Chemically induced cyclometalation of 2-(arylazo)phenols. Synthesis, characterization, and redox properties of a family of organoosmium complexes

Article abstract of DOI:10.1021/ic0343713

Reaction of 2-(arylazo)phenols (H₂ap-R; R = OCH₃, CH₃, H, Cl, and NO₂) with [Os(PPh₃) ₂(CO)₂(HCOO)₂] affords a family of organometallic complexes of osmium(II) of type [Os(PPh₃) ₂(CO)(ap-R)] where the 2-(arylazo)phenolate ligand is coordinated to the metal center as a tridentate C,N,O-donor. Structure of the [Os(PPh ₃)₂(CO)(ap-H)] complex has been determined by X-ray crystallography. All the [Os(PPh₃)₂(CO)(ap-R)] complexes are diamagnetic and show characteristic ¹H NMR signals and intense MLCT transitions in the visible region. They also show emission in the visible region at ambient temperature. Cyclic voltammetry on the [Os(PPh ₃)₂(CO)(ap-R)] complexes shows a reversible Os(II)-Os(III) oxidation within 0.39-0.73 V vs SCE, followed by a reversible Os(III)-Os(IV) oxidation within 1.06-1.61 V vs SCE. Coulometric oxidation of the [Os(PPh ₃)₂(CO)(ap-R)] complexes generates the [Os ^III-(PPh₃)₂(CO)(ap-R)]⁺ complexes, which have been isolated as the hexafluorophosphate salts. The [Os ^III(PPh₃)₂-(CO)(ap-R)]PF₆ complexes are one-electron paramagnetic and show axial ESR spectra. In solution they behave as 1:1 electrolytes and show intense LMCT transitions in the visible region. The [Os^III(PPh₃)₂(CO)(ap-R)]PF ₆ complexes have been observed to serve as mild one-electron oxidants in a nonaqueous medium.

Full text of DOI:10.1021/ic0343713

Inorg. Chem. 2003, 42, 5405−5411
Chemically Induced Cyclometalation of 2-(Arylazo)phenols. Synthesis,
Characterization, and Redox Properties of a Family of
Organoosmium Complexes
,1a
Parna Gupta,1a Ray J. Butcher,1b and Samaresh Bhattacharya*
Department of Chemistry, Inorganic Chemistry Section, JadaVpur UniVersity,
Kolkata 700 032, India, and Department of Chemistry, Howard UniVersity,
Washington, DC 20059
Received April 7, 2003
Reaction of 2-(arylazo)phenols (H ap-R; R ) OCH , CH , H, Cl, and NO ) with [Os(PPh3)2(CO)2(HCOO)2] affords
2
3
3
2
a family of organometallic complexes of osmium(II) of type [Os(PPh3)2(CO)(ap-R)] where the 2-(arylazo)phenolate
ligand is coordinated to the metal center as a tridentate C,N,O-donor. Structure of the [Os(PPh3)2(CO)(ap-H)]
complex has been determined by X-ray crystallography. All the [Os(PPh3)2(CO)(ap-R)] complexes are diamagnetic
and show characteristic 1H NMR signals and intense MLCT transitions in the visible region. They also show emission
in the visible region at ambient temperature. Cyclic voltammetry on the [Os(PPh3)2(CO)(ap-R)] complexes shows
a reversible Os(II)−Os(III) oxidation within 0.39−0.73 V vs SCE, followed by a reversible Os(III)−Os(IV) oxidation
III
within 1.06−1.61 V vs SCE. Coulometric oxidation of the [Os(PPh3)2(CO)(ap-R)] complexes generates the [Os -
III
(PPh3)2(CO)(ap-R)]+ complexes, which have been isolated as the hexafluorophosphate salts. The [Os (PPh3)2-
(CO)(ap-R)]PF complexes are one-electron paramagnetic and show axial ESR spectra. In solution they behave as
6
III
1:1 electrolytes and show intense LMCT transitions in the visible region. The [Os (PPh3)2(CO)(ap-R)]PF complexes
6
have been observed to serve as mild one-electron oxidants in a nonaqueous medium.
Introduction
inducing C-H activation of organic molecules and the
present work has originated from our search for such
complex molecules. We have recently synthesized an os-
mium complex, viz. [Os(PPh3)2(CO)2(HCOO)2], which upon
reaction with different bidentate acidic ligands (H-LL′)
affords complexes of type [Os(PPh3)2(CO)(LL′)(H)], in
which the Os(CO)(LL′)(H) fragment constitutes the equato-
rial plane and the PPh3 ligands occupy mutual trans positions
(1).4 The disposition of the Os-H bond in the equatorial
plane of 1 suggests that if a phenyl ring can be incorporated
in the bidentate ligand by linking it to the donor atom, which
is cis to the hydride, via one intervening atom X (2), then
C-H activation may be possible at the ortho position of the
pendant phenyl ring, leading to cyclometalation of the
modified ligand (3) via elimination of molecular hydrogen.
With this simple strategy of activating phenyl C-H bond in
mind, a group of 2-(arylazo)phenols (4) have been chosen
There has been considerable current interest in the utiliza-
tion of transition metal complexes for bringing about
interesting chemical transformations of organic substrates.2
Such reactions often proceed via a C-H activation of the
organic substrate,3 leading to the formation of reactive
organometallic complexes, which finally yields the desired
product. Thus, there has been a constant demand for
transition metal complexes that have the potential for
* Author to whom correspondence should be addressed.
(1) (a) Jadavpur University. (b) Howard University.
(2) (a) Collman, J. P., Hegedus, L. S., Norton, J. R., Finke, R. G., Eds.
Principles and Applications of Organotransition Metal Chemistry;
University Science Books: Mill Valley, CA, 1987. (b) Abel, E., Stone,
F. G. A., Wilkinson, G., Eds. ComprehensiVe Organometallic Chem-
istry; Pergamon Press: Oxford, 1995; Vol. 12. (c) Tsuji J. Transition
Metal Reagents and Catalysts; Wiley-VCH: Weinheim, 2000. (d)
Trost, B. M.; Verhoeven, T. R. In ComprehensiVe Organomettalic
Chemistry; Abel, E., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon
Press: Oxford, 1982; Vol. 8. (e) Hegedus, L. S. Transition Metals in
the Synthesis of Complex Organic Molecules; University Science
Books: Mill Valley, CA, 1994. (f) Hegedus, L. S. Coord. Chem. ReV.
1998, 168, 49. (g) Cornils, B., Hermann, W. A., Eds. Applied
Homogeneous Catalysis with Organometallic Compounds: A Com-
prehensiVe Handbook in Two Volumes; VCH: Weinheim, 1996. (h)
Liebeskind, L. S., Ed. AdVances in Metal-Organic Chemistry; Jai
Press: Greenwich, CT, 1996. (i) Bellar, M., Bolm, C., Eds. Transition
Metals for Organic Synthesis; Wiley-VCH: Weinheim, 1998; Vols.
1 and 2.
(3) (a) Slugovc, C.; Padilla-Martnez, I.; Sirol, S.; Carmona, E. Coord.
Chem ReV. 2001, 213, 129. (b) Jia, C.; Kitamura, T.; Fujiwara, Y.
Acc. Chem. Res. 2001, 34, 633. (c) Ritleng, V.; Sirlin, C.; Pfeffer, M.
Chem. ReV. 2002, 102, 1731. (d) Pamplin, C. B.; Legzdins, P. Acc.
Chem. Res. 2003, 36, 223.
(4) (a) Gupta, P.; Basuli, F.; Peng, S. M.; Lee, G. H.; Bhattacharya, S.
Indian J. Chem. 2003, 42A, in press. (b) Nag, S.; Gupta, P.; Peng, S.
M.; Lee, G. H.; Bhattacharya, S. Unpublished results.
10.1021/ic0343713 CCC: $25.00 © 2003 American Chemical Society
Published on Web 07/25/2003
Inorganic Chemistry, Vol. 42, No. 17, 2003 5405

Gupta et al.
Experimental Section
Materials. Commercial osmium tetroxide was purchased from
Arora Matthey, Kolkata, India, and was converted to [NH4]2[OsBr6]
by reduction with hydrobromic acid.7 [Os(PPh3)3Br2] was synthe-
sized, starting from [NH4]2[OsBr6], by following a reported
procedure.8 [Os(PPh3)2(CO)2(HCOO)2] was then prepared starting
from [Os(PPh3)3Br2] as before.4a The 2-(arylazo)phenol ligands were
prepared by coupling diazotized para-substituted anilines with
p-cresol. Purification of dichloromethane and acetonitrile and
preparation of tetrabutylammonium perchlorate (TBAP) for elec-
trochemical work were performed as reported in the literature.9 All
other chemicals and solvents were reagent-grade commercial
materials and were used as received.
as the target ligands for the present study. Though these
ligands (4) are known to bind to metal ions usually as
bidentate N,O-donors forming a six-membered chelate ring
(5),5 we have also observed them to bind to metal ions as
bidentate N,O-donors forming a five-membered ring (6).6 It
is this latter coordination mode (6) of the 2-(arylazo)-
phenolate ligands that is of relevance to this present study.
Because in this particular coordination mode (6) the phenyl
ring of the 2-(arylazo)phenolate ligand remains pendant as
shown in 2, it appears perfectly suitable for undergoing
orthometalation (7) upon reaction with [Os(PPh3)2(CO)2-
Preparations of Complexes
The [Os(PPh3)2(CO)(ap-R)] complexes were synthesized by
following a general procedure. Specific details are given below for
one complex.
[Os(PPh3)2(CO)(ap-H)]. To a solution of H2ap-H (25 mg, 0.11
mmol) in 2-methoxyethanol (40 mL) was added [Os(PPh3)2(CO)2-
(HCOO)2] (100 mg, 0.11 mmol). The resulting solution was heated
at reflux for 24 h to produce a deep green solution. The solvent
was then evaporated on a water bath to give a green residue, which
was purified by thin-layer chromatography on a silica plate. When
1:1 hexane-benzene was used as the eluant, a green band separated,
which was extracted with dichloromethane. Evaporation of this
extract afforded [Os(PPh3)2(CO)(ap-H)] as a green crystalline solid.
Yield: 60%.
All the[Os(PPh3)2(CO)(ap-R)]PF6 complexes were synthesized
by following a general procedure. Specific details are given below
for one complex.
[Os(PPh3)2(CO)(ap-H)]PF6. A solution of [Os(PPh3)2(CO)(ap-
H)] (50 mg, 0.05 mmol) in 50 mL of 1:9 dichloromethane-
acetonitrile (0.1 M NH4PF6) was coulometrically oxidized at 0.9
V vs SCE. Color of the solution gradually changed from bright
green to olive green during the oxidation. After the electrolysis
was complete (Qobserved ) 5.07 C, Qcalculated ) 5.06 C), the solution
containing the oxidized complex was evaporated to dryness under
reduced pressure. The dry residue was washed thoroughly with ice-
cold water to remove excess NH4PF6, leaving the [Os(PPh3)2(CO)-
(ap-H)]PF6 complex as an olive-green microcrystalline solid, which
was dried under vacuo over P4O10. The yield was quantitative.
Physical Measurements
Microanalyses (C, H, N) were performed using a Heraeus Carlo
Erba 1108 elemental analyzer. IR spectra were obtained on a Perkin-
Elmer 783 spectrometer with samples prepared as KBr pellets.
Solution electrical conductivities were measured using a Phillips
PR 9500 bridge with a solute concentration of 10-3 M. Electronic
spectra were recorded on a JASCO V-570 spectrophotometer.
Emission spectra were recorded on a Spex Flurolog spectrometer.
Magnetic susceptibilities were measured using a PAR 155 vibrating
sample magnetometer fitted with a Walker Scientific L75FBAL
magnet. 1H NMR spectra were obtained on a Bruker Avance DPX
300 NMR spectrometer using TMS as the internal standard. ESR
spectra were recorded with a Varian E-109C X-band spectrometer
fitted with a quartz Dewar for measurements at 77 K (liquid
(HCOO)2], affording organoosmium complexes. This strategy
has indeed worked well and reaction of the 2-(arylazo)-
phenols with [Os(PPh3)2(CO)2(HCOO)2] truly afforded a
group of organometallic complexes of the expected type, viz.
[Os(PPh3)2(CO)(ap-R)], where ap-R refers to the coordinated
2-(arylazo)phenolate ligand in 7. An account of the chemistry
of these complexes is presented here with special reference
to their synthesis, characterization, and redox properties.
(5) (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.
Org. Met. Chem. 1995, 81, 489. (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.
(7) Dwyer, F. P.; Hogarth, J. W. Inorg. Synth. 1957, 5, 204.
(8) Hoffman, P. R.; Caulton, K. G. J. Am. Chem. Soc. 1975, 97, 4221.
(9) (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) Basuli, F.; Peng, S. M.; Bhattacharya, S. Polyhedron 1998, 18, 391.
5406 Inorganic Chemistry, Vol. 42, No. 17, 2003

Cyclometalation of 2-(Arylazo)phenols
Table 1. Crystallographic Data for [Os(PPh3)2(CO)(ap-H)]
empirical formula
fw
C50H40N2O2OsP2
952.58
space group
a, Å
monoclinic, P21/c
12.262(3)
19.353(5)
18.410(4)
108.136(16)
4151.9(16)
4
b, Å
c, Å
â, deg
V, Å3
Z
λ, Å
0.71073
cryst size, mm
T, K
0.23 × 0.73 × 0.29
296(2)
µ, mm-1
R1a
3.191
0.0354
wR2b
GOFc
0.0745
1.020
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑{w(Fo2 - Fc2)2}/∑{w(Fo )}]1/2
.
2
2
c GOF ) [∑(w(Fo - Fc )2)/(M - N)]1/ 2, where M is the number of
reflections and N is the number of parameters refined.
dinitrogen). All spectra were calibrated with the aid of DPPH (g )
2.0037). 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. A platinum-wire gauge work-
ing electrode was used in the coulometric experiments. All
electrochemical experiments were performed under a dinitrogen
atmosphere. All electrochemical data were collected at 298 K and
are uncorrected for junction potentials.
Oxidation of N,N-dimethylaniline, 1,2-naphthaquinone-1-oxime,
and [OsII(bpy)2Br2] was carried out with all the [OsIII(PPh3)2(CO)-
(ap-R)]PF6 complexes. Experimental details for the reaction of each
reductant with one representative complex are given below.
(i) To a solution of [OsIII(PPh3)2(CO)(ap-H)]PF6 (11 mg, 0.01
mmol) in acetonitrile (10 mL) was added TBAP (0.34 g, 0.10 mol)
followed by N,N-dimethylaniline (2.4 mg, 0.02 mmol). An instant
color change from olive green to bright green took place and the
solution was studied by cyclic voltammetry.
(ii) To a solution of [OsIII(PPh3)2(CO)(ap-H)]PF6 (11 mg, 0.01
mmol) in acetonitrile (10 mL) was added 1,2-naphthaquinone-1-
oxime (3.2 mg, 0.02 mmol). The color of the solution immediately
changed from olive green to bright green. Toluene (10 mL) was
then added to the solution and the ESR spectrum of the resultant
solution was recorded.
(iii) [OsIII(PPh3)2(CO)(ap-H)]PF6 (11 mg, 0.01 mmol) and
[OsII(bpy)2Br2] (6.6 mg, 0.01 mmol) were taken together in
acetonitrile (20 mL). To the solution TBAP (0.68 g, 0.20 mol) was
added and the resultant solution was studied by cyclic voltammetry.
Crystallography of [Os(PPh3)2(CO)(ap-H)]. Single crystals of
[Os(PPh3)2(CO)(ap-H)] were obtained by slow diffusion of hexane
into a dichloromethane solution of the complex. Selected crystal
data and data collection parameters are given in Table 1. Data were
collected on a Bruker P4 diffractometer using graphite monochro-
mated Mo KR radiation (λ ) 0.71073 Å) by 2θ-ω scans within
the θ range of 2.41-27.50°. X-ray data reduction, structure solution,
and refinement were done using SHELXS-97 and SHELXL-97
programs.10 The structure was solved by the direct methods.
Figure 1. View of the [Os(PPh3)2(CO)(ap-H)] complex.
Table 2. Selected Bond Distances and Bond Angles for
[Os(PPh3)2(CO)(ap-H)]
Bond Distances (Å)
Os-C
1.863(4)
2.042(4)
2.054(3)
2.186(3)
2.3714(13)
2.3623(12)
C-O
1.144(5)
1.324(5)
1.282(5)
1.402(5)
1.410(5)
Os-C(13)
Os-N(1)
Os-O(1)
Os-P(1)
Os-P(2)
C(1)-O(1)
N(1)-N(2)
C(7)-N(1)
C(8)-N(2)
Bond Angles (deg)
P(1)-Os-P(2)
C-Os-N(1)
C(13)-Os-O(1)
174.53(4)
175.37(16)
154.30(14)
C(13)-Os-N(1)
N(1)-Os-O(1)
Os-C-O
76.55(16)
77.79(13)
178.4
oxyethanol to afford a group of green complexes of type
[Os(PPh3)2(CO)(ap-R)] in decent yields. The observed elem-
ental (C, H, N) analytical data of these complexes agree well
with their compositions (Table S1 in the Supporting Infor-
mation). To find out the coordination mode of the 2-(aryl-
azo)phenolate ligands in these complexes as well as the ster-
eochemistry of the complexes, the structure of one complex
of this family, viz. [Os(PPh3)2(CO)(ap-H)], has been deter-
mined by X-ray crystallography. The structure is shown in
Figure 1 and selected bond parameters are listed in Table 2.
The structure shows that the 2-(arylazo)phenolate ligand
is coordinated to osmium in the expected tridentate fashion
(7). Though relatively less common, this particular coordina-
tion mode of the 2-(arylazo)phenols was observed before in
their complexes of some heavier transition metal ions.11 Two
triphenylphosphines and a carbonyl are also coordinated to
osmium. Osmium is therefore sitting in a C2N1O1P2 coor-
dination environment, which is distorted octahedral in nature
as reflected in all the bond parameters around osmium. The
coordinated ap-H ligand and the carbonyl have constituted
one equatorial plane with the metal at the center, while the
PPh3 ligands have occupied the axial positions and hence
they are mutually trans. The Os-P and Os-C(carbonyl) and
C-O(carbonyl) distances are all quite normal.12 Within the
Os(ap-H) fragment, the Os-C, Os-N, and Os-O distances
are also quite normal.13,6 However, they are slightly longer
than those observed in the similar fragment of the [Os(PPh3)2-
Results and Discussion
Reaction of the 2-(arylazo)phenol ligands (4) with [Os(P-
Ph3)2(CO)2(HCOO)2] proceeds smoothly in refluxing 2-meth-
(10) Sheldrick, G. M. SHELXS-97 and SHELXL-97, Fortran programs for
crystal structure solution and refinement; University of Go¨ttingen:
Go¨ttingen, Germany, 1997.
Inorganic Chemistry, Vol. 42, No. 17, 2003 5407

Gupta et al.
Table 3. 1H NMR Data of the Complexes
chemical shift in ppma
aromatic protons
compound
PPh3
CH3
R
[Os(PPh3)2(CO)(ap-OCH3)]
[Os(PPh3)2(CO)(ap-CH3)]
[Os(PPh3)2(CO)(ap-H)]
[Os(PPh3)2(CO)(ap-Cl)]
[Os(PPh3)2(CO)(ap-NO2)]
7.12-7.40
7.14-7.45
7.18-7.50
7.18-7.55
7.20-7.60
1.63
1.68
1.75
1.76
1.78
5.80(s),b 6.15(d, 8.4),b 6.30(s),b 7.03 (d, 8.7)b
5.81(s),b 6.18(d, 8.4),b 6.32(s),b 7.09(d, 8.8)b
5.82-5.87(s + d),c 6.42-6.47(d + t),c 7.00(d, 7.5),b 7.11(d, 7.8)b
5.92(s),b 6.16(d, 8.4),b 6.33(s),b 7.13(d, 8.7)b
6.01(s),b 6.20(d, 8.3),b 6.40(s)b
3.50
2.04
6.45 (t, 7.5)
s
s
a Multiplicity (s ) singlet, d ) doublet, t ) triplet) of the signal and the coupling constant (in Hz) are given in parentheses. b Integration ) 1H. c Integration
) 2H.
Table 4. Electronic Spectral and Cyclic Voltammetric Data
electronic spectral data (emission)b
λ
max/nm
electronic spectral data (absorption)
quantum cyclic voltammetric dataa,c
yield (φ)
E1/2e,V (∆Ep , mV)
a
f
compound
λ
max/nm (ꢀ/M-1 cm-1
)
excitation emission
[Os(PPh3)2(CO)(ap-OCH3)] 718(3800), 396(7600), 352(10000), 330d(9000), 258d(23000)
[Os(PPh3)2(CO)(ap-CH3)] 730(4200), 432(5400), 390d(7200), 356(14200), 256d(19000)
352
356
356
358
338
400
420
420
412
375
1.76 × 10-3
0.98 × 10-3
1.83 × 10-3
1.12 × 10-3
1.84 × 10-3
0.39(70), 1.06(80)
0.47(70), 1.15(78)
0.51(70), 1.25(82)
0.54(70), 1.29(80)
0.73(70), 1.61(85)
[Os(PPh3)2(CO)(ap-H)]
[Os(PPh3)2(CO)(ap-Cl)]
738(4200), 434(5000), 356(15000), 264d(32000)
738(4000), 428d(4000), 396d(6200), 358(14100), 276d(26000)
[Os(PPh3)2(CO)(ap-NO2)] 706(3200), 428(6400), 338(6200), 258d(15500)
a In dichloromethane. b 1:9 dichloromethane-ethanol. c Supporting electrolyte, TBAP, reference electrode SCE; scan rate 50 mV s-1
.
d Shoulder. E1/2
e
) 0.5(Epa + Epc), where Epa and Epc are anodic peak potential and cathodic peak potential respectively. f ∆Ep ) (Epa - Epc).
(ap-Cl)Br] complex11a and this difference in bond lengths is
attributable to the difference in oxidation state of osmium
in the two types of complexes. As all the [Os(PPh3)2(CO)-
(ap-R)] complexes have been synthesized similarly and they
show similar properties (vide infra), the other four [Os-
(PPh3)2(CO)(ap-R)] (R * H) complexes are assumed to have
structure similar to the [Os(PPh3)2(CO)(ap-H)] complex. It
may be noted here that, though cyclometalated osmium(III)
complexes of the 2-(arylazo)phenols of type [Os(PPh3)2(ap-
R)Br] were synthesized before in our laboratory,11a there is
no precedence in the literature of similar cyclometalated
complexes of osmium(II).
The [Os(PPh3)2(CO)(ap-R)] complexes are soluble in
acetone, dichloromethane, and chloroform, producing bright
green solutions. Electronic spectra of the complexes have
been recorded in dichloromethane solution. Spectral data are
presented in Table 4, and a selected spectrum is shown in
Figure S2 in the Supporting Information. All the [Os(PPh3)2-
(CO)(ap-R)] complexes show several intense absorptions in
the visible and ultraviolet regions. The absorptions 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 transitions. Multiple
charge-transfer transitions in such mixed-ligand complexes
may result from the lower symmetry splitting of the metal
level, the presence of different acceptor orbitals, and the
mixing of singlet and triplet configurations in the excited
state through spin-orbit coupling.16 To have better insight
into the nature of the absorptions in the visible region,
qualitative EHMO calculations have been performed17 on
Magnetic susceptibility measurements show that the [Os-
(PPh3)2(CO)(ap-R)] complexes are diamagnetic, which cor-
responds to the +2 oxidation state of osmium (low spin d6,
1
S ) 0) in these complexes. H NMR spectra of the [Os-
(PPh3)2(CO)(ap-R)] complexes have been recorded in CDCl3
solution. Spectral data are given in Table 3, and a representa-
tive spectrum is shown in Figure S1 in the Supporting
Information. Each complex shows broad signals within 7.1-
7.5 ppm for the presence of coordinated PPh3 ligands. The
methyl signal of the ap-R ligand is uniformly observed as a
distinct peak near 1.6 ppm. Among the expected aromatic
proton signals for the coordinated ap-R ligand, most have
been clearly observed while a few could not be detected due
to their overlap with the PPh3 signals. In the infrared
spectrum each [Os(PPh3)2(CO)(ap-R)] complex shows a
strong band near 1900 cm-1 (Table S1 in the Supporting
Information) due to the coordinated carbon monoxide14 and
three strong bands near 520, 695, and 745 cm-1, indicating
the presence of coordinated PPh3 ligands.15 Sharp bands are
also observed near 1210, 1540, and 1580 cm-1, which were
absent in the [Os(PPh3)2(CO)2(HCOO)2] complex, and hence
these are attributed to the coordinated 2-(arylazo)phenolate
(11) (a) Majumder, K.; Peng, S. M.; Bhattacharya, S. J. Chem. Soc., Dalton
Trans. 2001, 284. (b) Dutta, S.; Peng, S. M.; Bhattacharya, S. J. Chem.
Soc., Dalton Trans. 2000, 4623. (a) Lahiri, G, K.; Bhattacharya, S.;
Mukherjee, M.; Mukherjee, A.; Chakravorty, A. Inorg. Chem. 1987,
26, 3359.
(12) (a) Rickard, C. E. F.; Roper, W. R.; Williamson, A.; Wright, L. J.
Organometallics 2002, 21, 4862. (b) Rickard, C. E. F.; Roper, W. R.;
Williamson, A.; Wright, L. J. Organometallics 2002, 21, 1714. (c)
Rickard, C. E. F.; Roper, W. R.; Woodgate, S. D.; Wright, L. J. J.
Organomet. Chem. 2002, 643-644, 169. (d) Pramanik, K.; Shivaku-
mar, M.; Ghosh, P.; Chakravorty, A. Inorg. Chem. 2000, 39, 195. (e)
Das, A.; Basuli, F.; Peng, S. M.; Bhattacharya, S. Polyhedron 1999,
18, 2729. (f) Robinson, P. D.; Hinckley, C. C.; Ikuo, A. Acta
Crystallogr. 1988, C44, 1491. (g) Bellachioma, G.; Cardaci, G.;
Macchioni, A.; Zanazzi, P. Inorg. Chem. 1993, 32, 547.
(13) (a) Gusev, D. G.; Dolgushin, F. M.; Antipin, M. Y. Organometallics
2002, 21, 1001. (b) Barrio, P.; Esteruelas, M.; A.; O˜ nate, E.
Organometallics 2002, 21, 1714.
(14) Diluzio, J. W.; Vaska, L. J. Am. Chem. Soc. 1961, 83, 1262.
(15) (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.
1
ligand. The H NMR and infrared spectral data of the [Os-
(PPh3)2(CO)(ap-R)] complexes are therefore consistent with
their compositions.
5408 Inorganic Chemistry, Vol. 42, No. 17, 2003

Cyclometalation of 2-(Arylazo)phenols
excitation wavelength corresponding to the lowest energy
absorption (∼700 nm) was used, no emission was observed.
However, when an excitation wavelength of ∼350 nm was
used, all the [Os(PPh3)2(CO)(ap-R)] complexes showed
strong emission (Table 4; Figure S2 in the Supporting
Information) within 375-420 nm. This indicates that transi-
tion of a metal electron to LUMO+2 (or LUMO+3) leads
to a photochemically excited state, which is much more stable
compared to excitation of a metal electron to LUMO (π* of
NdN) or LUMO+1 (π* of CO). The higher stability of the
photochemically excited state involving LUMO+2 (or
LUMO+3) may be attributed to the possible delocalization
of the excited electron over the metalated phenyl ring (or
p-cresol ring). Quantum yields (φ) of these emissions,
evaluated with reference to [Ru(bpy)3](ClO4)2,19 indicate that
these complexes are relatively poor emitters.
Electrochemical properties of all the [Os(PPh3)2(CO)(ap-
R)] complexes have been studied by cyclic voltammetry in
dichloromethane solution (0.1 M TBAP). Voltammetric data
are given in Table 4, and a selected cyclic voltammogram
is shown in Figure S3 in the Supporting Information. Each
complex shows an oxidative response within 0.39-0.73 V
vs SCE, which is assigned to the osmium(II)-osmium(III)
oxidation. This oxidation is reversible, characterized by a
peak-to-peak separation (∆Ep) of 70 mV, which remains
unchanged upon changing the scan rate, and the anodic peak
current (ipa) is almost equal to the cathodic peak current (ipc),
as expected for a reversible electron-transfer process. Com-
parison of the potential of the osmium(II)-osmium(III)
couple in the [OsIII(PPh3)2(ap-R)Br]11a and [OsII(PPh3)2(CO)-
(ap-R)] complexes shows that the bivalent state of osmium
is much more stabilized (more than 1.1 V) in the present
group of complexes, which may be attributed to the presence
of carbon monoxide in the coordination sphere instead of
the bromide. CO being a strong π-acid ligand stabilizes the
+2 oxidation state of osmium through effective π-interaction.
The [Os(PPh3)2(CO)(ap-R)] complexes show a second oxida-
tive response within 1.06-1.61 V vs SCE, which is also
reversible in nature (∆Ep ∼ 80 mV; ipa ∼ ipc) and is assigned
to the osmium(III)-osmium(IV) oxidation. The one-electron
nature of both the oxidations has been verified by comparing
their current heights (ipa) with that of the standard ferrocene/
ferrocenium couple under identical experimental conditions.
The potential of both the osmium(II)-osmium(III) and
osmium(III)-osmium(IV) oxidations has been observed to
be sensitive to the nature of the substituent R in the
2-(arylazo)phenolate ligand. The potential (E1/2) increases
with increasing electron-withdrawing character of the sub-
stituent R. The plot of E1/2 versus σ [σ ) Hammett constant
of R;20 OCH3 ) -0.27, CH3 ) -0.17, H ) 0.00, Cl ) 0.23,
Figure 2. Partial molecular orbital diagram of [Os(PPh3)2(CO)(ap-H)].
computer-generated models of the [Os(PPh3)2(CO)(ap-R)]
complexes. The results obtained are similar for all the
complexes. A partial MO diagram for a representative
complex is shown in Figure 2, and composition of selected
molecular orbitals is given in Table S2 in the Supporting
Information. The highest occupied molecular orbital (HOMO)
and the next two filled orbitals (HOMO-1 and HOMO-2)
have major (>50%) contributions from the osmium dxy, dyz,
and dzx orbitals. These three occupied orbitals may therefore
be regarded as the osmium t2 orbitals. The lowest unoccupied
molecular orbital (LUMO), which is rather isolated, has more
than 80% contribution from the 2-(arylazo)phenolate ligand
and is concentrated heavily on the azo (NdN) fragment.
Among the next few vacant orbitals (LUMO+1, LUMO+2,
LUMO+3, etc.), LUMO+1 is primarily centered on the
carbonyl ligand, while LUMO+2 and LUMO+3 are delo-
calized over the metalated phenyl ring and the p-cresol ring,
respectively. The intense and rather isolated absorption in
the 700-740-nm region may therefore be assigned to the
charge-transfer transition taking place from the highest filled
osmium t2 orbital (HOMO) to the vacant π*(NdN) orbital
of the 2-(arylazo)phenolate ligand (LUMO). The other
intense absorptions at higher energies may be assigned to
charge-transfer transitions from the osmium t2 orbitals to the
higher energy vacant orbitals. As osmium(II) complexes of
ligands containing strong chromophoric groups often show,
in addition to intense MLCT transitions, interesting lumi-
nescence properties,18 such properties have also been ex-
plored in the [Os(PPh3)2(CO)(ap-R)] complexes. The lumi-
nescence studies were carried out in 1:9 dichloromethane-
ethanol solution at ambient temperature (298 K). When an
(18) (a) Gao, F. G.; Bard, A. J. Chem. Mater. 2002, 14, 3465. (b) Baudin,
H. B.; Davidsson, J.; Serroni, S.; Juris, A.; Balzani, V.; Campagna,
S.; Hammarstron, l. J. Phys. Chem. A 2002, 106, 4312. (c) Staffilani,
M.; Belser, P.; Hartl, F., Kleverlaan, C. J.; De Cola L. J. Phys. Chem.
A 2002, 106, 9242. (d) Ortega, J. V.; Khin, K.; van der Veer, W. E.;
Ziller, J.; Hong, B. Inorg. Chem. 2000, 39, 6038. (e) Juris, A.; Prodi,
L.; Harriman, A.; Ziessel, R.; Hissler, M.; El-ghayoury, A.; Wu, F.;
Riesgo, E. C.; Thummel, R. O. Inorg. Chem. 2000, 39, 3590. (f)
Holmlin, R. E.; Yao, J. A.; Barton, J. K. Inorg. Chem. 1999, 38, 174.
(19) Alsfasser, R.; Eldik, R. V. Inorg. Chem. 1996, 35, 628.
(16) (a) Pankuch, B. J.; Lacky, D. E.; Crosby, G. A. J. Phys. Chem. 1980,
84, 2061. (b) Ceulemans, A.; Vanquickenborne, L. G. J. Am. Chem.
Soc. 1981, 103, 2238. (c) decuritus, S.; Felix, F.; Ferguson, J.; Gudel,
H. U.; Ludi, A. J. Am. Chem. Soc. 1980, 103, 4102. (d) Kober, E. M.;
Meyer, T. J. Inorg. Chem. 1982, 21, 3967.
(17) (a) Mealli, C.; Proserpio, D. M. CACAO Version 4.0; Italy, 1994. (b)
Mealli, C.; Proserpio, D. M. J. Chem. Educ. 1990, 67, 399.
Inorganic Chemistry, Vol. 42, No. 17, 2003 5409

Gupta et al.
Table 5. ESR g Valuesa and Electronic Spectral Values of Osmium(III) Complexes
electronic spectral data (absorption)
b
compound
g
g|
λ )
max/nm (ꢀ/M-1 cm-1
[Os(PPh3)2(CO)(ap-OCH3)]PF6
[Os(PPh3)2(CO)(ap-CH3)]PF6
[Os(PPh3)2(CO)(ap-H)]PF6
[Os(PPh3)2(CO)(ap-Cl)]PF6
[Os(PPh3)2(CO)(ap-NO2)]PF6
2.100
2.125
2.138
2.117
2.078
1.989
1.981
1.974
1.985
1.992
1220c(800), 944c(1300), 746(3100), 420(5100), 378(5400), 344c(4500)
1220c(800), 988(1100), 742(2200), 466c(2400), 376(5900), 342c(4800)
1210c(800), 994(1300), 738(2200), 468c(2100), 374(6000), 342c(5500)
1220c(900), 980(1200), 746(2400), 466c(2300), 376(7000), 346(6300)
1220c(1700), 944c(2500), 738(3600), 474(5400), 360(4600)
a In 1:1 dichloromethane-toluene at 77 K. b In dichloromethane solution. c Shoulder.
and NO2 ) 0.78] is linear for both the oxidations (Figure
S3 in the Supporting Information) with a slope (F) of 0.30
and 0.49 V, respectively (F ) reaction constant of this
couple21). This linear correlation of E1/2 versus σ with
reasonable slopes (F) clearly shows that a single substituent
on the 2-(arylazo)phenolate ligand can influence the metal-
centered oxidation potential in these complexes in a predict-
able manner.
The electrochemical reversibility of the osmium(II)-
osmium(III) couple indicate that the [OsIII(PPh3)2(CO)(ap-
R)]+ complexes are stable on a cyclic voltammetric time
scale. The relatively low positive potential of this oxidation
suggests that the oxidized complexes should also be stable
on a much longer time scale. To investigate this, all the [Os-
(PPh3)2(CO)(ap-R)] complexes have been coulometrically
oxidized at appropriate potential in dichloromethane solution
(0.1 M TBAP). The oxidations have been smooth and
quantitative, associated with a color change of bright green
to olive green. The solutions containing the oxidized
complexes show identical cyclic voltammetric responses such
as their respective precursors, except that the couple near
0.4 V now appears as a reductive response. The olive-green
solutions of [OsIII(PPh3)2(CO)(ap-R)]+ have been quantita-
Figure 3. ESR spectrum of [OsIII(PPh3)2(CO)(ap-H)]PF6 in 1:1 dichlo-
tively converted back, upon coulometric reduction at 0.2 V,
to the bright green solutions of [OsII(PPh3)2(CO)(ap-R)],
identified by their characteristic electronic spectra. This
shows that in these [Os(PPh3)2(CO)(ap-R)]n+ (n ) 0, 1)
complexes, interconversion between the +2 and +3 states
of osmium takes place without any gross change in stereo-
chemistry.
Encouraged by the observed stability of the oxidized
complexes, they have been generated in a much larger scale
by coulometric oxidation of the [OsII(PPh3)2(CO)(ap-R)]
complexes in 1:9 dichloromethane-acetonitrile solution
using NH4PF6 as the supporting electrolyte and the cationic
[OsIII(PPh3)2(CO)(ap-R)]+ complexes have been isolated as
the hexafluorophosphate salt in quantitative yield. The [OsIII-
(PPh3)2(CO)(ap-R)]PF6 complexes show satisfactory mi-
croanalytical data (Table S1 in the Supporting Information).
Magnetic susceptibility measurements show that these [Os-
(PPh3)2(CO)(ap-R)]PF6 complexes are one-electron para-
magnetic (µeff ) 1.89-1.97 µB), which corresponds to the
+3 state of osmium (low-spin d5, S ) 1/2) in these complexes.
All the [Os(PPh3)2(CO)(ap-R)]PF6 complexes show axial
ESR spectra in 1:1 dichloromethane-toluene solution at 77
romethane-toluene solution at 77 K.
K with distinct g and g| signals. A selected spectrum is
shown in Figure 3 and the spectral data are given in Table
5. Infrared spectrum of each [OsIII(PPh3)2(CO)(ap-R)]PF6
complex is mostly similar to that of its [OsII(PPh3)2(CO)-
(ap-R)] precursor. The νCO stretch is, however, shifted to
ca. 1960 cm-1 (Table S1 in the Supporting Information). A
strong band near 840 cm-1 is displayed by all the [OsIII-
(PPh3)2(CO)(ap-R)]PF6 complexes, which was absent in the
[OsII(PPh3)2(CO)(ap-R)] complexes, indicating the presence
of the hexafluorophosphate anion. The [Os(PPh3)2(CO)(ap-
R)]PF6 complexes are soluble in acetonitrile, acetone, dichlo-
romethane, and chloroform, producing olive-green solutions.
Conductivity studies in acetonitrile solution shows that these
complexes behave as 1:1 electrolytes (ΛM ) 145-160 Ω-1
cm2 M-1), as expected. Electronic spectra of these complexes
have been recorded in acetonitrile solution. Each complex
shows six22 absorptions, two in the near-IR region, two in
the visible region, and two22 in the ultraviolet region (Table
5; Figure S4 in the Supporting Information). The two
absorptions displayed by all the complexes within 1300-
(20) Hammett, L. P. Physical Organic Chemistry, 2nd ed.; McGraw-Hill:
New York, 1970.
(22) In the [OsIII(PPh3)2(CO)(ap-NO2)]PF6 complex a total of five absorp-
tions was observed (two in the near-IR, two in the visible, and one in
the UV region).
(21) Mukherjee, R. N.; Rajan, O. A.; Chakravorty, A. Inorg. Chem. 1982,
21, 785.
5410 Inorganic Chemistry, Vol. 42, No. 17, 2003

Cyclometalation of 2-(Arylazo)phenols
900 nm are relatively much weaker in intensity and these
are assigned to transitions within the split components of
the osmium t2 orbitals. The two intense absorptions in the
visible region are believed to be due to ligand-to-metal
charge-transfer transitions, and those in the ultraviolet region
are attributable, as before, to intraligand transitions.
The stability of the [OsIII(PPh3)2(CO)(ap-R)]+ complexes
and their reduction potential suggest that they might serve
as mild one-electron oxidants. To check this, reaction of all
the [OsIII(PPh3)2(CO)(ap-R)]+ complexes was first carried
out with N,N-dimethylaniline (DMA). Upon addition of
DMA to a solution of any [OsIII(PPh3)2(CO)(ap-R)]+ complex
in acetonitrile (0.1 M TBAP), an instant color change from
olive green to bright green takes place and cyclic voltam-
metry on the resultant solution shows a new response at 0.34
V, which is characteristic of N,N,N′,N′-tetramethylbenzidine
(TMB, 8).23 Evidently, TMB has been produced by one-
Aneasilyoxidizabletransitionmetalcomplex,viz.[OsII(bpy)2-
Br2], has also been used as the reductant. Equimolar
quantities of [OsII(bpy)2Br2] and any [OsIII(PPh3)2(CO)(ap-
R)]+ complex were taken together in acetonitrile (0.1 M
TBAP) and cyclic voltammetry on the resultant solution
confirms that the expected redox reaction (eq 3) has indeed
taken place. A reversible one-electron reduction at 0.01 V,
[OsII(bpy)2Br2] + [OsIII(PPh3)2(CO)(ap-R)]+ f
[OsIII(bpy)2Br2]+ + [OsII(PPh3)2(CO)(ap-R)] (3)
characteristic of [OsIII(bpy)2Br2]+,26 is observed together with
the reversible one-electron oxidation of [OsII(PPh3)2(CO)-
(ap-R)] within 0.39-0.73 V. These redox reactions show
that the [OsIII(PPh3)2(CO)(ap-R)]+ complexes can indeed
function as efficient one-electron oxidants in a nonaqueous
medium.
Conclusions
The present study shows that C-H activation of selective
organic substrates may be easily achieved by their reaction
with [Os(PPh3)2(CO)2(HCOO)2], leading to the formation of
interesting organoosmium complexes. Such possibilities are
currently under exploration.
electron oxidation of DMA, followed by a coupling reaction
(eq 1). Conversion of [OsIII(PPh3)2(CO)(ap-R)]+ to [OsII-
Acknowledgment. Financial assistance received from the
Department of Science and Technology, New Delhi [Grant
No. SP/S1/F33/98], the Council of Scientific and Industrial
Research, New Delhi [Grant No. 01(1675)/00/EMR-II], and
the University Grants Commission, New Delhi [Grant No.
F.12-56/2001(SR-I)], is gratefully acknowledged. The authors
thank Prof. Nitin Chattopadhyay and Dr. Pradipta Purkay-
astha for their help with the fluorescence spectral measure-
ments, and the RSIC at Central Drug Research Institute,
Lucknow, India, for the C,H,N analysis data. P.G. thanks
the CSIR for her fellowship. The authors thank the reviewers
for their critical comments and constructive suggestions,
which have been helpful in preparing the revised version.
2DMA + 2[OsIII(PPh3)2(CO)(ap-R)]+ f
TMB + 2 [OsII(PPh3)2(CO)(ap-R)] + 2H+ (1)
(PPh3)2(CO)(ap-R)] has been confirmed by their spectra in
the visible region. It appears from the current height
measurement of the TMB response that about 45% of the
oxidant has been utilized in the coupling reaction. The
remainder has probably been wasted in the side reactions.24
Another organic compound, viz. 1,2-naphthaquinone-1-oxime
(Hnqo, 9), was tried as the next reductant. Addition of Hnqo
to an acetonitrile solution of any [OsIII(PPh3)2(CO)(ap-R)]+
complex also causes the expected color change as before
and the ESR spectrum recorded on the resultant solution at
ambient temperature shows an intense signal at g ) 2.006
with three nitrogen hyperfine lines (Figure S5 in the
Supporting Information), characteristic of the iminoxy radical
(nqo•).25 This shows that [OsIII(PPh3)2(CO)(ap-R)]+ truly
oxidizes Hnqo to generate the nqo• radical (eq 2).
Supporting Information Available: 1H NMR spectrum of [Os-
(PPh3)2(CO)(ap-OCH3)] (Figure S1), absorption and emission
spectra of [Os(PPh3)2(CO)(ap-CH3)] (Figure S2), cyclic voltam-
mogram of [Os(PPh3)2(CO)(ap-OCH3)] and least-squares plot of
E1/2 values of Os(II)-Os(III) and Os(III)-Os(IV) couples versus
σ (Figure S3), absorption spectrum of [Os(PPh3)2(CO)(ap-H)]PF6
(Figure S4), and ESR spectrum of the iminoxy radical (nqo•) (Figure
S5), and tables containing microanalytical and νCO data of the [Os-
(PPh3)2(CO)(ap-R)] and [Os(PPh3)2(CO)(ap-R)]PF6 complexes (Table
S1) and composition of selected molecular orbitals of the [Os-
(PPh3)2(CO)(ap-R)] complexes (Table S2). X-ray crystallographic
data in CIF format. This material is available free of charge via
the Internet at http://pubs.acs.org.
Hnqo + [OsIII(PPh3)2(CO)(ap-R)]+ f
nqo• + [OsII(PPh3)2(CO)(ap-R)] + H+ (2)
IC0343713
(23) Mizoguchi, T.; Adams, R. N. J. Am. Chem. Soc. 1962, 84, 2058.
(24) (a) Galus, Z.; Adams, R. N. J. Am. Chem. Soc. 1962, 84, 2061. (b)
Galus, Z.; White, R. M.; Rowland, F. S.; Adams, R. N. J. Am. Chem.
Soc. 1962, 84, 2065.
(26) (a) Kober, E. M.; Casper, J. V.; Sullivan, B. P.; Meyer, T. J. Inorg.
Chem. 1988, 27, 4587. (b) Cyclic voltammetry on [Os(bpy)2Br2] has
been performed by us.
(25) Fox, W. M.; Symons, M. C. R. J. Chem. Soc. A 1966, 1503.
Inorganic Chemistry, Vol. 42, No. 17, 2003 5411