Chemistry of 2-(arylazo) phenolate complexes of osmium. Synthesis, structure and redox properties

Article abstract of DOI:10.1016/S0277-5387(98)00278-2

Reaction of five 2-(arylazo)phenol ligands (abbreviated in general as Hap-R, where H stands for the phenolic proton) with [Os_II(bpy)₂Br₂] has afforded complexes of type [Os^II(bpy)₂(ap-R)]⁺ , which have been isolated as the perchlorate salts. The complexes are diamagnetic (low-spin d⁶, S = 0) and in acetonitrile solution shows several MLCT transitions in the visible region. Structure of the [Os(bpy)₂(ap-Me))ClO₄ complex has been determined by X-ray crystallography. The 2-(arylazo)phenolate anion is coordinated to osmium as a bidentate N,O-donor forming a five-membered chelate ring and the OsN₅O coordination sphere is distorted octahedral. Cyclic voltammetry shows a reversible osmium(II)-osmium(III) oxidation in the range of 0.37-0.51 V vs SCE followed by an irreversible osmium(III)-osmium(IV) oxidation in the range of 1.36-1.50 V vs SCE. These oxidation potentials are sensitive to the electronic nature of the substituent R in the 2-(arylazo)phenolate ligands. Three one-electron reductions of the coordinated bpy ligands are also displayed on the negative side of SCE below - 1.0V. Chemical or electrochemical oxidation of the [Os^II(bpy)₂(ap-R)]ClO₄ complexes affords brownish-yellow [Os^III(bpy)₂(ap-R)]²⁺ species, which have been isolated as the perchlorate salts. These complexes are one-electron paramagnetic (low-spin d⁵, 5=1/2) and in acetonitrile solution show LMCT transitions in the visible region. Reduction of the brownish-yellow [Os^III(bpy)₂(ap-R)](ClO₄)₂ complexes gives back the respective brown [Os^II(bpy)₂(ap-R)]C1O₄ complexes.

Full text of DOI:10.1016/S0277-5387(98)00278-2

\
Polyhedron 07 "0888# 280Ð391
PERGAMON
Chemistry of 1!"arylazo# phenolate complexes of osmium[
Synthesis\ structure and redox properties
Falguni Basulia\ Shie!Ming Pengb\ Samaresh Bhattacharyaa\ ꢀ
aDepartment of Chemistry\ Inorganic Chemistry Section\ Jadavpur University\ Calcutta 699921\ India
bDepartment of Chemistry\ National Taiwan University\ Taipei\ Taiwan\ R[O[C[
Received 10 January 0887^ accepted 7 July 0887
Abstract
Reaction of _ve 1!"arylazo#phenol ligands "abbreviated in general as Hap!R\ where H stands for the phenolic proton# with
ðOs"bpy#1Br1Ł has a}orded complexes of type ðOsII"bpy#1"ap!R#Ł¦\ which have been isolated as the perchlorate salts[ The complexes
are diamagnetic "low!spin d5\ Sꢁ9# and in acetonitrile solution shows several MLCT transitions in the visible region[ Structure of
the ðOs"bpy#1"ap!Me##ClO3 complex has been determined by X!ray crystallography[ The 1!"arylazo#phenolate anion is coordinated
to osmium as a bidentate N\O!donor forming a _ve!membered chelate ring and the OsN4O coordination sphere is distorted
octahedral[ Cyclic voltammetry shows a reversible osmium"II#Ðosmium"III# oxidation in the range of 9[26Ð9[40 V vs SCE followed
by an irreversible osmium"III#!osmium"IV# oxidation in the range of 0[25Ð0[49 V vs SCE[ These oxidation potentials are sensitive to
the electronic nature of the substituent R in the 1!"arylazo#phenolate ligands[ Three one!electron reductions of the coordinated bpy
ligands are also displayed on the negative side of SCE below −0[9 V[ Chemical or electrochemical oxidation of the ðOsII"bpy#1"ap!
R#ŁClO3 complexes a}ords brownish!yellow ðOsIII"bpy#1"ap!R#Ł1¦ species\ which have been isolated as the perchlorate salts[ These
complexes are one!electron paramagnetic "low!spin d4\ Sꢁ0:1# and in acetonitrile solution show LMCT transitions in the visible
region[ Reduction of the brownish!yellow ðOsIII"bpy#1"ap!R#Ł"ClO3#1 complexes gives back the respective brown ðOsII"bpy#1"ap!
R#ŁClO3 complexes[ Þ 0888 Elsevier Science Ltd[ All rights reserved[
Keywords] Osmium^ 1!"arylazo#phenolates^ synthesis^ Structure^ Redox properties
0[ Introduction
exception where the ring size is speculated to be either six
or _ve "2# ð20Ł[ However\ instances are also known where
these ligands coordinate metal ions as dianionic tri!
dentate C\N\O!donors "3# a}ording organometallic com!
plexes ð21Ł[ It may be noted here that while the ruthenium
chemistry of these ligands has been studied well ð21Ð
25Ł\ the analogous osmium chemistry appears to remain
unexplored[ Herein we wish to report our studies on a
group of osmium complexes having only one coordinated
1!"arylazo#phenolate ligand[ To satisfy the remaining four
coordination sites of osmium in this Os"ap!R# moiety\ 1\1?!
bipyridine "bpy# has been used as the coligand[ The syn!
thesis and characterization of a group of ðOs"bpy#1"ap!
R#ŁClO3 complexes and their spectroscopic and electron!
transfer properties are described in this paper[
The chemistry of osmium has been receiving con!
tinuous attention ð0Ð07Ł largely because of the interesting
redox properties exhibited by its complexes[ Osmium
o}ers a wide range of oxidation states\ the stability and
interconvertibility of which are directed by the coor!
dination environment around the metal ion[ Com!
plexation of osmium by ligands of di}erent types is of
particular importance in this respect[ In the present study\
which has originated from our recent interest in the chem!
istry of osmium ð08Ð10Ł\ 1!"arylazo#phenols "0# have been
used as the principal ligand[ These ligands are abbrevi!
ated in general as Hap!R\ where H stands for the dis!
sociable phenolic proton and R is the substituent on
the 1!"arylazo#phenol ligand[ The 1!"arylazo#phenolate
anions usually bind metal ions as bidentate N\O!coor!
dinator forming stable chelate ring\ the size of which is
reported to be six "1# in almost all cases ð11Ð29Ł with one
1[ Experimental
1[0[ Materials
Osmium tetroxide was purchased from Arora
Matthey\ Calcutta\ India and was converted into
ꢀ Corresponding author[
9166!4276:88:, ! see front matter Þ 0888 Elsevier Science Ltd[ All rights reserved[
PII] S9166!4276 "87#99167!1

281
F[ Basuli et al[ : Polyhedron 07 "0888# 280Ð391
ðNH3Ł1ðOsBr5Ł by reduction with HBr ð26Ł[ ðOs"bpy#1Br1Ł
was synthesized from ðNH3Ł1ðOsBr5Ł using a literature
method ð27Ł[ 1\1?!Bipyridine was purchased from Loba
Chemie\ Mumbai\ India[ The p!substituted anilines and
p!cresol were obtained from S[D[ "Mumbai#\ India[ The
1!"arylazo#phenol ligands were prepared by coupling
diazotized p!substituted anilines with p!cresol[ All other
chemicals and solvents were reagent grade commercial
materials and were used as received[ Puri_cation of ace!
tonitrile and preparation of tetraethylammonium per!
chlorate "TEAP# for electrochemical work were
performed as reported in the literature ð28\39Ł[
9[05 mmol#[ The solution was then heated at re~ux for
5 h[ After being cooled to room temperature\ a saturated
solution of NaClO3 "9[4 cm2# was added to it and the
volume of the solution was reduced to about 04 cm2 under
reduced pressure[ The solution was then kept in the
refrigerator for 13 h[ ðOs"bpy#1"ap!H#ŁClO3=H1O pre!
cipitated as a dark brown microcrystalline solid\ which
was collected by _ltration\ washed with little ice!cold
water and dried in vacuo over P3O09[ Puri_cation of the
product was done by recrystallization from dichlo!
romethaneÐhexane solution[ The yield was 099 mg
"70)#[
1[1[ Preparation of complexes
1[1[1[ ðOsIII"bpy#1"ap!H#Ł"ClO3#1=H1O
The ðOsII"bpy#1"L#ŁClO3 and ðOsIII"bpy#1"L#Ł"ClO3#1
complexes\ reported in this work\ were prepared by fol!
lowing two general methods[ Speci_c details are given
below for two representative cases[
To an acetonitrile solution of ðOs"bpy#1"ap!
H#ŁClO3=H1O "099 mg\ 9[01 mmol# was added one drop
of bromine[ The colour of the solution immediately chan!
ged to brownish!yellow[ A saturated aqueous solution of
NaClO3 "9[4 cm2# was then added[ Upon partial evap!
oration of the solvents dark crystalline precipitate of
ðOs"bpy#1"ap!H#Ł"ClO3#1=H1O separated out\ which was
collected by _ltration\ washed with little ice!cold water
and dried in vacuo over P3O09[ Puri_cation of the product
1[1[0[ ðOsII"bpy#1"ap!H#ŁClO3=H1O
ðOs"bpy#1Br1Ł "099 mg\ 9[04 mmol# was dissolved in 2]0
EtOHÐwater "39 cm2# and to the solution was added Hap!
H
"24 mg\ 9[05 mmol# followed by NEt2 "05 mg\

F[ Basuli et al[ : Polyhedron 07 "0888# 280Ð391
282
was done by recrystallization from acetonitrileÐbenzene
solution[ The yield was 74 mg "64)#[
unit cell dimensions were determined by a least!squares _t
of 14 machine!centered re~ections "08[29³1u³15[43>#[
Data were collected on an Enraf!Nonjus CAD!3
di}ractometer using graphite monochromated Mo Ka
radiation "lꢁ9[6096 A# by uÐ1u scans within the angular
range 2[9Ð49[9>[ Three standard re~ections\ measured
every 2599 s of X!ray exposure\ showed no signi_cant
intensity variation over the course of data collection[ X!
ray data reduction and structure solution and re_nement
were done using the NRCVAX package[ The structure
was solved by the Patterson method[ Final cycles of
re_nement converged with discrepancy indices of
Rfꢁ9[927 and Rwꢁ9[922[
1[2[ Physical measurements
Microanalyses "C\ H\ N# were performed using a Per!
kin!Elmer 139C elemental analyser[ Infrared spectra were
obtained on a Perkin!Elmer 672 spectrometer with sam!
ples prepared as KBr pellets[ Electronic spectra were
recorded on a Simadzu UV 0590 spectrophotometer[
Magnetic susceptibilities were measured using a PAR
044 vibrating sample magnetometer _tted with a Walker
Scienti_c L64FBAL magnet[ 0H NMR spectra were
obtained on a Brucker AC!199 NMR spectrometer using
TMS as the internal standard[ ESR spectral studies were
done using a varian E!098C spectrometer _tted with a
quartz Dewar for measurements at 66 K "liquid nitrogen#[
Solution electrical conductivities were measured using a
Philips PR 8499 bridge with a solute concentration of
09−2 M[ Electrochemical measurements were made using
a PAR model 162 potentiostat[ A platinum disc or graph!
ite working electrode\ a platinum wire auxiliary electrode
and an aqueous saturated calomel reference electrode
"SCE# were used in a three electrode con_guration[ A
platinum wire gauze working electrode was used in the
coulometric experiments[ A RE 9963 XÐY recorder was
used to trace the voltammograms[ Electrochemical
measurements were performed under a dinitrogen atmo!
sphere[ All electrochemical data were collected at 187 K
and are uncorrected for junction potentials[
2[ Results and discussion
2[0[ ðOsII"bpy#1"ap!R#Ł¦ complexes
Displacement of the two bromide ligands from the
coordination sphere of ðOs"bpy#1Br1Ł by the 1!"ary!
lazo#phenolate ligands in 2]0 ethanolÐwater a}orded
complexes of type ðOs"bpy#1"ap!R#Ł¦ in decent yields\
which were isolated as perchlorate salts in the solid state[
Some characterization data of the complexes are given in
Table 1[ Elemental "C\ H\ N# analytical data are in good
agreement with the compositions of the complexes[ The
ðOs"bpy#1"ap!R#Ł¦ complexes are diamagnetic\ which
corresponds to the ¦1 state of osmium "low!spin d5\
Sꢁ9# in these complexes[ 0H NMR spectra of the
ðOs"bpy#1"ap!R#Ł¦ complexes were recorded in CDCl2
solution[ The aromatic region "5[9Ð7[5 ppm# of these
spectra is rather complex in nature due to overlap of
signals and hence assignment of the signals in this region
to speci_c protons has not been possible[ However\ inten!
sity measurement of these signals corresponds to the total
number of aromatic protons present in the respective
complexes[ In all _ve ðOs"bpy#1"ap!R#Ł¦ complexes\ a
distinct methyl resonance is observed near 1[2 ppm which
is assigned to the methyl group in the p!cresol fragment
of ap!R ligands[ Additional methyl signals are observed
in ðOs"bpy#1"ap!Me#Ł¦ and ðOs"bpy#1"ap!OMe#Ł¦\
respectively\ at 1[93 and 2[56 ppm\ which are due to the
methyl and methoxy!methyl group in the arylazo frag!
ment of the ap!Me and ap!OMe ligands[
1[3[ Crystallography
Single crystals of ðOs"bpy#1"ap!Me#ŁClO3=H1O were
grown by slow di}usion of hexane into a dichlo!
romethane solution of the complex[ Selected crystal data
and data collection parameters are given in Table 0[ The
Table 0
Crystallographic data for ðOs"bpy#1"ap!Me#ŁClO3=H1O
Formula
fw
C23H20N5O5ClOs
734[29
Space group
monoclinic\ P10:c
02[4780"12#
06[679"2#
03[8939"06#
003[368"02#
2166[2"8#
3
Ä
a "A#
Ä
b "A#
The
molecular
structure
of
ðOs"bpy#1"ap!
Ä
c "A#
Me#ŁClO3=H1O was determined by X!ray crystallography[
A view of the complex cation is shown in Fig[ 0 and
selected bond distances and angles are listed in Table
2[ The coordination sphere around osmium is distorted
octahedral\ which is re~ected in the three trans!angles
and twelve cis!angles[ The 1!"arylazo#phenolate ligand is
coordinated to osmium as a bidentate N\O!donor for!
ming a _ve!membered chelate ring with a bite angle of
68[7>[ To the best of our knowledge\ this represents the
_rst example of a structurally characterized 1!"aryl!
b ">#
V "A #
2
Ä
Z
Crystal size "mm#
9[19×9[14×9[39
187
39[136
T "K#
m "cm−0
Rf
#
9[927
Rw
9[922
GOF
0[12
RfꢁS==Fo=−=Fc==:S=Fo=[ RwꢁðSw"=Fo=−=Fc=#1:Sw"Fo#1Ł0:1
[

283
F[ Basuli et al[ : Polyhedron 07 "0888# 280Ð391
azo#phenolate complex forming a _ve!membered chelate
ring[ To investigate the origin of the formation of such
_ve!membered chelate ring\ instead of the rather usual
six!membered ring ð11Ð29Ł\ a computer model0 of
ðOs"bpy#1"ap!R#Ł¦ was constructed forcing a six!mem!
bered chelate ring formation by the 1!"arylazo#!phenolate
ligand and assuming the ap!R ligand to be planar in this
coordination mode ð13Ł[ The model "Figure 1# clearly
shows that in this coordination mode\ the phenyl ring of
the arylazo fragment of the ap!R ligand comes in contact
with one pyridine ring of one bpy ligand causing insta!
bility to the complex[ The observed coordination mode
of the 1!"arylazo# phenolate ligand therefore appears to
be directed by its steric interaction with one bpy ligand[
The OsÐN and OsÐO distances are quite normal and so
is the phenolic CÐO distance ð19Ł[ However\ the NÐN
II
Ä
distance is short\ only 0[130"8# A[ In Os !azo complexes\
where the azo function is part of the chelate ring\ the azo
NÐN distance is usually longer than ideal N1N double
bond due to back donation from the low!spin d5 metal
center to the pꢀ!orbital of the azo ligand ð32Ł[ The short
NÐN distance observed in this ðOs"bpy#1"ap!Me#Ł¦ com!
plex is attributable to the non!participation of this azo
group in p!bonding with the metal t1 orbitals[ As the
properties of all _ve ðOs"bpy#1"ap!R#Ł¦ complexes are
similar "vide infra#\ the other four ðOs"bpy#1"ap!R#Ł¦
complexes are assumed to have a similar structure as
ðOs"bpy#1"ap!Me#Ł¦[
Infrared spectra of the ðOs"bpy#1"ap!R#ŁClO3 com!
plexes show many vibrations of di}erent intensities from
0599 cm−0 downwards[ Assignment of all bands to spec!
i_c vibrations has not been attempted[ However\ com!
parison of these spectra with the spectrum of
ðOs"bpy#1Br1Ł shows the presence of some common
vibrations "e[g[ vibrations near 0599\ 0379\ 0359\ 0339\
0319\ 0919\ 654\ 629 and 559 cm−0#\ which are probably
due to the common Os"bpy#1 moiety[ Some additional
vibrations are observed in the spectra of the
ðOs"bpy#1"ap!R#ŁClO3 complexes "e[g[ vibrations near
0389\ 0249\ 0099\ 859\ 724 and 519 cm−0#[ Of these
additional vibrations\ the two intense ones observed near
0099 and 519 cm−0 in all these complexes\ are assigned to
the perchlorate ion[ The other new vibrations are obvi!
ously due to the coordinated 1!"arylazo# phenolate
ligand[
The ðOs"bpy#1"ap!R#ŁClO3 complexes are soluble in
common polar organic solvents like ethanol\ acetone\
dichloromethane\ acetonitrile etc[\ producing brown
solutions[ Conductance measurement in acetonitrile solu!
tion shows that these complexes behave as 0]0 electrolytes
"Table 1#\ as expected[ Electronic spectra of the
ðOs"bpy#1"ap!R#ŁClO3 complexes have been recorded in
acetonitrile solution[ Spectral data are presented in Table
0 Computer modelling was done by using part of a software package
described later "see end of this section and Ref[ ð30\31Ł#[

F[ Basuli et al[ : Polyhedron 07 "0888# 280Ð391
284
Fig[ 0[ Structure of ðOs"bpy#1"ap!Me#Ł¦ cation[
Table 2
Selected bond distances and bond angles for ðOs"bpy#1"ap!Me#ŁClO3=H1O
Ä
Bond distances "A#
OsÐO0
OsÐN0
OsÐN2
OsÐN3
OsÐN4
OsÐN5
1[967"4#
1[927"5#
1[961"5#
1[956"5#
1[921"5#
1[952"5#
N0ÐN1
C6ÐN0
C7ÐN1
C0ÐO0
0[130"8#
0[330"09#
0[315"00#
0[212"8#
Bond angles ">#
N2ÐOsÐN5
O0ÐOsÐN4
N0ÐOsÐN3
O0ÐOsÐN0
O0ÐOsÐN2
N0ÐOsÐN2
N0ÐOsÐN4
N2ÐOsÐN3
N2ÐOsÐN4
061[8"2#
060[58"13#
056[43"14#
68[70"11#
78[98"10#
82[87"12#
094[35"14#
66[3"2#
O0ÐOsÐN5
O0ÐOsÐN3
N4ÐOsÐN5
N0ÐOsÐN5
N3ÐOsÐN4
N3ÐOsÐN5
84[3"2#
89[82"10#
67[1"2#
81[17"12#
73[62"13#
85[8"2#
85[7"2#
1 and a representative spectrum is displayed in Fig[ 2[
Each complex systematically shows _ve intense absorp!
tions in the visible region and three absorptions of very
high intensity in the ultraviolet region[ The absorptions
in the ultraviolet region are assigned to transitions within
the ligand orbitals[ The intense absorptions in the visible
region are probably due to allowed metal!to!ligand
charge!transfer transitions[ Multiple charge!transfer
transitions in such mixed!ligand complexes may result
from lower symmetry splitting of the metal level\ the
presence of di}erent acceptor orbitals and from the mix!
ing of singlet and triplet con_gurations in the excited
state through spin!orbit coupling ð33Ð36Ł[ To have a bet!
ter insight into the nature of these observed electronic
transitions\ qualitative EHMO calculations have been
performed ð30\31Ł on a model of the ðOs"bpy#1"ap!R#Ł¦

285
F[ Basuli et al[ : Polyhedron 07 "0888# 280Ð391
Fig[ 1[ Computer model of ðOs"bpy#1"ap!H#Ł¦ forcing a six!membered chelate ring formation by the ap!H ligand[ Some ring carbon atoms and
hydrogen atoms of one bpy ligand have been omitted for clarity[
complexes which was computer generated from
ðOs"bpy#1"ap!R#Ł¦ by replacing the aryl group of the
arylazo fragment of ap!R ligand by H and imposing a C0
symmetry[ Partial MO diagram is shown in Fig[ 3[
The HOMO\ HOMO!0 and HOMO!1 of this model
are predominantly osmium t1g in character[ The LUMO
and LUMO¦0 are basically pꢀ!orbitals of bpy while the
LUMO¦1 has almost equal contribution from both bpy
and the 1!"arylazo# phenolate ligand[ The lowest energy
HOMO:LUMO transition is therefore a OS"t1g#:
bpy"pꢀ# transition[ Speci_c assignment of all observed
absorptions based on this MO diagram has not been
attempted[ However from the MO diagram it is clear that
from the three _lled orbitals "viz[ HOMO\ HOMO!0 and
HOMO!1# multiple charge transfer transitions may take
place to the vacant accepting orbitals "viz[ LUMO\
LUMO¦0 and LUMO¦1#[
2[1[ Cyclic voltammetric studies
Electron!transfer properties of the ðOs"bpy#1"ap!R#Ł¦
complexes have been studied in acetonitrile solution
"9[0 M TEAP# by cyclic voltammetry[ All the complexes
show two oxidative responses on the positive side of
SCE and three reductive responses on the negative side[
Voltammetric data are presented in Table 3 and a selected
voltammogram is displayed in Fig[ 4[
The _rst oxidative response exhibited by each complex
in the range of 9[26Ð9[40 V "all potentials are referenced
to SCE# is assigned to the osmium"II#Ðosmium"III# oxi!
dation eq[ "0#[ This oxidation is
ðOsII"bpy#1"ap!R#Ł¦_ðOsIII"bpy#1"ap!R#Ł1¦¦e−
"0#
reversible\ characterized by a peak!to!peak separation

F[ Basuli et al[ : Polyhedron 07 "0888# 280Ð391
286
Fig[ 2[ Electronic spectra of ðOsII"bpy#1"ap!H#ŁClO3 "***# and ðOsIII"bpy#1"ap!H#Ł"ClO3#1 "! ! !# in acetonitrile solution[
"DEp# of 59 mV which remains unchanged upon changing
the scan rate[ The anodic peak current "ipa# is almost
equal to the cathodic peak current "ipc#\ as expected for a
reversible couple[ The one!electron nature of this oxi!
dation has been con_rmed by constant potential coul!
ometric experiments "vide infra#[ The osmium"II#Ð
osmium"III# oxidation potential in these ðOs"bpy#1"ap!
R#Ł¦ complexes is observed to be sensitive to the nature
of the substituent R in the ap!R ligand\ the potential
increases with increasing electron!withdrawing character
of R[ The plot of oxidation potentials vs Hammett con!
stant "s# of the substituent R "the s values ð37Ł used are]
OMeꢁ−9[16\ Meꢁ−9[06\ Hꢁ9[99\ Clꢁ9[12\
NO1ꢁ9[67# is linear "Fig[ 5# with a r value "rꢁreaction
constant of this redox couple ð38Ł# of 9[02 V[ This shows
that a single substituent\ which is _ve bonds away from
the electroactive metal center\ can in~uence the redox
potential in a predictable manner[ Comparison of the
osmium"II#Ðosmium"III# oxidation potential in the
ðOs"bpy#1"ap!R#Ł¦ complexes with that in ðOs"bpy#2Ł1¦
"9[73 V#1 shows that in the ðOs"bpy#1ap!R#Ł¦ complexes\
osmium"II#Ðosmium"III# oxidation is taking place at
much lower potentials[ This observed lowering of oxi!
dation potential upon replacing one bpy by one 1!"ary!
lazo# phenolate ligand re~ects the ability of these
phenolate ligands to stabilize the trivalent state of
osmium[ Similar lowering of redox potentials upon
replacing bpy by phenolate ligands in both ruthenium
and osmium complexes is documented in literature ð19\
25\ 49\ 40Ł[ This further points to the fact that inspite of
coordination by the soft azo!nitrogen\ coordination by
phenolate oxygen has been very e}ective in lowering the
osmium"II#Ðosmium"III# oxidation potential[
A second oxidative response\ quasi!reversible in
nature\ is shown by all the ðOs"bpy#1"ap!R#Ł¦ complexes
in the range of 0[25Ð0[49 V and is assigned to the osmium!
"III#Ðosmium"IV# oxidation ðeq[ "1#Ł[
ðOsIII"bpy#1"ap!R#Ł1¦:ðOsIV"bpy#1"ap!R#Ł2¦¦e−
"1#
The one!electron nature of this oxidation is established
by comparing its current height "ipa# with that of the
osmium"II#Ðosmium"III# couple[ These oxidation poten!
tials "Epa# also correlate linearly with s of substituent R
"Fig[ 5# and the observed r value "9[02 V# is same as
observed in the case of osmium"II#Ðosmium"III#
oxidation\ indicating a similar in~uence of the substituent
R on this osmium"III#Ðosmium"IV# oxidation[
The three reductive responses\ exhibited by the
ðOs"bpy#1"ap!R#Ł¦ complexes on the negative side of
SCE\ are assigned to reductions of the coordinated bpy
ligands as shown in eqs[ "2Ð4#[ It is
ðOsII"bpy#1"ap!R#Ł¦¦e−_ðOsII"bpy#"bpy#"ap!R#Ł "2#
ðOsII"bpy#"bpy#"ap!R#Ł¦e−_ðOsII"bpy#1"ap!R#Ł− "3#
ðOsII"bpy#1"ap!R#Ł−¦e−
_ðOsII"bpy#"bpy#"ap!R#Ł1− "4#
well documented in the literature that each bpy ligand can
successively accept two electrons in its lowest unoccupied
molecular orbital ð41\42Ł[ Hence\ in these ðOs"bpy#1"ap!
R#Ł¦ complexes four successive one!electron reductions
1 This oxidation potential has been determined by us[

287
F[ Basuli et al[ : Polyhedron 07 "0888# 280Ð391
Fig[ 3[ Qualitative molecular orbital diagram of ðOs"bpy#1"ap!R#Ł¦[
may be expected\ of which three have been experimentally
observed[ The fourth reduction could not be observed
due to solvent cut!o}[
oxidized complexes might also be stable on a longer time
scale[ To investigate the stability of the ðOsIII"bpy#1"ap!
R#Ł1¦ complexes\ they were electrochemically generated
by coulometric oxidation of the ðOsII"bpy#1"ap!R#Ł¦ spec!
ies at 9[6 V[ The oxidations were smooth and quan!
titative\ resulting in a colour change of brown to
brownish!yellow[ The brownish!yellow solutions of the
ðOsIII"bpy#1"ap!R#Ł1¦ complexes show identical cyclic vol!
tammograms as their respective precursors\ except that
the osmium"II#Ðosmium"III# couple now appears as a
reductive response[ This indicates that no gross change
2[2[ ðOsIII"bpy#1"ap!R#Ł1¦ complexes
The reversible nature of osmium"II#Ðosmium"III# oxi!
dation in the ðOs"bpy#1"ap!R#Ł¦ complexes shows that
the ðOsIII"bpy#1"ap!R#Ł1¦ species are stable\ at least on
the cyclic voltammetric time scale[ The oxidation poten!
tials are relatively low\ which further suggest that the

F[ Basuli et al[ : Polyhedron 07 "0888# 280Ð391
288
Table 3
Cyclic voltammetric dataa of the ðOs"bpy#1"ap!R#ŁClO3 complexes
Compound
E0:1b V vs SCE "DEp c\ mV#
OsII:III
OsIII:IV
bpy reductions
ðOs"bpy#1"ap!OMe#ŁClO3
ðOs"bpy#1"ap!Me#ŁClO3
ðOs"bpy#1"ap!H#ŁClO3
ðOs"bpy#1"ap!Cl#ŁClO3
ðOs"bpy#1"ap!NO1#ŁClO3
9[26"59#
9[27"59#
9[30"59#
9[32"59#
9[40"59#
0[25"89#
0[26"89#
0[31"099#
0[33"89#
0[49"099#
−0[06"019#\ −0[60"89#\ −1[91"019#
−0[08"099#\ −0[61"89#\ −1[93"019#
−0[07"79#\ −0[58"099#\ −1[94"099#
−0[04"69#\ −0[69"79#\ −1[92"099#
−0[96"59#\ −0[57"79#\ −1[97"099#
a Solvent\ acetonitrile^ supporting electrolyte\ TEAP^ scan rate\ 49 mV s−0
[
b E0:1ꢁ9[4"Epa¦Epc#\ where Epa and Epc are anodic and cathodic peak potentials\ respectively[
c DEpꢁEpa−Epc[
Fig[ 4[ Cyclic voltammogram of ðOs"bpy#1"ap!Cl#ŁClO3 in acetonitrile solution "9[0 M TEAP# at a scan rate of 49 mV s−0[ A platinum working electrode
was used for scanning the positive side of SCE and a graphite working electrode was used for scanning the negative side[
in the coordination environment around osmium took
place during the oxidation[ Coulometric reduction of the
brownish!yellow solutions at 9[0 V gave back brown solu!
tions of the respective ðOsII"bpy#1"ap!R#Ł¦ complexes\
which were identi_ed by their characteristic electronic
spectra[ Chemical oxidation of the ðOsII"bpy#1"ap!R#Ł¦
complexes by bromine in acetonitrile solution also
a}orded the ðOsIII"bpy#1"ap!R#Ł1¦ species\ which were
isolated as the perchlorate salt in the solid state[ Charac!
terization data of the ðOsIII"bpy#1"ap!R#Ł"ClO3#1 com!
plexes are given in Table 4[ Composition of these
complexes have been con_rmed by their microanalytical
data[ Except small shifts in band positions\ the IR spectra
of these oxidized complexes are almost identical to their
respective ðOsII"bpy#1"ap!R#ŁClO3 precursors[ The ðOsIII
"bpy#1"ap!R#Ł"ClO3#1 complexes are one!electron para!
magnetic\ which is in accordance with the ¦2 oxidation
state of osmium "iow!spin d4\ Sꢁ0:1# in these complexes[
However\ ESR studies show that these ðOsIII"bpy#1"ap!
R#Ł"ClO3#1 complexes are ESR!silent[ The ESR!inactivity
in low!spin d4 complexes is known to result from exten!
sive mixing of the Kramers doublets by strong spin!orbit
coupling which gives rise to short electronic relaxation
time ð19\ 43Ł[ In acetonitrile solution these ðOsIII
"bpy#1"ap!R#Ł"ClO3#1 complexes behave as 0]1 elec!
trolytes\ as expected[ Electronic spectra of the ðOsIII
"bpy#1"ap!R#Ł"ClO3#1 complexes have been recorded in
acetonitrile solution[ Each complex shows several
absorptions in the UV\ visible and lower energy regions
"Table 4 and Fig[ 2#[ The intense absorptions in the
UV region are assigned to transitions within the ligand
orbitals and those in the visible region to ligand!to!metal
charge!transfer transitions[ The two relatively less intense
absorptions in the lower energy region "792Ð730 and 805Ð
843 nm# could be due to crystal _eld transitions within
the three split t1 levels ð44Ł[ Chemical reduction of the

399
F[ Basuli et al[ : Polyhedron 07 "0888# 280Ð391
Fig[ 5[ Least!squares plot of E0:1 values of "a# OsII:OsIII couple vs s and "b# OsIII:OsIV couple vs s[
brownish!yellow ðOsIII"bpy#1"ap!R#Ł"ClO3#1 complexes in
acetonitrile solution by hydrazine quantitatively a}ords
the respective brown ðOsII"bpy#1"ap!R#Ł¦ complexes[ This
shows that the osmium"II#ÐOsmium"III# oxidation eq[
"0# is chemically reversible as well[
osmium[ The observed shift of about 399 mV in
osmium"II#Ðosmium"III# oxidation potential on going
from ðOs"bpy#2Ł1¦ to ðOs"bpy#1"ap!R#Ł¦ indicates that in
the osmium complexes having more of these phenolate
ligands\ this oxidation will be much easier[ For example\ in
ðOsII"bpy#"ap!R#1Ł and ðOsII"ap!R#2Ł! complexes\ the same
osmium"II#Ðosmium"III# oxidation may be expected to
appear near 9[9 and −9[3 V\ respectively[ Therefore the
air!stable oxidation state of osmium in these two com!
plexes will probably be ¦2[ Studies on these two and other
osmiumÐphenolate complexes are currently in progress[
3[ Conclusion
The present study on the ðOs"bpy#1"ap!R#ŁClO3 com!
plexes reveals that coordination by phenolate oxygen is
very e}ective in stabilizing the higher oxidation states of

F[ Basuli et al[ : Polyhedron 07 "0888# 280Ð391
390
Acknowledgements
Financial assistance received from the Council of Scien!
ti_c and Industrial Research\ New Delhi ðGrant No[
90"0397#:085:EMR!IIŁ is gratefully acknowledged[
Thanks are also due to the Third World Academy of
Sciences for _nancial support for the purchase of an elec!
trochemical cell system[ The authors thank Dr Surajit
Chattopadhyay of Vidyasagar University\ Midnapore\
West Bengal and Dr Rupendranath Banerjee of Jadavpur
University\ Calcutta\ for their help[ F[ B[ thanks the Uni!
versity Grants Commission\ New Delhi\ for her fellow!
ship[
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