Chemistry of osmium in N2P2Br2 coordination sphere: Synthesis, structure and reactivities

Article abstract of DOI:10.1016/S0277-5387(99)00178-3

A family of three mixed-ligand osmium complexes of type [Os(PPh₃)₂(N-N)Br₂], where N-N=2,2′-bipyridine (bpy), 4,4′-dimethyl-2,2′-bipyridine (Me₂bpy) and 1,10-phenanthroline (phen), have been synthesized and characterized. The complexes are diamagnetic (low-spin d⁶, S=0) and in dichloromethane solution they show intense MLCT transitions in the visible region. The two bromide ligands have been replaced from the coordination sphere of [Os(PPh₃)₂(phen)Br₂] under mild conditions by a series of anionic ligands L (where L=quinolin-8-olate (q), picolinate (pic), oxalate (Hox) and 1-nitroso-2-naphtholate (nn)) to afford complexes of type [Os(PPh₃)₂(phen)(L)]⁺, which have been isolated and characterized as the perchlorate salt. The structure of the [Os(PPh₃)₂(phen)(pic)]ClO₄ complex has been determined by X-ray crystallography. The PPh₃ ligands occupy trans positions and the picolinate anion is coordinated to osmium as a bidentate N,O-donor forming a five-membered chelate ring. The [Os(PPh₃)₂(phen)(L)]⁺ complexes are diamagnetic and show multiple MLCT transitions in the visible region. The [Os(PPh₃)₂(N-N)Br₂] complexes show an osmium(II)-osmium(III) oxidation (-0.02 to 0.12 V vs. SCE) followed by an osmium(III)-osmium(IV) oxidation (1.31 to 1.43 V vs. SCE). The [Os(PPh₃)₂(phen)(L)]⁺ complexes display the osmium (II)-osmium (III) oxidation (0.26 to 0.84 V vs. SCE) and one reduction of phen (-1.50 to -1.79 V vs. SCE). The osmium (III)-osmium (IV) oxidation has been observed only for the L=q and L=Hox complexes at 1.38 V vs. SCE and 1.42 V vs. SCE respectively. The osmium(III) species, viz. [Os^III(PPh₃)₂(N-N)Br₂]⁺ and [Os^III(PPh₃)₂(phen)(L)]²⁺, have been generated both chemically and electrochemically and characterized in solution by electronic spectroscopy and cyclic voltammetry. Elsevier Science Ltd.

Full text of DOI:10.1016/S0277-5387(99)00178-3

Polyhedron 18 (1999) 2729–2736
www.elsevier.nl/locate/poly
Chemistry of osmium in N2P2Br2 coordination sphere
Synthesis, structure and reactivities
Anindya Dasa, Falguni Basulia, Shie-Ming Pengb, Samaresh Bhattacharyaa ,
*
aDepartment of Chemistry, Inorganic Chemistry Section, Jadavpur University, Calcutta 700032, India
bDepartment of Chemistry, National Taiwan University, Taipei, Taiwan, ROC
Received 11 December 1998; accepted 26 May 1999
Abstract
A family of three mixed-ligand osmium complexes of type [Os(PPh3)2(N-N)Br2], where N-N52,29-bipyridine (bpy), 4,49-dimethyl-
2,29-bipyridine (Me2bpy) and 1,10-phenanthroline (phen), have been synthesized and characterized. The complexes are diamagnetic
(low-spin d6, S50) and in dichloromethane solution they show intense MLCT transitions in the visible region. The two bromide ligands
have been replaced from the coordination sphere of [Os(PPh3)2(phen)Br2] under mild conditions by a series of anionic ligands L (where
L5quinolin-8-olate (q), picolinate (pic), oxalate (Hox) and 1-nitroso-2-naphtholate (nn)) to afford complexes of type
[Os(PPh3)2(phen)(L)]1, which have been isolated and characterized as the perchlorate salt. The structure of the
[Os(PPh3)2(phen)(pic)]ClO4 complex has been determined by X-ray crystallography. The PPh3 ligands occupy trans positions and the
picolinate anion is coordinated to osmium as a bidentate N,O-donor forming a five-membered chelate ring. The [Os(PPh3)2(phen)(L)]1
complexes are diamagnetic and show multiple MLCT transitions in the visible region. The [Os(PPh3)2(N-N)Br2] complexes show an
osmium(II)–osmium(III) oxidation (20.02 to 0.12 V vs. SCE) followed by an osmium(III)–osmium(IV) oxidation (1.31 to 1.43 V vs.
SCE). The [Os(PPh3)2(phen)(L)]1 complexes display the osmium (II)–osmium (III) oxidation (0.26 to 0.84 V vs. SCE) and one
reduction of phen (21.50 to 21.79 V vs. SCE). The osmium (III)–osmium (IV) oxidation has been observed only for the L5q and
L5Hox complexes at 1.38 V vs. SCE and 1.42 V vs. SCE respectively. The osmium(III) species, viz. [OsIII(PPh3)2(N-N)Br2]1 and
[OsIII(PPh3)2(phen)(L)]21, have been generated both chemically and electrochemically and characterized in solution by electronic
spectroscopy and cyclic voltammetry.
1999 Elsevier Science Ltd. All rights reserved.
Keywords: Osmium; Triphenylphosphine; Diimine; Synthesis; Structure; Reactivities
1. Introduction
bond dissociation [e.g. displacement reactions, insertion
(into the metal–ligand bond) reactions]. For the reactions
of type (ii) to occur, the complexes must contain one or
more labile metal–ligand bond(s). Osmium complexes with
Os–X (X5Cl, Br) bonds are of particular attraction in this
context because the Os–X bonds are known to be poten-
tially labile. However, the degree of lability and hence
reactivities arising therefrom, is a function of the oxidation
state of osmium and nature of ligands occupying the other
coordination positions. In the present study, which is part
of our recent activities in osmium chemistry [19–23], we
have chosen a cis-OsBr2 moiety as the reactive fragment.
To satisfy the remaining four coordination sites of os-
mium, triphenylphosphine (PPh3) and some N–N donor
ligands (viz. 2,29-bipyridine (bpy,1); 4,49-dimethyl-2,29-
bipyridine (Me2bpy, 2) and 1,10-phenanthroline (phen, 3))
have been used.
The chemistry of osmium has been of much current
interest [1–18] largely because of the variety of reactions
exhibited by its complexes. Reactivity of osmium com-
plexes is directed by the coordination environment around
osmium. Hence variation of the coordination sphere brings
about corresponding variation in the reactivities of the
complexes. The reactions displayed by transition metal
complexes can be broadly classified into two categories: (i)
reactions where no metal–ligand bond breaking is involved
[e.g. stereoisomerization, outer-sphere electron-transfer
reaction, etc.] and (ii) reactions occurring via metal–ligand
*Corresponding author. Tel.: 191-33-483-8398.
E-mail address: icsg@mahendra.iacs.res.in (S. Bhattacharya)
0277-5387/99/$ – see front matter
PII: S0277-5387(99)00178-3
1999 Elsevier Science Ltd. All rights reserved.

2730
A. Das et al. / Polyhedron 18 (1999) 2729–2736
and 1,10-phenanthroline (18 mg, 0.09 mmol) was added
2-methoxyethanol (40 cm3). The resulting mixture was
refluxed for 2 h. Upon cooling [Os(PPh3)2(phen)Br2]
started to separate out as a greenish-brown solid. The
deposited solid was collected by filtration, washed thor-
oughly with 2-methoxyethanol and dried in vacuo over
P4O10. Recrystallization from dichloromethane–hexane
solution gave the pure product as a microcrystalline solid.
The yield was 68 mg (73%).
The [OsII(PPh3)2(phen)(L)]ClO4 (L5pic, q, Hox, nn)
complexes were synthesized by using a general procedure.
Yields varied in the range of 60–70%. Specific details are
given below for one complex.
All these ligands belong to the diimine family and they
have recognized p-acid character. Hence their coordination
is expected to stabilize the bivalent state of osmium, which
is a lower oxidation state of this metal and thus it is
expected to favour dissociation of the Os–Br bonds.
Herein we wish to report the synthesis and characterization
of a family of osmium complexes of type [Os(PPh3)2(N-
N)Br2] and their spectroscopic and electron-transfer prop-
erties with special reference to their reactions involving the
Os–Br bond cleavage.
2.2.2. [OsII(PPh3 )2(phen)(pic)](ClO4 )
[Os(PPh3)2(phen)Br2] (100 mg, 0.10 mmol) and
picolinic acid (12 mg, 0.10 mmol) were dissolved together
in dichloromethane (30 cm3) and triethylamine (10 mg,
0.10 mmol) was added to it followed by ethanol (40 cm3).
The solution was boiled for few minutes in air to remove
dichloromethane as much as possible and it was heated at
reflux for 6 h. After being cooled to room temperature, a
saturated aqueous solution of NaClO4 (0.5 cm3) was added
to it. [Os(PPh3)2(phen)(pic)]ClO4 precipitated as a brown
solid which was collected by filtration, washed with ice-
cold water and dried in vacuo over P4O10. Purification of
the product was done by recrystallization from dichlorome-
thane–hexane solution. The yield was 70 mg (66%).
2. Experimental
2.1. Materials
2.3. Physical measurements
Osmium tetroxide was purchased from Arora Matthey,
Calcutta, India and was converted into [NH4]2[OsBr6] by
reduction with HBr [24]. [Os(PPh3)3Br2] was synthesized
from [NH4]2[OsBr6] using a previously published litera-
ture method [25]. Triphenylphosphine, 2,29-bipyridine,
oxalic acid (H2ox) and 1-nitroso-2-naphthol (Hnn) were
purchased from Loba Chemie, Mumbai, India. 1,10-Phen-
anthroline, picolinic acid (Hpic) and quinoline-8-ol (Hq)
were purchased from E. Merck (India) Pvt. Ltd., Mumbai.
4,49-Dimethyl-2,29-bipyridine was purchased from Al-
drich. All other chemicals and solvents were reagent grade
commercial materials and were used as received. Purifica-
tion of acetonitrile and dichloromethane, and preparation
of tetraethylammonium perchlorate (TEAP) for electro-
chemical work were performed as reported in the literature
[26,27].
Microanalyses (C, H, N) were performed using a
Perkin-Elmer 240C elemental analyzer. Infrared spectra
were obtained on a Perkin-Elmer 783 spectrometer with
samples prepared as KBr pellets. Electronic spectra were
recorded on Shimadzu UV 1601 and Hitachi U 3501
spectrophotometers. Magnetic susceptibilities were mea-
sured using a PAR 155 vibrating sample magnetometer
fitted with a Walker Scientific L75FBAL magnet. 1H NMR
spectra were obtained on a Brucker AC-200 NMR spec-
trometer using TMS as the internal standard. Solution
electrical conductivities were measured using a Philips PR
9500 bridge with a solute concentration of 1023 M.
Electrochemical measurements were made using a PAR
model 273 potentiostat. A platinum disc or graphite
working electrode, a platinum wire auxiliary electrode and
an aqueous saturated calomel reference electrode (SCE)
were used in a three electrode configuration. A platinum
wire gauze working electrode was used in the coulometric
experiments. A RE 0074 X–Y recorder was used to trace
the voltammograms. Dinitrogen gas was purified by suc-
cessively bubbling it through alkaline dithionite and con-
centrated sulphuric acid. Electrochemical measurements
were performed under a dinitrogen atmosphere. All elec-
trochemical data were collected at 298 K and are un-
corrected for junction potentials.
2.2. Preparation of complexes
The [OsII(PPh3)2(N-N)Br2] (N-N5bpy, Me2bpy, phen)
complexes were synthesized by using a general procedure.
Yields varied in the range of 70–75%. Specific details are
given below for one complex.
2.2.1. [Os(PPh3 )2(phen)Br2 ]
To a mixture of [Os(PPh3)3Br2] (100 mg, 0.09 mmol)

A. Das et al. / Polyhedron 18 (1999) 2729–2736
2731
Table 1
data collection parameters are given in Table 1. The unit
cell dimensions were determined by a least-squares fit of
25 machine-centred reflections (14.90,2u ,22.108). Data
were collected on an Enraf-Nonius CAD-4 diffractometer
using graphite monochromated MoKa radiation (l5
Crystallographic data for [Os(PPh3)2(phen)(pic)]ClO4?3C6H6
Formula
fw
C72H60N3O6ClP2Os
1350.87
Monoclinic, P21 /n
14.815(6)
19.575(3)
20.939(4)
91.58(3)
6070(3)
4
0.2530.2530.20
298
22.527
0.043
0.040
Space group
˚
a (A)
˚
0.7107 A) by u22u scans with a maximum 2u angle of
˚
b (A)
˚
458. Three standard reflections, measured every 3600 s of
X-ray exposure, showed no significant intensity variation
over the course of data collection. X-ray data reduction and
structure solution and refinement were carried out using
the NRCVAX package. The structure was solved by the
Patterson method.
c (A)
b (8)
V (A )
3
˚
Z
Crystal size (mm)
T (K)
m (cm21
)
a
Rf
b
Rw
GOF
1.34
3. Results and discussion
a Rf 5o iFou2uFci/o uFou.
b Rw 5[o w(uFou2uFcu)2 /o w(Fo)2]1 / 2
.
The [Os(PPh3)2(N-N)Br2] (N-N5bpy, Me2bpy, phen)
complexes were synthesized in decent yields by the
smooth displacement of one PPh3 from [Os(PPh3)3Br2]
with the nitrogen donor ligands in refluxing 2-methox-
yethanol. Composition of the complexes were confirmed
by their elemental (C, H, N) analytical data (Table 2). All
three complexes are diamagnetic, which is in accordance
with the 12 state of osmium (low-spin d6, S50) in these
2.4. Crystallography
Single crystals of [Os(PPh3)2(phen)(pic)]ClO4.3C6H6
were grown by slow diffusion of benzene into a dichloro-
methane solution of the complex. Selected crystal data and
Table 2
Characterization data of the [Os(PPh3)2(N-N)Br2] and [Os(PPh3)2(phen)(L)]ClO4 complexes
Compound
Yield
%
Analytical dataa
Electronic spectral data
Cyclic voltammetric datac
E1 / 2,V vs. SCE (DEp, mV)
0.11(60), 1.43(70)
%C
%H
%N
lmax, nm (e, M21 cm21
)
[Os(PPh3)2(bpy)Br2]d
[Os(PPh3)2(Me2bpy)Br2]d
[Os(PPh3)2(phen)Br2]d
[Os(PPh3)2(phen)(q)]ClO4
70
53.39
3.65
2.66
(2.72)
492b(2100), 452b(2200),
348(4300), 296(12900),
248b(18800), 224(22700)
492(3800), 464b(3800),
344(7600), 292(26600),
248b(35900), 224(53100)
684b(800), 480b(5100),
448(5900), 288b(13200),
252(33600), 224(45400)
660b(1100), 520b(2600),
456(4200), 356(3700),
260b(21400), 240b(27900),
212(44200)
(53.59)
(3.69)
68
73
62
54.22
3.90
2.61
(2.65)
20.02(70), 1.31(80)
(54.43)
(3.97)
54.58
3.60
2.67
(2.66)
0.12(70), 1.42(70)
(54.65)
(3.61)
e
60.19
3.79
3.66
(3.69)
0.39(70),1.38(160),
21.66(120)
(60.12)
(3.87)
e
[Os(PPh3)2(phen)(pic)]ClO4
66
67
58.07
3.74
3.71
(3.76)
596b(1000), 416(6500),
260b(30900), 244b(39600),
208(66900)
0.71(80), 21.55(60)
(58.08)
(3.76)
e
[Os(PPh3)2(phen)(Hox)]ClO4
55.52
3.46
2.57
(2.59)
676b(800), 456b(5900),
436(6400), 312b(8300),
276b(19400), 260b(28500),
244b(34700), 208(71500)
516b(11700), 484(16200),
392b(6300), 360(8100),
260b(35200), 244b(44200),
212(78500)
0.26(70), 1.42(150),
21.79(90)
(55.47)
(3.51)
e
[Os(PPh3)2(phen)(nn)]ClO4
63
59.76
3.81
3.58
(3.60)
0.84(70), 21.50(150)
(59.70)
(3.77)
a Calculated values are in parentheses.
b Shoulder.
c Supporting electrolyte, TEAP; scan rate, 50 mVs21; E1 / 2 5 0.5(Epa 1 Epc), where Epa and Epc are anodic and cathodic peak potentials respectively;
DEp 5Epa 2Epc.
d Solvent, dichloromethane.
e Solvent, acetonitrile.

2732
A. Das et al. / Polyhedron 18 (1999) 2729–2736
complexes. As all the N–N ligands are symmetrical
bidentate ligands, these [Os(PPh3)2(N-N)Br2] complexes
may, in principle, exist in three geometrical isomeric forms
(4–6). Structures 4 and 5 have a C2 axis, and hence only
are very similar. Each complex shows many vibrations
below 1600 cm21, some of which (e.g. strong vibrations
near 1470, 1420, 1085, 835, 740, 690, 495 and 400 cm21
)
are also observed in the spectrum of [Os(PPh3)3Br2] while
some others (e.g. sharp vibrations near 1400, 1280 and
1240 cm21) are new and may be attributed to the
coordinated N–N ligands. The [Os(PPh3)2(N-N)Br2] com-
plexes are moderately soluble in dichloromethane, chloro-
form and much less soluble in acetonitrile, acetone and
ethanol. The solutions are brown in colour. Electronic
spectra of the complexes have been recorded in dichloro-
methane solution. Spectral data are presented in Table 2.
All three complexes show several intense absorptions in
the visible region and some very intense absorptions in the
UV region. The absorptions in the UV region are attribut-
able to transitions within the ligand orbitals, whereas the
absorptions in the visible region are probably due to
allowed metal-to-ligand charge-transfer transitions. Multi-
ple charge-transfer transitions in such mixed-ligand com-
plexes may result from lower symmetry splitting of the
metal level, the presence of different acceptor orbitals and
from the mixing of singlet and triplet configurations in the
excited state through spin-orbit coupling [28–31]. To have
a better insight into the nature of these observed electronic
transitions, qualitative EHMO calculations have been
performed [32,33] on a model of the [Os(PPh3)2(N-N)Br2]
complexes which was computer generated from
[Os(PPh3)2(bpy)Br2] by replacing the phenyl groups of
the two trans PPh3 by H and imposing a C2 symmetry.
Partial MO diagram is shown in Fig. 1. The highest
occupied molecular orbital (HOMO) and the next two
occupied orbitals (HOMO-1 and HOMO-2) are predomi-
nantly osmium in character (with minor contribution from
the bromides) and attributable to the components of
osmium t2 level. The lowest unoccupied molecular orbital
(LUMO) and the next two unoccupied orbitals (LUMO11
and LUMO12) are basically p* orbitals of bpy. Accord-
ing to this model, several electronic transitions are there-
fore possible within these orbitals which are indeed
experimentally observed. Hence the observed absorptions
in the visible region may be assigned to charge-transfer
transitions occurring from the filled metal t2 orbitals to
empty p* orbitals of the diimine ligands.
1
seven H NMR signals (three signals from PPh3 and four
from N–N ligands) may be expected for these isomers,
provided no overlap of signals occurs. As structure 6 does
not have any C2 symmetry, it is expected to show 14
resonances. 1H NMR spectrum recorded on the
[Os(PPh3)2(Me2bpy)Br2] complex showed only one sharp
methyl resonance at 1.24 ppm.
In the aromatic region four intense and broad signals have
been observed at 5.52, 6.10, 6.41 and 6.67 ppm, which
indicate fluxional character of these complexes. However,
intensity measurements agree with the number of aromatic
protons present in the complex molecule. 1H NMR spectral
data clearly indicate that the complex molecule has a C2
symmetry and thus exclude structure 6. It has not been
possible to further distinguish between structures 4 and 5
on the basis of 1H NMR spectra. However, we assume
structure 4 for all the three complexes where the bulky
PPh3 ligands are trans to each other and hence sterically
would be more stable. This structure seems likely also
from the p-bonding view point. The N–N ligands are well
known for their strong p-acidity and PPh3 is also an
efficient p-acceptor by virtue of the dp–dp interaction. In
structure 5, the PPh3 ligands are mutually cis but trans to
the pyridine nitrogens. Hence each P and N trans to it
would be competing for the same t2g orbital for p-inter-
action, hence structure 5 is unfavourable. Structure 4 also
seems probable for these [Os(PPh3)2(N-N)Br2] complexes
in view of the crystal structure of [Os(PPh3)2(phen)(pic)]1
(vide infra), which has been prepared by replacing the two
bromides of [Os(PPh3)2(phen)Br2] with a picolinate lig-
and.
Reactivity studies on the [Os(PPh3)2(N-N)Br2] com-
plexes involving dissociation of the Os–Br bonds was
restricted only to the N-N5phen complex because of its
better solubility. Reactions of [Os(PPh3)2(phen)Br2] were
studied with four selected acidic ligands, viz. quinolin-8-ol
(Hq, 7), picolinic acid (Hpic, 8), oxalic acid (H2ox, 9) and
1-nitroso-2-naphthol (Hnn, 10). The reactions proceeded
smoothly in ethanol medium (a little dichloromethane was
necessary initially to take [Os(PPh3)2(phen)Br2] into
ethanolic medium) in the presence of a base to afford the
expected complexes of type [Os(PPh3)2(phen)(L)]1 (L5q,
pic, Hox and nn). It is interesting to note here that during
the reaction with oxalic acid, only one proton is lost from
H2ox and the monoanionic oxalate ion (Hox) coordinates
Infrared spectra of the [Os(PPh3)2(N-N)Br2] complexes

A. Das et al. / Polyhedron 18 (1999) 2729–2736
2733
Fig. 1. Qualitative molecular orbital diagram of [Os(PPh3)2(bpy)Br2].
to osmium. The complex cations were isolated as per-
chlorate salts in the solid state. Microanalytical data of
these four complexes agreed well with their compositions
(Table 2). These complexes are diamagnetic which shows
that the 12 state of osmium is retained in them. Molecular
structure of [Os(PPh3)2(phen)(pic)]ClO4?3C6H6 was de-
termined by X-ray crystallography. The structure is shown
in Fig. 2 and selected bond distances and angles are
presented in Table 3. The coordination sphere around
osmium is N3O1P2 which is slightly distorted octahedral in
nature as reflected in all the bond angles with osmium as
the central atom. Picolinic acid is coordinated as a N,O-
donor with a bite angle of 768. The two PPh3 ligands are
trans as assumed in the parent [Os(PPh3)2(phen)Br2]
complex (see structure 4). The Os–N(phen), Os–N(pic),
Os–O and Os–P distances are quite normal [19–23]. The
synthesis
of
the
[Os(PPh3)2(phen)(L)]1
from
[Os(PPh3)2(phen)Br2] is therefore stereoretentive. The
other three [Os(PPh3)2(phen)(L)]1 complexes, presented
in this paper, have the same [Os(PPh3)2(phen)] fragment
and hence they are assumed to have similar structures with
the PPh3 ligands in trans positions.
Infrared spectra of the [Os(PPh3)2(phen)(L)]ClO4 com-
plexes are largely similar to the spectrum of the parent
[Os(PPh3)2(phen)Br2] complex. Some additional vibra-

2734
A. Das et al. / Polyhedron 18 (1999) 2729–2736
Fig. 2. Structure of the [Os(PPh3)2(phen)(pic)]1 complex.
tions are observed in all the [Os(PPh3)2(phen)(L)]ClO4
complexes, of which the two intense ones observed near
1100 and 625 cm21 indicate the presence of perchlorate
ion. The nCO(carboxylate) vibration is observed at 1640 cm21
in the L5pic complex and at 1650 cm21 in the L5Hox
complex. In the L5nn complex the nNO vibration is
observed at 1500 cm21. The [Os(PPh3)2(phen)(L)]ClO4
complexes are soluble in acetonitrile solution producing
brown solutions. Conductivity measurements in acetoni-
trile solution shows that these complexes behave as 1:1
absorptions in the UV region (due to transitions within the
ligand orbitals).
Cyclic voltammetry on the [Os(PPh3)2(N-N)Br2] com-
plexes show two reversible oxidations (Table 2, Fig. 3)
eletrolytes (LM 5150–160 V21 cm2
M
21). Electronic
spectra of the [Os(PPh3)2(N-N)(L)]ClO4 complexes, re-
corded in acetonitrile solution (Table 2), show several
intense absorptions (due to metal-to-ligand charge-transfer
transitions) in the visible region together with very intense
Table 3
Selected
bond
distances
and
bond
angles
for
[Os(PPh3)2(phen)(pic)]ClO4?3C6H6
˚
Bond distances (A)
Os–P1
Os–P2
Os–O1
Os–N1
2.381(3)
2.387(3)
2.138(6)
2.091(7)
Os–N2
2.102(7)
Os–N3
O1–C6
O2–C6
2.088(8)
1.273(12)
1.211(13)
Bond angles (8)
P1–Os–P2
O1–Os–N3
N1–Os–N2
P1–Os–O1
P1–Os–N1
P1–Os–N2
P1–Os–N3
P2–Os–O1
P2–Os–N1
178.40(12)
176.5(3)
174.9(3)
91.07(19)
89.19(24)
86.50(22)
91.67(23)
87.34(19)
90.54(24)
P2–Os–N2
P2–Os–N3
O1–Os–N1
O1–Os–N2
N1–Os–N3
N2–Os–N3
93.70(22)
89.92(23)
76.0(3)
101.4(3)
106.2(3)
76.6(3)
Fig. 3. Cyclic voltammograms of (a) [Os(PPh3)2(phen)Br2] in dichloro-
methane solution (0.1 M TEAP) and (b) [Os(PPh3)2(phen)(q)]1 in
acetonitrile solution (0.1 M TEAP) at a scan rate of 50 m Vs21
.

A. Das et al. / Polyhedron 18 (1999) 2729–2736
2735
within 20.02 to 0.12 V (all potentials are referenced to
SCE) and 1.31 to 1.43 V, respectively. The first oxidation
is assigned to the osmium(II)–osmium(III) oxidation and
the second one to osmium(III)–osmium(IV) oxidation.
The osmium(II)–osmium(III) oxidation potential in the
[Os(PPh3)2(Me2bpy)Br2] complex is 130 mV more nega-
tive than that in the [Os(PPh3)2(bpy)Br2] complex, which
may be attributed to the electron-donating character of the
methyl groups of the Me2bpy ligand. It is interesting to
note that the osmium(II)–osmium(III) oxidation potential
in the [Os(PPh3)2(N-N)Br2] complexes is about 150 mV
more positive than that in the corresponding [Os(N-
N)2Br2] complexes [34], which indicates that replacement
of one N–N ligand by two PPh3 has stabilized the bivalent
state of osmium.
oxidized complexes could be air-stable for a much longer
time. To investigate the stability of these oxidized species,
the [Os(PPh3)2(N-N)Br2] complexes have been
coulometrically oxidized in dichloromethane solution at
0.4 V. The oxidations have been smooth and quantitative
with n|1 [n(number of electron-transfer)5Qf /Qc, where
Qf 5observed coulomb count and Qc 5calculated coulomb
count]. Solutions of the oxidized complexes are green in
colour and display identical voltammograms as their
respective [OsII(PPh3)2(N-N)Br2] precursors (except that
the osmium(II)–osmium(III) couple now appears as a
reductive response). This indicates that the osmium(II)–
osmium(III) oxidation takes place without any gross
change in stereochemistry. Coulometric reduction of the
green solution at 20.2 V quantitatively afforded back
brown solutions of the respective [OsII(PPh3)2(N-N)Br2]
complexes, which have been identified by their characteris-
tic electronic spectrum. The [OsIII(PPh3)2(N-N)Br2]1
complexes were also generated by chemical oxidation of
the [OsII(PPh3)2(N-N)Br2] complexes by bromine. Elec-
tronic spectra of the green [OsIII(PPh3)2(N-N)Br2]1 com-
plexes in acetonitrile solution show several intense absorp-
tions in the visible region (probably ligand-to-metal
charge-transfer transitions) together with very intense
transitions in the UV region (Table 4). Chemical reduction
of the oxidized green solutions gives back brown solutions
Cyclic voltammetry on the [Os(PPh3)2(phen)(L)]1 com-
plexes show a reversible osmium(II)–osmium(III) oxida-
tion, whose potential varies significantly (0.26–0.84 V) as
L is varied. A second quasi-reversible oxidation, assumed
to be the osmium(III)–osmium(IV) oxidation, has been
observed only for the L5q and L5Hox complexes at 1.38
V and 1.42 V, respectively. A reversible/quasi-reversible
reduction
has
been
displayed
by
all
four
[Os(PPh3)2(phen)(L)]1 complexes below 21.5 V which is
assigned to reduction of the coordinated phen ligand.
These N–N ligands are known to undergo two successive
one-electron reductions [35,36], the first of which has been
experimentally observed while the second reduction could
not be observed due to solvent cut-off. In the
[Os(PPh3)2(N-N)Br2] complexes, even the first reduction
of the N–N ligand could not be observed due to the
smaller voltage window of dichloromethane.
of
the
osmium(II)
complexes.
The
[OsII(PPh3)2(phen)(L)]1 complexes have also been oxi-
dised in acetonitrile solution by bromine to afford solutions
of respective [OsIII(PPh3)2(phen)(L)]21 complexes. Elec-
tronic spectra of these solutions (Table 4) show intense
absorptions in the visible and UV region. Addition of
hydrazine to these oxidized solutions immediately brings
about reduction to the brown osmium(II) complexes. This
indicates that the osmium(II)–osmium(III) oxidation is
chemically reversible. However, stability of the
[OsIII(PPh3)2(phen)(L)]21 complexes (towards prolonged
The reversible nature of the osmium(II)–osmium(III)
oxidation in [Os(PPh3)2(N-N)Br2] complexes indicates
that the oxidized species, viz. [OsIII(PPh3)2(N-N)Br2]1,
are stable, at least on the cyclic voltammetric time-scale.
The low oxidation potentials further indicate that the
Table 4
Electronic spectral data of the [OsIII(PPh3)2(N-N)Br2]1 and [OsIII(PPh3)2(phen)(L)]21 complexes in dichloromethane solution
Compound
lmax, nm (e, M21 cm21
)
[OsIII(PPh3)2(bpy)Br2]1
232(29200), 264a(13400), 296a(7700),
409(2400), 475(1800), 608a(770)
229(99800), 295(23000), 426(4900),
433a(4000), 588a(2300), 608(4200),
782a(2700), 895a(1800), 1003a(1200)
262(72500), 325(15600), 375(5500),
405a(5000), 476(3400), 609(2800),
852a(2200)
[OsIII(PPh3)2(Me2bpy)Br2]1
[OsIII(PPh3)2(phen)Br2]1
[OsIII(PPh3)2(phen)(q)]21
[OsIII(PPh3)2(phen)(pic)]21
[OsIII(PPh3)2(phen)(Hox)]21
[OsIII(PPh3)2(phen)(nn)]21
267(29200), 386(4600), 488(4400),
549a(2200), 676a(1100)
254(30900), 275a(21200), 300a(9600),
443(5300), 600(800)
263(52400), 277a(35600), 298a(15000),
403(5100), 619(900)
217(138500), 268(202600), 401a(7900),
490(15900), 635a(870), 718(930), 939(830)
a Shoulder.

2736
A. Das et al. / Polyhedron 18 (1999) 2729–2736
exposure to air) is less than that of the [OsIII(PPh3)2(N-
N)Br2]1 complexes.
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Acknowledgements
Financial assistance received from the Council of Sci-
entific and Industrial Research, New Delhi [Grant No.
01(1408)/96/EMR-II] is gratefully acknowledged. Thanks
are also due to the Third World Academy of Sciences for
financial support for the purchase of an electrochemical
cell system. The authors thank Professor Gurunath
Mukherjee of University of Calcutta, Dr. Rupendranath
Banerjee of Jadavpur University and Dr. Surajit Chat-
topadhyay of Vidyasagar University, Midnapore, West
Bengal, for their help. FB thanks the University Grants
Commission, New Delhi, for her fellowship.
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