [Ir(PPh3)2(NO-R)(H)Cl] complex obtained from the
ethanol reaction. These complexes then undergo cyclometa-
lation affording the [Ir(PPh3)2(CNO-R)(H)] and [Ir(PPh3)2-
(CNO-R)Cl] complexes via elimination of HCl and H2
respectively. The speculated intermediates could not be
isolated in these reactions, probably because they undergo
rapid irreversible conversion into the cyclometalated species
in refluxing toluene.
Infrared spectra of the [Ir(PPh3)2(NO-R)(H)Cl], [Ir(PPh3)2-
(NO-R)(H)2], and [Ir(PPh3)2(CNO-R)(H)] complexes re-
spectively show one, two, and one strong Ir-H stretch/
stretches within 2070-2250 cm-1.8a,c Three strong bands near
520, 695, and 745 cm-1 are observed in all these complexes
due to the coordinated PPh3 ligands.4,12 Few sharp bands are
also displayed by all these complexes within 1200-1400
cm-1, which are absent in the spectrum of [Ir(PPh3)3Cl], and
hence these are attributable to the coordinated 2-(arylazo)-
phenolate ligands. Besides absence of the Ir-H stretch,
infrared spectra of the [Ir(PPh3)2(CNO-R)Cl] complexes are
mostly similar to those of the [Ir(PPh3)2(CNO-R)(H)] com-
1
plexes. H NMR spectra of the [Ir(PPh3)2(NO-R)(H)Cl]
complexes show broad signals within 6.92-7.48 ppm due
to the coordinated PPh3 ligands. In each complex, the hydride
signal is observed as a distinct triplet, due to coupling with
the two magnetically equivalent phosphorus nuclei, near
-21.0 ppm. The methyl signal of the NO-R ligand is dis-
played near 1.9 ppm. Most of the expected aromatic proton
signals for the coordinated NO-R ligand have been clearly
observed in these complexes, while few signals could not
be detected due to their overlap with other signals. Signals
(9) (a) Warner, H.; Papenfubs, B.; Steinert, P. Z. Anorg. Allg. Chem. 2001,
627, 1807. (b) Dorta, R.; Togni, A. Organometallics 1998, 17, 3423.
(c) Dobbs, D. A.; Bergmann, R. G. J. Am. Chem. Soc. 1993, 115,
3836. (d) Eberhardt, G. C.; Tadros, M. E.; Vaska, L. J. Chem. Soc.,
Chem. Commun. 1972, 5, 290.
(10) (a) Empsall, H. D.; Johnson, S.; Shaw, B. L. J. Chem. Soc., Dalton
Trans. 1980, 302. (b) Empsall, H. D.; Hyde, E. M.; Mentzer, E.; Shaw,
B. L.; Uttley, M. F. J. Chem. Soc. A 1976, 2069. (c) Chatt, J.; Johnson,
N. P.; Shaw, B. L. J. Chem. Soc. A 1964, 1625.
(11) (a) Dorta, R.; Goikhman, R.; Milstein, D. Organometallics 2003, 22,
2806. (b) Klei, S. R.; Tilley, T. D.; Bergman, R. G. Organometallics
2002, 21, 4905. (c) Tellers, D. M.; Yung, C. M.; Arndtsen, B. A.;
Adamson, D. R.; Bergman, R. G. J. Am. Chem. Soc. 2002, 124, 1400.
(d) Klei, S. R.; Tilley, T. D.; Bergman, R. G. J. Am. Chem. Soc. 2000,
122, 1816. (e) Sjoevall, S.; Johansson, M.; Andersson, C. Organo-
metallics 1999, 18, 2198. (f) Niu, S.; Hall, M. B. J. Am. Chem. Soc.
1998, 120, 6169. (g) Hinderling, C.; Plattner, D. A.; Chen, P. Angew.
Chem. 1997, 36, 243. (h) Luecke, H. F.; Arndtsen, B. A.; Burger, P.;
Bergman, R. G. J. Am. Chem. Soc. 1996, 118, 2517.
(12) (a) Dutta, S.; Basuli, F.; Peng, S. M.; Lee, G. H.; Bhattacharya, S.
New. J. Chem. 2002, 26, 1607. (b) Basuli, F.; Peng, S. M.;
Bhattacharya, S. Inorg. Chem. 2001, 40, 1126. (c) Das, A. K.; Peng,
S. M.; Bhattacharya, S. J. Chem. Soc., Dalton Trans. 2000, 181. (d)
Dutta, S.; Peng, S. M.; Lee, G. H.; Bhattacharya, S. Inorg. Chem.
2000, 39, 2231.
Inorganic Chemistry, Vol. 43, No. 2, 2004 709

Acharyya et al.
Table 3. Electronic Spectral and Cyclic Voltammetric Data of the Complexes
electronic spectral data
cyclic voltammetric datab
a
compound
λ
max/nm (ꢀ/M-1 cm-1
)
E, V vs SCE
[Ir(PPh3)2(NO-OCH3)(H)Cl]
[Ir(PPh3)2(NO- CH3)(H)Cl]
[Ir(PPh3)2(NO-H)(H)Cl]
[Ir(PPh3)2(NO-Cl)(H)Cl]
[Ir(PPh3)2(NO-NO2)(H)Cl]
[Ir(PPh3)2(NO-OCH3)(H)2]
[Ir(PPh3)2(NO- CH3)(H)2]
[Ir(PPh3)2(NO-H)(H)2]
554 (7700), 432 (6100),c 338 (16000), 262 (53700)c
0.91,d -1.00e
558 (7400), 430 (5500),c 346 (13300), 258 (43800)c
1.00,d -0.83e
552 (8300), 424 (4200),c 346 (24100), 256 (50800)c
1.11,d -0.72e
558 (6800), 426 (3900),c 342 (10800), 256 (42000)c
1.06,d -0.72e
592 (8100), 352 (13000), 256 (45500)c
1.10,d -0.87e
540 (2400), 394 (2400),c 334 (4600), 262 (13300)c
1.25,d 0.72,d -0.97e
1.29,d 0.73,d -0.91e
1.28,d 0.91,d -0.81e
1.29,d 0.92,d -0.82e
1.33,d 0.93,d -0.98e
1.02,d 0.57 (72), -0.94e
1.09,d 0.64 (70), -1.02e
1.08,d 0.68 (78), -1.04
1.23,d 0.72 (80), -1.08e
1.68,d 0.88 (78), -0.96e
1.36,d 0.79 (80), -1.03e
1.27,d 0.83 (60), -1.20e
1.31,d 0.89 (64), -1.16e
1.25,d 0.94 (94), -1,12e
1.48,d 1.10 (69), -0.95e
544 (3100), 396 (3600),c 332 (8500), 268 (21300)c
528 (8700), 376 (5000)c, 328 (10900), 268 (29500)c
[Ir(PPh3)2(NO-Cl)(H)2]
[Ir(PPh3)2(NO-NO2)(H)2]
[Ir(PPh3)2(CNO-OCH3)(H)]
[Ir(PPh3)2(CNO-CH3)(H)]
[Ir(PPh3)2(CNO-H)(H)]
[Ir(PPh3)2(CNO-Cl)(H)]
[Ir(PPh3)2(CNO-NO2)(H)]
[Ir(PPh3)2(CNO-OCH3)Cl]
[Ir(PPh3)2(CNO- CH3)Cl]
[Ir(PPh3)2(CNO-H)Cl]
534 (12100), 376 (8000),c 330 (14700), 266 (39400)c
556 (8900), 372 (11400),c 336 (13500), 264(30800)c
604 (7800), 566 (6900),c 456 (1600),c 382 (5200),c 340 (10200), 258 (24500)c
606 (8800), 574 (8100),c 466 (1900),c 392 (5000),c 388 (14500), 266 (35800)c
610 (7400), 576 (7000),c 478 (2000),c 398 (3900),c 336 (13200), 266 (31800)c
614 (7700), 582 (7200),c 468 (1700),c 394 (4000),c 340 (12900), 274 (27000)c
678 (12200), 638 (11300),c 532 (3400),c 434 (5300),c 364 (19000), 266 (44500)c
660 (4100), 622 (4000),c 386 (7400), 338 (7100), 270 (17400)c
666 (3900), 632 (3800),c 382 (7600), 340 (9200), 270 (26100)c
672 (4800), 636 (4600),c 378 (7800),c 354 (10500),c 338 (9900),c 270 (21700)c
678 (6500), 640 (6200),c 382 (10200),c 360 (12100),c 342 (11200),c 276 (16300)c
744 (7500), 692 (7100),c 412 (10400),c 382 (14100),c 270 (20500)c
[Ir(PPh3)2(CNO-Cl)Cl]
[Ir(PPh3)2(CNO-NO2)Cl]
d
e
a In dichloromethane. b Solvent, acetonitrile; supporting electrolyte, TBAP; scan rate 50 mV s-1
.
c Shoulder. Epa value. Epc value.
Table 4. Composition of Selected Molecular Orbitals
% contribution of fragments to
compounds
contributing fragments
HOMO
HOMO-1
HOMO-2
LUMO
LUMO+1
LUMO+2
[Ir(PPh3)2(CNO-H)(H)]
Ir
ap-H
82
9
56
40
62
27
12
81
98
92
(NdN, 53)
[Ir(PPh3)2(NO-H)(H)2]
[Ir(PPh3)2(NO-H)(H)Cl]
cis-[Ir(PPh3)2(CNO-H)Cl]
Ir
ap-H
89
90
70
23
78
12
9
85
96
96
97
96
96
95
(NdN, 64)
10
82
(NdN, 63)
11
85
Ir
ap-H
73
20
75
18
Ir
ap-H
80
8
64
26
75
15
(NdN, 51)
for the hydrogen-containing substituents in the NO-R lig-
and (R ) OCH3 and CH3) are also observed in the ex-
pected region. Besides showing two hydride signals (near
phenyl rings of the triphenylphosphines have been replaced
by hydrogens. The results are found to be similar for all the
complexes. Compositions of selected molecular orbitals are
given in Table 4, and a partial MO diagram of a selected
complex is shown in Figure 5. MO diagrams of the other
complexes are deposited as Supporting Information (Figures
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. These three occupied orbitals may therefore be
regarded as the iridium t2 orbitals. The lowest unoccupied
molecular orbital (LUMO) has >80% contribution from the
2-(arylazo)phenolate ligand and is concentrated mostly on
the azo (NdN) fragment. The LUMO+1 and LUMO+2 are
localized on other parts of the 2-(arylazo)phenolate ligand.
The lowest energy absorption in the visible region is therefore
assignable to an allowed charge-transfer transition from the
filled iridium t2-orbital (HOMO) to the vacant π*(azo)-orbital
of the 2-(arylazo)phenolate ligand (LUMO). The other
intense absorptions in the visible region may be assigned to
charge-transfer transitions from the iridium t2-orbitals to the
higher energy vacant orbitals.
1
-19.0 and -23.0 ppm) H NMR spectra of the [Ir(PPh3)2-
(NO-R)(H)2] complexes are similar to those of the
[Ir(PPh3)2(NO-R)(H)Cl] complexes. 1H NMR spectra of the
[Ir(PPh3)2(CNO-R)(H)] complexes show a hydride signal
near -12.0 ppm, while the other spectral features are quali-
tatively similar to those of the previous complexes. Except
for the absence of the hydride signal, 1H NMR spectrum of
each [Ir(PPh3)2(CNO-R)Cl] complex is similar to that of
its corresponding [Ir(PPh3)2(CNO-R)(H)] complex.
Electronic spectra of all the complexes, recorded in
dichloromethane solution, show several intense absorptions
in the visible and ultraviolet regions (Table 3). The absorp-
tions in the ultraviolet region are attributable to transitions
within the ligand orbitals, and those in the visible region
are probably due to metal-to-ligand charge-transfer transition.
To have an insight into the nature of the absorptions in the
visible region, qualitative EHMO calculations have been
performed13 on computer-generated models of four repre-
sentative members of the four types of complexes, where
Electrochemical properties of all the complexes have been
studied by cyclic voltammetry in acetonitrile solution (0.1
(13) (a) Mealli, C.; Proserpio, D. M. CACAO Version 4.0, Firenze, Italy,
1994. (b) Mealli, C.; Proserpio, D. M. J. Chem. Educ. 1990, 67, 399.
710 Inorganic Chemistry, Vol. 43, No. 2, 2004

Iridium(III) Complexes
variation with the electron-withdrawing character of the
substituent R in the 2-(arylazo)phenolate ligand. However,
potential of the reversible Ir(III)-Ir(IV) oxidation in the
[Ir(PPh3)2(CNO-R)(H)] and [Ir(PPh3)2(CNO-R)Cl] com-
plexes has been found to be sensitive to the nature of the
substituent R. The potential increases with increasing
electron-withdrawing character of the substituent R. The plots
of oxidation potential vs σ [σ ) Hammett constant of R;14
OCH3 ) -0.27, CH3 ) -0.17, H ) 0.00, Cl ) 0.23, and
NO2 ) 0.78] are linear for both the [Ir(PPh3)2(CNO-R)-
(H)] and [Ir(PPh3)2(CNO-R)Cl] complexes (Figures S4 and
S5) with slopes (F) of 0.27 and 0.29 V respectively (F )
reaction constant of this couple15), which shows that the para-
substituent (R) on the 2-(arylazo)phenolate ligand, which is
four bonds away from the metal center, can still influence
the metal-centered oxidation potential in a predictable
manner.
Conclusions
The present study shows that the 2-(arylazo)phenols (1)
undergo facile phenolic O-H activation and/or C-H activa-
tion at one ortho position of the phenyl ring in the arylazo
fragment, mediated by [Ir(PPh3)3Cl]. Studies on such activa-
tion of some other organic molecules, having structural
similarity to the 2-(arylazo)phenols, is in progress. Dispo-
sition of the Ir-H bond in the equatorial plane of the
[Ir(PPh3)2(NO-R)(H)Cl] complexes (Scheme 1, Figure 1)
suggests that C-H activation, similar to that observed in
the [Ir(PPh3)2(NO-R)(H)2] complexes, may also be possible
in these complexes under suitable reaction conditions. Such
a possibility is currently under exploration.
Figure 5. Partial molecular orbital diagram of [Ir(PPh3)2(NO-H)(H)Cl].
M TBAP). Voltammetric data are given in Table 3. All the
complexes show an oxidative response on the positive side
of SCE and a reductive response on the negative side, which
are respectively assigned to Ir(III)-Ir(IV) oxidation and
Ir(III)-Ir(II) reduction. One-electron stoichiometry of these
responses has been established by comparing their cur-
rent heights with those of standard ferrocene/ferrocenium
couple under identical experimental conditions. Except the
[Ir(PPh3)2(NO-R)(H)Cl] complexes, complexes of the other
three types also show a second oxidative response, which is
assigned to oxidation of the coordinated 2-(arylazo)phen-
olate ligand. Cyclic voltammetric behavior of the [Ir(PPh3)2-
(NO-R)(H)2] complexes is similar to that of the [Ir(PPh3)2-
(NO-R)(H)Cl] complexes, except that potential of both
Ir(III)-Ir(IV) oxidation and Ir(III)-Ir(II) reduction is shifted
to less positive values. The observed shift is probably due
to displacement of the chloride by the hydride. A similar
trend is observed within the [Ir(PPh3)2(CNO-R)(H)] and
[Ir(PPh3)2(CNO-R)Cl] series. Except for the Ir(III)-Ir(IV)
oxidation in the [Ir(PPh3)2(CNO-R)(H)] and [Ir(PPh3)2-
(CNO-R)Cl] complexes, all the redox responses are found
to be irreversible in all the complexes. Potential of the
irreversible redox responses did not show any systematic
Acknowledgment. Financial assistance received from the
Council of Scientific and Industrial Research, New Delhi
[Grant No. 01(1675)/00/EMR-II] and the Department of
Science and Technology, New Delhi [Grant No. SP/S1/F33/
98] is gratefully acknowledged. The RSIC at Central Drug
Research Institute, Lucknow, India, is thanked for the C, H,
and N analysis data. The authors thank the reviewers for
their critical comments and constructive suggestions, which
have been helpful in preparing the revised version.
Supporting Information Available: Partial molecular orbi-
tal diagrams of [Ir(PPh3)2(NO-H)(H)2] (Figure S1), [Ir(PPh3)2-
(CNO-H)(H)] (Figure S2) and [Ir(PPh3)2(CNO-H)Cl] (Figure S3),
least-squares plots of E1/2 values of Ir(III)-Ir(IV) couple vs σ for
the [Ir(PPh3)2(CNO-R)(H)] and [Ir(PPh3)2(CNO-R)Cl] complexes
(Figure S4 and S5), and X-ray crystallographic data in CIF format.
This material is available free of charge via the Internet at
http://pubs.acs.org.
IC034785W
(14) Hammett, L. P. Physical Organic Chemistry, McGraw-Hill: New
York, 2nd ed. 1970.
(15) Mukherjee, R. N.; Rajan, O. A.; Chakravorty, A. Inorg. Chem. 1982,
21, 785.
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