\
PERGAMON Polyhedron 07 "0888# 280Ð391
9166!4276:88:, ! see front matter Þ 0888 Elsevier Science Ltd[ All rights reserved[ PII] S 9 1 6 6 ! 4 2 7 6 " 8 7 # 9 9 1 6 7 ! 1
Chemistry of 1!"arylazo# phenolate complexes of osmium[
Synthesis\ structure and redox properties
Falguni Basuli
a\ Shie!Ming Peng
b\ Samaresh Bhattacharya
a\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 ðOs
II
"bpy#1"ap!R#Ł
¦
\ which have been isolated as the perchlorate salts[ The complexes
are diamagnetic "low!spin d5\ S 9# and in acetonitrile solution shows several MLCT transitions in the visible region[ Structure of
the ðOs"bpy#1"ap!Me##ClO3complex 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#ŁClO3complexes a}ords brownish!yellow ðOs
III"bpy#
1"ap!R#Ł
1¦species\ which have been isolated as the perchlorate salts[ These
complexes are one!electron paramagnetic "low!spin d4
\ S 0:1# 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#ŁClO3complexes[ Þ 0888 Elsevier Science Ltd[ All rights reserved[
Keywords]Osmium^ 1!"arylazo#phenolates^ synthesis^ Structure^ Redox properties
0[ Introduction
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
Corresponding author[
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#ŁClO3complexes and their spectroscopic and electron!
transfer properties are described in this paper[ 1[ Experimental
1[0[ Materials
Osmium tetroxide was purchased from Arora
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[
1[1[ Preparation of complexes The ðOsII
"bpy#1"L#ŁClO3 and ðOs III
"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[
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\
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 cm
2
# 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[1[ ðOsIII
"bpy#1"ap!H#Ł"ClO3#1=H1O
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 cm
2
# 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
was done by recrystallization from acetonitrileÐbenzene solution[ The yield was 74 mg "64)#[
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[ 0
H 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[
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 C23H20N5O5ClOs
fw 734[29
Space group monoclinic\ P10:c
a"A # 02[4780"12# b"A # 06[679"2# c"A # 03[8939"06# b "># 003[368"02# V"A2# 2166[2"8# Z 3 Crystal size "mm# 9[19×9[14×9[39 T"K# 187 m "cm−0# 39[136 Rf 9[927 Rw 9[922 GOF 0[12 Rf S==Fo=−=Fc==:S=Fo=[ Rw ðSw"=Fo=−=Fc=#1:Sw"Fo#1Ł0:1[
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 "l 9[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 Rf 9[927 and Rw 9[922[
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
\ S 9# in these complexes[ 0
H 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[
The molecular structure of ðOs"bpy#1"ap!
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!
Table 1 Characterization data of the ðOs"bpy# 1 "ap!R#ŁClO 3 complexes Compound Analytical data a LM b"V −0 cm 1M −0 # Electronic spectral data bl max "nm# "o \M −0 cm −0 # ðOs"bpy# 1 "ap!OMe#ŁClO 3 =H 1 O 36[41 "36[30# 2[50 "2[59# 8[66 "8[65# 034 481 c"2399#\ 491 c"5099#\ 332 "7699#\ 278 c"5599#\ 245 c"5799#\ 183 "25399#\ 131 c"18699#\ 103 "31399# ðOs"bpy# 1 "ap!Me#ClO 3 =H 1 O 37[31 "37[20# 2[60 "2[56# 8[88 "8[84# 049 484 c"5199#\ 385 c"09399#\ 314 "05099#\ 278 c"03099#\ 245 c"09699#\ 183 "23499#\ 131 c"16299#\ 103 "34499# ðOs"bpy# 1 "ap!H#ClO 3 =H 1 O 36[50 "36[57# 2[33 "2[38# 09[04 "09[00# 034 507 c"3099#\ 376 "09299#\ 317 "00099#\ 287 c "09999#\ 245 c "7099#\ 183 "16099#\ 131 c"08499#\ 109 "21899# ðOs"bpy# 1 "ap!Cl#ŁClO 3 =H 1 O 34[77 "34[67# 2[11 "2[13# 8[58 "8[60# 044 501 c"3499#\ 499 c "00999#\ 317 "02299#\ 281 c"00499#\ 245 c "8999#\ 183 "17099#\ 131 c"11299#\ 103 "18799# "Os"bpy# 1 "ap!NO 1 #ŁClO 3 =H 1 O 34[08 "34[12# 2[10 "2[08# 00[11 "00[08# 039 537 c"4399#\ 499 c"04199#\ 317 "07399#\ 287 c"06299#\ 249 c"04799#\ 183 "31699#\ 127 c"26099#\ 109 "48999# aCalculated values are in parentheses[ bIn acetonitrile solution[ cShoulder[
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 distance is short\ only 0[130"8# A [ In OsII
!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 p !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−0in 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#ŁClO3complexes have been recorded in
acetonitrile solution[ Spectral data are presented in Table
0Computer modelling was done by using part of a software package
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
Bond distances "A #
OsÐO0 1[967"4# N0ÐN1 0[130"8# OsÐN0 1[927"5# C6ÐN0 0[330"09# OsÐN2 1[961"5# C7ÐN1 0[315"00# OsÐN3 1[956"5# C0ÐO0 0[212"8# OsÐN4 1[921"5# OsÐN5 1[952"5# Bond angles "># N2ÐOsÐN5 061[8"2# O0ÐOsÐN4 060[58"13# N0ÐOsÐN3 056[43"14# O0ÐOsÐN0 68[70"11# O0ÐOsÐN5 84[3"2# O0ÐOsÐN2 78[98"10# O0ÐOsÐN3 89[82"10# N0ÐOsÐN2 82[87"12# N4ÐOsÐN5 67[1"2# N0ÐOsÐN4 094[35"14# N0ÐOsÐN5 81[17"12# N2ÐOsÐN3 66[3"2# N3ÐOsÐN4 73[62"13# N2ÐOsÐN4 85[7"2# N3ÐOsÐN5 85[8"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¦
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 t1gin character[ The LUMO
and LUMO¦0 are basically p !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"p # 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#Ł 1¦ ¦e− "0# reversible\ characterized by a peak!to!peak separation
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]
OMe −9[16\ Me −9[06\ H 9[99\ Cl 9[12\
NO1 9[67# is linear "Fig[ 5# with a r value "r reaction
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#1shows 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
1This oxidation potential has been determined by us[
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#Ł 1¦
:ðOsIV"bpy#1"ap!R#Ł 2¦
¦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!
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}[
2[2[ ðOsIII
"bpy#1"ap!R#Ł 1¦
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
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
Table 3
Cyclic voltammetric dataaof the ðOs"bpy#
1"ap!R#ŁClO3complexes
Compound E0:1bV vs SCE "DEpc\ mV#
OsII:III OsIII:IV bpy reductions
ðOs"bpy#1"ap!OMe#ŁClO3 9[26"59# 0[25"89# −0[06"019#\ −0[60"89#\ −1[91"019#
ðOs"bpy#1"ap!Me#ŁClO3 9[27"59# 0[26"89# −0[08"099#\ −0[61"89#\ −1[93"019#
ðOs"bpy#1"ap!H#ŁClO3 9[30"59# 0[31"099# −0[07"79#\ −0[58"099#\ −1[94"099#
ðOs"bpy#1"ap!Cl#ŁClO3 9[32"59# 0[33"89# −0[04"69#\ −0[69"79#\ −1[92"099#
ðOs"bpy#1"ap!NO1#ŁClO3 9[40"59# 0[49"099# −0[96"59#\ −0[57"79#\ −1[97"099#
aSolvent\ acetonitrile^ supporting electrolyte\ TEAP^ scan rate\ 49 mV s−0[
bE
0:1 9[4"Epa¦Epc#\ where Epaand Epcare anodic and cathodic peak potentials\ respectively[
cDE
p Epa−Epc[
Fig[ 4[ Cyclic voltammogram of ðOs"bpy#1"ap!Cl#ŁClO3in 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#Ł 1¦
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#ŁClO3precursors[ 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\ S 0:1# in these complexes[
However\ ESR studies show that these ðOsIII"bpy# 1"ap!
R#Ł"ClO3#1complexes are ESR!silent[ The ESR!inactivity
in low!spin d4complexes 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
Fig[ 5[ Least!squares plot of E0:1values of "a# OsII:OsIIIcouple vs s and "b# OsIII:OsIVcouple vs s[
brownish!yellow ðOsIII
"bpy#1"ap!R#Ł"ClO3#1complexes 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[
3[ Conclusion
The present study on the ðOs"bpy#1"ap!R#ŁClO3com!
plexes reveals that coordination by phenolate oxygen is very e}ective in stabilizing the higher oxidation states of
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[
Table 4 Characterization data of the ðOs III "bpy# 1 "ap!R#Ł"ClO 3 #1 complexes Compound Analytical data a me} b\ mB Electronic spectral data cl max "nm# "o \M −0 cm −0 # ðOs"bpy# 1 "ap!OMe#Ł"ClO 3 #1 =H 1 O 31[41 "31[49# 2[19 "2[12# 7[68 "7[64# 0[81 843 d"799#\ 792 d"0599#\ 558 d"1999#\ 491 d"7399#\ 287 "03199#\ 185 "20199#\ 137 "5599#\ 197 "15999# ðOs"bpy# 1 "ap!Me#Ł"ClO 3 #1 =H 1 O 32[14 "32[11# 2[14 "2[17# 7[76 "7[78# 0[82 811 d"0199#\ 705 "0699#\ 579 "0599#\ 496 d"5899#\ 283 "05799#\ 183 "21299#\ 135 "6099#\ 198 "16699# ðOs"bpy# 1 "ap!H#Ł"ClO 3 #1 =H 1 9 31[52 "31[47# 2[03 "2[01# 8[97 "8[92# 0[74 827 d"899#\ 794 "1999#\ 579 d"0499#\ 491 d"6799#\ 399 "04399#\ 185 "22199#\ 137 "01599#\ 105 "37999# ðOs"bpy# 1 "ap!Cl#Ł"ClO 3 #1 =H 1 O 30[98 "30[94# 1[82 "1[89# 7[64 "7[60# 0[77 816 d"0099#\ 703 "0599#\ 491 d"5599#\ 390 "03799#\ 184 "23499#\ 136 "07899#\ 105 "42699# ðOs"bpy# 1 "ap!NO 1 #Ł"ClO 3 #1 =H 1 O 39[55 "39[50# 1[89 "1[76# 09[97 "09[94# 0[89 805 d"0499#\ 730 "0699#\ 401 d"00199#\ 398 "05999#\ 177 "40899#\ 137 d"30799#\ 107 "67699# aCalculated values are in parentheses[ bIn the solid state at 187 K^ mB ½ 8[16391×09 −13 JT −0 [ cIn dichloromethane solution[ dShoulder[ 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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