Chemistry of 2-(arylazo)phenolate complexes of ruthenium. Synthesis, structure and reactivities

Article abstract of DOI:10.1016/S0277-5387(98)00308-8

A group of six ruthenium(III) complexes of type [Ru(acac)(L)₂], where acac = acetylacetonate anion and L=2-(arylazo)-4-methylphenolate anion or 1-(phenylazo)-2-naphtholate anion, have been synthesized and characterized. Structural characterization of a representative complex, where L=1-(phenylazo)-2-naphtholate anion, shows that the azophenolate ligands are coordinated as N,O-donor ligands forming six-membered chelate rings. The complexes are paramagnetic (low-spin d⁵, S=1/2) and show rhombic ESR spectra in 1:1 dichloromethane-toluene solution at 77 K. In carbon tetrachloride solution, these complexes show intense LMCT transitions in the visible region together with weak ligand-field transitions in the near-IR region. All the complexes display two cyclic voltammetric responses, a ruthenium(III)-ruthenium(IV) oxidation in the range of 0.83 to 1.03V vs SCE and a ruthenium(III)-ruthenium(II) reduction in the range of -0.24 to -0.52 V vs SCE. Formal potentials of both the couples correlate linearly with the Hammett constant of the para substituent in the arylazo fragment of the 2-(arylazo)-4-methylphenolate ligand. The ruthenimn(IV) and ruthenium(II) congeners of the [Ru^III(acac)(L)₂J complexes have been generated by chemical or electrochemical methods and they have been characterized by electronic spectroscopy and cyclic voltammetry.

Full text of DOI:10.1016/S0277-5387(98)00308-8

\
Polyhedron 07 "0888# 520Ð539
PERGAMON
Chemistry of 1!"arylazo#phenolate complexes of ruthenium[
Synthesis\ structure and reactivities
Kanchana Suia\ Shie!Ming Pengb\ Samaresh Bhattacharyaa\ꢀ
a Department of Chemistry\ Inorganic Chemistry Section\ Jadavpur University\ Calcutta 699 921\ India
b Department of Chemistry\ National Taiwan University\ Taipei\ Taiwan\ R[O[C[
Received 05 June 0887^ accepted 5 August 0887
Abstract
A group of six ruthenium"III# complexes of type ðRu"acac#"L#1Ł\ where acacꢁacetylacetonate anion and Lꢁ1!"arylazo#!3!
methylphenolate anion or 0!"phenylazo#!1!naphtholate anion\ have been synthesized and characterized[ Structural characterization
of a representative complex\ where Lꢁ0!"phenylazo#!1!naphtholate anion\ shows that the azophenolate ligands are coordinated as
N\O!donor ligands forming six!membered chelate rings[ The complexes are paramagnetic "low!spin d4\ Sꢁ0:1# and show rhombic
ESR spectra in 0]0 dichloromethaneÐtoluene solution at 66 K[ In carbon tetrachloride solution\ these complexes show intense LMCT
transitions in the visible region together with weak ligand!_eld transitions in the near!IR region[ All the complexes display two cyclic
voltammetric responses\ a ruthenium"III#Ðruthenium"IV# oxidation in the range of 9[72 to 0[92 V vs SCE and a ruthenium"III#Ð
ruthenium"II# reduction in the range of −9[13 to −9[41 V vs SCE[ Formal potentials of both the couples correlate linearly with the
Hammett constant of the para substituent in the arylazo fragment of the 1!"arylazo#!3!methylphenolate ligand[ The ruthenimn"IV#
and ruthenium"II# congeners of the ðRuIII"acac#"L#1Ł complexes have been generated by chemical or electrochemical methods and
they have been characterized by electronic spectroscopy and cyclic voltammetry[ Þ 0888 Elsevier Science Ltd[ All rights reserved[
Keywords] Ruthenium^ 1!"arylazo#phenolates^ Synthesis^ Structure^ Reactivities
0[ Introduction
ligands display di}erent coordination modes upon com!
plex formation[ The 1!"arylazo#phenol ligands are known
to coordinate metal ions usually as a bidentate N\O!
donor\ via dissociation of the phenolic proton\ forming
a six!membered chelate ring "1# ð4Ł[ However\ we have
recently observed that they also form _ve!membered che!
late ring "2# under sterically forced conditions ð5Ł[ These
ligands are also known to coordinate metal ions as
dianionic tridentate C\N\O!donors "3# a}ording organ!
ometallic complexes ð6Ł[ It may be mentioned here that
the ruthenium chemistry of these ligands has not been
much explored ð7Ł[ Herein\ we have restricted our studies
to some bis!1!"arylazo#phenolate complexes of
ruthenium\ where acetylacetonate anion has served as the
coligand[ The synthesis of a series of mixed tris
complexes\ their characterization including structure
determination and their reactivities are described in this
paper[ Six 1!"arylazo#phenols\ which are abbreviated in
general as HL\ where H stands for dissociable phenolic
proton\ have been used in the present study[ Speci_c
abbreviations are shown in structure 4[ In the 1!"arylazo#!
3!methylphenols\ the substituent R is systematically
varied in order to study their in~uence on the redox
properties of the complexes[
The chemistry of ruthenium has been a center of
immense interest ð0Ł\ largely because of the fascinating
reactivities exhibited by its complexes[ It is the nature of
the ligands\ which surround the metal\ that dictates the
properties of the complexes[ Coordination of ruthenium
by ligands of di}erent types is of signi_cant importance
in this respect[ In the present work\ which has originated
from our interest in the chemistry of ruthenium in di}er!
ent coordination environments ð1Ł\ we have used 1!"ary!
lazo# phenols "0# as the principal ligand[ The interest
behind the choice of these ligands is two!fold[ Firstly\ it
contains two potential donor sites\ viz[ azo nitrogen and
phenolate oxygen\ which are of opposite natures[ The
azo group has a strong p!acid character and coordination
by azo nitrogen stabilizes ruthenium in the lower oxi!
dation states ð2Ł\ while phenolate oxygen is a hard base
and coordination by it is known to stabilize the higher
oxidation states of the same metal ð3Ł[ Secondly\ these
ꢀ Corresponding author[
9166!4276:88:, ! see front matter Þ 0888 Elsevier Science Ltd[ All rights reserved[
PII] S9166!4276"87#99297!7

521
K[ Sui et al[ : Polyhedron 07 "0888# 520Ð539

K[ Sui et al[ : Polyhedron 07 "0888# 520Ð539
522
1[ Experimental
gradually became brown[ The solvent was then evap!
orated under reduced pressure[ The crude product was
dissolved in hexane and was subjected to chro!
matography on a silica gel "59Ð019 mesh# column[ On
elution with hexane and 1]0 hexane]benzene mixture as
eluent\ yellow and light red bands came out and were
rejected[ With a 0]3 hexane]benzene mixture as the next
eluent\ a deep brown band came out[ This was collected
and evaporation of the eluate gave ðRu"acac#"ap!H#1Ł as
a dark brown microcrystalline solid[ The yield was 54)
"091 mg#[
1[0[ Materials
Commercial ruthenium trichloride\ purchased from
Arora Matthey\ Calcutta\ India was converted to
RuCl2 = 2H1O by repeated evaporation with concentrated
hydrochloric
acid[
ðRu"acac#2Ł
"acacꢁacetyl!
acetonate ion# was prepared by following a reported pro!
cedure ð8Ł[ The 1!"arylazo#phenol ligands were prepared
by coupling diazotized anilines with p!cresol or b!naph!
thol[ Puri_cation of acetonitrile and preparation of tetra!
ethylammonium perchlorate for electrochemical work
were performed as before ð1Ł[ All other chemicals and
solvents were reagent grade commercial materials and
were used as received[
1[2[1[ ðRu"acac#"an!H#1Ł
ðRu"acac#"an!H#1Ł was prepared using the same above
procedure with slight modi_cation in the puri_cation
method[ The crude sample was dissolved in a minimum
amount of dichloromethane and chromatographed
through a silica gel column[ Initial yellow and faint red
bands\ which came out with hexane and a 2]0 hexane]
benzene mixture as eluents\ were rejected[ With 1]0 hexane]
benzene as the eluent\ a deep brown band came out and
was collected[ Evaporation of the eluate gave the pure
compound[ The yield was 57) "008 mg#[
1[1[ Physical measurements
Microanalyses "C\ H\ N# were performed using a Per!
kin!Elmer 139C elemental analyzer[ IR spectra were
obtained on a Perkin!Elmer 672 spectrometer with sam!
ples prepared as KBR pellets[ Electronic spectra were
recorded on Hitachi U 2490 and Shimadzu UV 139 spec!
trophotometers[ Magnetic susceptibilities were measured
using a PAR 044 Vibrating sample magnetometer[ X!
band ESR spectra were recorded on a Varian E!098C
spectrometer _tted with a quartz Dewar for measure!
ments at 66 K "liquid nitrogen# and the spectra were
calibrated with DPPH "`ꢁ1[9926#[ Electrochemical
measurements were made using a PAR model 162 Potent!
iostat[ A platinum disc working electrode "of area
9[91 cm1#\ a platinum wire auxiliary electrode and an
aqueous saturated calomel reference electrode "SCE#
were used in a three electrode con_guration[ A platinum!
wire gauge working electrode was used in the coulometric
experiments[ Dinitrogen gas was puri_ed by successively
bubbling it through alkaline dithionite and concentrated
sulphuric acid[ All electrochemical experiments were per!
formed under a dinitrogen atmosphere and elec!
trochemical data were collected at 187 K and are
uncorrected for junction potentials[ A RE 9978 XÐY
recorder was used to trace the voltammograms[
1[3[ Crystallography
Single crystals of ðRu"acac#"an!H#1Ł were grown by
slow evaporation of a 0]0 acetoneÐisopropanol solution
of the complex[ Selected crystal data and data collection
parameters are given in Table 0[ Data were collected
on a Siemens Smart CCD di}ractometer using graphite!
Ä
monochromated Mo!Ka radiation "lꢁ9[60962 A# by uÐ
1u scans[ The standard re~ections\ used to check the
crystal stability towards X!ray exposure\ showed no sig!
Table 0
Crystallographic data for ðRu"acac#"an!H#1Ł
Empirical formula
fw
C26H18N3O3Ru
583[60
Space group
P10:n
Ä
a "A#
00[8512"1#
02[6706"1#
08[4414"2#
89
092[363"0#
89
2023[61"7#
3
0[361
9[60962
9[19×9[97×9[94
184"1#
9[437
9[9378
Ä
b "A#
Ä
c "A#
1[2[ Preparations
a ">#
b ">#
g ">#
The ðRu"acae#"L#1Ł complexes were synthesized fol!
lowing a general procedure[ Speci_c details for one com!
plex are given in next section[
2
Ä
V "A #
Z
rcalc "mg m−0
#
Ä
l "A#
1[2[0[ ðRu"acac#"ap!H#1Ł
Crystal size "mm#
T "K#
ðRu"acac#2Ł "099 mg\ 9[14 mmol# and Hap!H "219 mg\
0[4 mmol# were taken in 19 cm2 ethylbenzoate[ The mix!
ture was heated at 059>C with continuous passage of
nitrogen to remove the volatile acetylacetone[ Heating
was continued for 5 h and the colour of the solution
m "mm−0
R0
#
wR1
9[9763
GOF
0[943

523
K[ Sui et al[ : Polyhedron 07 "0888# 520Ð539
ni_cant intensity variation over the entire course of data
collection[ X!ray data reduction and structure solution
and re_nement were done using the NRCVAX package[
The structure was re_ned by full!matrix least!squares on
F1[ Final cycles of re_nement converged with discrepancy
indices of R0ꢁ9[9378 and wR1ꢁ9[9763[
ruthenium is usually octahedrally coordinated\ this indi!
cates that the 1!"arylazo#phenolates are coordinated as
bidentate ligands "as in 1 or 2#[ As the 1!"ary!
lazo#phenolates are unsymmetrical bidentate ligands\ the
ðRu"acac#"L#1Ł complexes may\ in principle\ exist in three
di}erent geometrical isomeric forms "5\ 6 and 7#[
Molecular structure of a representative complex\ viz[
ðRu"acac#"an!H#1Ł has been solved by X!ray crys!
tallography[ The structure is shown in Fig[ 0 and selected
bond distances and angles are given in Table 2[ The 0!
"phenylazo#naphtholate ligands are coordinated as
bidentate N\O!donor ligand forming six!membered che!
late rings "as in 1# with a bite angle of ½89>[ The coor!
dination sphere around ruthenium is N1O3 which is
slightly distorted from ideal octahedral geometry as
re~ected in the bond angles[ The azo nitrogens occupied
trans positions and the phenolate oxygens\ cis positions[
Therefore\ structure determination shows that ðRu "acac#
"an!H#1Ł has structure 5[ The reason for the nitrogens to
be in the trans position is probably steric in nature[ A
bulky phenyl group is linked to this nitrogen and rotation
2[ Results and discussion
Reaction of the 1!"arylazo#phenol ligands "HL# with
tris!"acetylacetonato# ruthenium"III# a}orded bis!com!
plexes of type ðRu"acac#"L#1Ł where two acetylacetonate
ligands were replaced by the 1!"arylazo#phenolate ligands
while one acetylacetonate is still retained[ Formation of
similar mixed ligand tris chelates\ via incomplete dis!
sociation of acetylacetonates from ðM"acac#2Ł\ is known
for ruthenium ð09Ł as well as other metals ð00Ł[ Elemental
"C\ H\ N# analytical data "Table 1# of these ðRu"acac#"L#1Ł
complexes are in good agreement with their compo!
sitions[ As acetylacetonate is a bidentate ligand and
Table 1
Microanalytical\ magnetic moment and electronic spectral data of the ðRuIII"acac#"L#1Ł complexes
Compound
Microanalytical dataa ")#
me}
Electronic spectral datab l "nm# "o\ M−0 cm−0
#
"BM#
C
H
N
ðRu"acac#"ap!OCH2#1Ł
ðRu"acac#"ap!CH2#1Ł
ðRu"acac#"ap!H#1Ł
ðRu"acac#"ap!Cl#1Ł
ðRu"acac#"ap!NO1#1Ł
ðRu"acac#"an!H#1Ł
46[7 "47[0#
59[4 "59[8#
59[3 "48[7#
42[0 "42[7#
41[5 "41[1#
53[3 "53[9#
3[7 "3[7#
4[9 "4[0#
3[8 "3[6#
2[6 "2[8#
3[9 "2[7#
3[1 "3[1#
7[9 "7[1#
7[4 "7[5#
8[0 "8[9#
7[9 "7[0#
00[8 "00[7#
7[2 "7[0#
0[82
0[76
0[89
0[74
0[78
0[89
197 "62399#\ 149c "20999#\ 234 "06499#\ 316 "03999#\
620c "1399#\ 0119 "103#\ 0449c "098#
109 "28999#\ 149c "14299#\ 205 "06299#\ 311 "00299#\
579c "1299#\ 0159 "110#\ 0560c "022#
101 "32299#\ 141c "1399#\ 291 "19099#\ 315 "00599#\
573c "1099#\ 0225 "082#\ 0699c "014#
107 "22199#\ 145c "10999#\ 183 "08999#\ 325 "00099#\
695c "1099#\ 0129 "040#\ 0507c "70#
109 "27399#\ 141c "11299#\ 219 "19599#\ 342 "8999#\
633c "0399#\ 0299 "72#\ 0699 "46#
117 "55199#\ 159c "28099#\ 205 "16299#\ 269 "05699#\
354 "06999#\ 797c "0899#\ 0324 "235#\ 0899 "182#
aCalculated values are in parentheses[
bIn carbon tetrachloride solution[
cShoulder[

K[ Sui et al[ : Polyhedron 07 "0888# 520Ð539
524
of this ring around the CÐN bond is allowed[ With two
such nitrogens in the cis positions "as in 6 and 7#\ severe
steric interaction between these phenyl rings appears to
be highly probable[ The RuÐN and RuÐO distances are
as usually observed in complexes of ruthenium"III# ð3bŁ[
The azo NÐN distances in ðRu"acac#"an!H#1Ł are longer
producing solutions with di}erent shades of brown[ Elec!
tronic spectra of these complexes were recorded in carbon
tetrachloride solution[ Spectral data are displayed in
Table 1 and a selected spectrum is shown in Fig[ 1[ The
spectral properties of all six complexes are similar[ Each
complex shows several intense absorptions in the visible
region which are probably due to ligand!to!metal charge!
transfer transitions[ They also show two weak transitions
in the near!IR region\ the origin of which will be discussed
below[
Magnetic susceptibility measurements show that the
ðRu"acac#"L#1Ł complexes are one!electron paramagnetic
"Table 1#\ which is in accordance with the ¦2 state of
ruthenium "low!spin d4\ Sꢁ0:1# in these complexes[ ESR
spectra of the ðRu"acac#"L#1Ł complexes were recorded
in a 0]0 dichloromethaneÐtoluene solution at 66 K[ A
representative spectrum is shown in Fig[ 2[ Each complex
shows a rhombic spectrum with three distinct g!values
"Table 3#[ However\ all the spectra may be regarded as
pseudo!axial\ consisting of two very closely spaced signals
"g0 and g1\ rhombic component of g_# and a rather iso!
lated signal "g2\ g> in the axial case#[ Hence the axial
distortion "D# that splits the t1 level into a and e
components\ is expected to be much larger than the rhom!
bic distortion "V# which splits e[ This splitting pattern is
illustrated as an inset in Fig[ 2[ Spin!orbit coupling causes
than uncoordinated azo "N N# distances ð01Ł[ However\
1
they are similar to those observed in other structurally
characterised azo complexes of trivalent ruthenium
ð6\02Ł[ The phenolic CÐO distances are also quite normal
ð3b\6Ł[ The bond distances in the Ru"acae# fragment com!
pare well with those found in other complexes containing
the RuIII"acac# moiety ð03a\03bŁ[ As the properties of all
six ðRu"acac#"L#1Ł complexes are similar "vide infra#\ the
other _ve ðRu "acac#"L#1Ł complexes are assumed to have
a similar structure as ðRu"acac#"an!H#1Ł[
Infrared spectra of the ðRu"acac#"L#1Ł complexes are
very similar[ Each shows many vibrations of di}erent
intensities below 0699 cm−0[ No attempt has been made
for individual band assignment[ However\ comparison
with the infrared spectrum of ðRu"acac#2Ł shows the pres!
ence of additional vibrations in the spectra of ðRu
"acac#"L#1Ł complexes\ which are indicative of the pres!
ence of 1!"arylazo#phenolate ligands[ The ðRu"acac#"L#1Ł
complexes are soluble in common organic solvents like
acetonitrile\ acetone\ hexane\ carbon tetrachloride\ etc[\
Fig[ 0[ View of the ðRu"acac#"an!H#1Ł molecule[

525
K[ Sui et al[ : Polyhedron 07 "0888# 520Ð539
Table 2
Selected bond distances and bond angles for ðRu"acac#"an!H#1Ł
Ä
Bond distances "A#
RuÐO"0#
RuÐO"1#
RuÐO"2#
RuÐO"3#
RuÐN"0#
RuÐN"2#
0[862"1#
C"23#ÐO"2#
C"23#ÐC"24#
C"24#ÐC"25#
C"25#ÐO"3#
0[Ð166"3#
0[279"5#
0[271"5#
0[179"3#
0[862"1#
1[908"1#
1[913"1#
1[925"2#
1[926"2#
Bond angles ">#
O"1#ÐRuÐO"2#
O"0#ÐRuÐO"3#
N"0#ÐRuÐN"2#
O"0#ÐRuÐO"1#
O"0#ÐRuÐO"2#
O"1#ÐRuÐO"3#
O"2#ÐRuÐO"3#
O"0#ÐRuÐN"0#
O"1#ÐRuÐN"0#
067[03"8#
064[52"09#
068[24"00#
84[53"09#
74[38"8#
77[27"09#
89[42"09#
89[97"09#
78[64"09#
O"2#ÐRuÐN"0#
O"3#ÐRuÐN"0#
O"0#ÐRuÐN"2#
O"1#ÐRuÐN"2#
O"2#ÐRuÐN"2#
O"3#ÐRuÐN"2#
77[65"09#
80[55"09#
78[37"09#
78[71"8#
80[57"8#
77[70"09#
Fig[ 1[ Electronic spectra of ðRuIII"acac#"ap!H#1Ł "***\ in carbon tetrachloride solution#\ ðRuII"acac#"ap!H#1Ł− "! ! !\ in acetonitrile solution# and
ðRuIV"acac#1"ap!H#1Ł¦ "Ð= Ð\ in acetonitrile solution#[
further changes in the energy gaps[ Two electronic tran!
sitions "transition energies DE0 and DE1^ DE0³DE1# are
therefore possible within these three levels[ All the energy
parameters have been computed "Table 3# using the
observed g!values\ g!tensor theory of low!spin d4 com!
plexes ð04a\04bŁ and a reported method ð05Ł[ The axial
distortion is indeed much larger than the rhombic one
for all the six complexes[ Taking the spin!orbit coupling
constant "l# of complexed ruthenium"III# to be 0999 cm−0
ð05Ł\ the DE0 and DE1 transitions are expected to appear
near 5999 and 7999 cm−0\ respectively[ Both transitions
were experimentally observed\ at energies very close to
the theoretically predicted values "Table 3#\ as two rela!
tively weak absorptions "Table 1\ Fig[ 1#[ The ESR data
analysis thus indicates that the ðRu"acac#"L#1Ł complexes
are distorted from ideal octahedral geometry\ as also
observed from the X!ray structure determination of
ðRu"acac#"an!H#1Ł[
Electron!transfer properties of the ðRu"acac#"L#1Ł com!
plexes were studied in acetonitrile solution "9[0 M TEAP#
by cyclic voltammetry[ Voltammetric data are presented
in Table 4 and a selected voltammogram is shown in Fig[
3[ Each complex shows two voltammetric responses\ one
oxidative response on the positive side of SCE and
another reductive response on the negative side\ which
are respectively assigned to the ruthenium"III#Ð
ruthenium"IV#
and
ruthenium"III#Ðruthenimn"II#
couples "eqs[ "0# and "1##[

K[ Sui et al[ : Polyhedron 07 "0888# 520Ð539
526
Table 3
ESR g!valuesa and derived parametersb of the ðRu"acac#"L#1Ł complexes
Compound
g0
g1
g2
D:l
V:l
DE0:lc
DE1:lc
ðRu"acac#"ap!OCH2#1Ł
ðRu"acac#"ap!CH2#1Ł
ðRu"acac#"ap!H#1Ł
ðRu"acac#"ap!Cl#1Ł
ðRu"acac#"ap!NO1#1Ł
ðRu"acac#"an!H#1Ł
1[030
1[041
1[041
1[055
1[030
1[050
1[006
1[019
1[013
1[003
1[003
1[006
0[827
0[827
0[839
0[827
0[830
0[828
5[65
5[72
5[82
5[83
5[84
5[82
9[76
0[00
0[99
0[74
0[94
0[45
5[14 "5[34#
5[12 "4[87#
5[26 "4[77#
5[91 "5[07#
5[26 "4[77#
5[03 "4[15#
6[49 "7[19#
6[55 "6[83#
6[62 "6[37#
7[97 "7[02#
6[66 "6[58#
6[84 "5[86#
aIn 0]0 dichloromethaneÐtoluene solution at 66 K[
bSpin!orbit coupling constant "l# for complexed Ru"III# is ½0999 cm−0
cThe experimentally observed values "see Table 1# are given in parentheses[
[
ðRuIII"acac#"L#1Ł gh ðRuIV"acac#"L#1Ł¦¦e−
ðRuIII"acac#"L#1Ł¦e− gh ðRuII"acac#"L#1Ł−
"0#
"1#
Both responses are reversible with a peak!to!peak sep!
aration of 59Ð69 mV[ The one!electron nature of these
couples are established by constant potential coulometry
"vide infra#[ It may be noted here that the ruthenium"III#Ð
ruthenium"IV# and ruthenium"III#Ðruthenium"II# poten!
tials of these ðRu"acac#"L#1Ł complexes are much lower
than those in the ðRu"bpy#"L#1Ł "bpyꢁ1\1?!bipyridine#
complexes ð7cŁ\ which is due to the hard nature of the
acetylacetonate oxygens compared to the soft nitrogens in
bpy[
Potentials of both the ruthenium"III#Ðruthenium"II#
and ruthenimn"III#Ðruthenium"IV# couples in these
ðRu"acac#"L#1Ł "Lꢁthe 1!"arylazo#!3!methylphenolate
ligands# complexes are found to be sensitive to the nature
of the substituent R[ With increasing electron!with!
drawing character of R\ the formal potentials "E0:1# of
both couples increase[ The plot of the E0:1 values vs 1s
"where s is the Hammett constant of R ð06Ł# are linear
"Fig[ 4#[ The slope of these lines\ which is known as the
reaction constant r ð07Ł and is a measure of the sensitivity
of E0:1 with R\ is 9[97 for the ruthenium"III#Ð
ruthenium"IV# couple and 9[09 for the ruthenium"III#Ð
ruthenium"II# couple[ It is clear from these plots that the
ruthenium"III#Ðruthenium"II# couple is more in~uenced
by R compared to the ruthenium"III#Ðruthenium"IV#
couple[ It is also interesting to note that a single substitu!
ent\ which is six bonds away from the electroactive metal
center\ can in~uence the metal!centred redox potentials
in a predictable manner[
Fig[ 2[ ESR spechm of ðRu"acac#"ap!CH2#1Ł in 0]0 dichlo!
romethane]toluene at 66 K[
Table 4
Cyclic voltammetric dataa
Compound
E0:1 "V# "DEp\ mV#
RuII:RuIII
RuIII:RuIV
ðRu"acac#"ap!OCH2#1Ł
ðRu"acac#"ap!CH2#1Ł
ðRu"acac#"ap!H#1Ł
ðRu"acac#"ap!Cl#1Ł
ðRu"acac#"ap!NO1#1Ł
ðRu"acac#"an!H#1Ł
−9[41"59#
−9[49"69#
−9[36"59#
−9[30"69#
−9[20"59#
−9[13"59#
9[76"69#
9[78"59#
9[80"59#
9[84"59#
0[92"69#
9[72"59#
From the reversible nature of both ruthenium"III#Ð
ruthenium"IV#
and
ruthenium"III#Ðruthenium"II#
couples\ it appeared that both the oxidized species\
i[e[ðRuIV"acac#"L#1Ł¦ and the reduced species\ i[e[ðRuII
"acac#"L#1Ł−\ might be stable on longer time scales[ To
investigate this\ ðRu"acac#"L#1Ł was coulometrically oxi!
dized near 0[1 V vs SCE[ The oxidations have been
smooth and quantitative with n½0 ðn"number of electron
transfer#ꢁQf:Qc\ where Qcꢁcalculated Coulomb count
aConditions] solvent\ acetonitrile^ supporting electrolyte\ TEAP
"9[0 M#^ working electrode\ platinum^ auxiliary electrode\ platinum]
reference electrode\ SCE] solute concentration\ ½09−2 M^
E0:1ꢁ9[4"Epa¦Epc#\ where Epa and Epc are anodic and cathodic peak
potentials^ DEpꢁEpa−Epc^ scan rate 49 mV S−0
[

527
K[ Sui et al[ : Polyhedron 07 "0888# 520Ð539
Fig[ 3[ Cyclic voltammogram of ðRu"acae#"ap!NO1#1Ł in acetonitrile solution "9[0 M TEAP# at a scan rate of 49 mV S−0
[
Table 5
Electronic spectral dataa of the ðRuII"acac#"L#1Ł− and ðRuIV"acac#"L#1Ł¦ complexes l "nm# "o\ M−0 cm−0
#
L
ðRuII"acac#"L#1Ł−
ðRuIV"acac#"L#1Ł¦
ap!OCH2
ap!CH2
ap!H
129 "89699#\ 219b "11399#\ 393 "06399#\ 414 "00799#\ 693b "4299#
121 "095099#\ 297b "14099#\ 399 "05399#\ 429 "00699#\ 699b "4599#
114 "089999#\ 293b "11199#\ 397 "00399#\ 439 "7199#\ 697b "2599#
117 "09399#\ 173b "27599#\ 285 "06499#\ 421 "04299#\ 693b "6299#
129 "091599#\ 181b "13199#\ 399 "02499#\ 437 "7499#\ 639b "3299#
121 "82199#\ 293 "22999#\ 279 "11499#\ 445 "06199#\ 619b "6299#
101 "35799#\ 149b "18999#\ 233 "05599#\ 313 "03499#\ 579b
"1299#
101 "35799#\ 141b "17199#\ 205 "08499#\ 319 "02399#\ 541b
"1599#
101 "62099#\ 149b "28399#\ 293 "18999#\ 313 "04599#\ 545b
"2299#
ap!Cl
ap!NO1
an!H
101 "45199#\ 141b "20499#\ 299 "13299#\ 313 "04999#\ 579b
"2699#
101 "51699#\ 137 "32399#\ 185 "25499#\ 339b "06099#\ 621b
"2799#
105 "59999#\ 149b "26899#\ 299b "05699#\ 253 "05699#\ 359
"04399#\ 645b "0899#
aIn acetonitrile solution[
bShoulder[
and Qfꢁobserved Coulomb countŁ\ a}ording yellowish!
brown solutions of ðRuIV"aeac#"L#1Ł¦[ These solutions
show identical cyclic voltammograms as their respective
precursors\ the only exception being the ruthenium"III#Ð
ruthenium"IV# couple appears as a reductive response[
The same oxidations were achieved chemically by a solu!
tion of Ce3¦ in 0 M HClO3[ Electronic spectra of the
ðRuIV"acac#"L#1Ł¦ complexes in acetonitrile solution show
several intense absorptions in the visible and UV region
"Table 5\ Fig[ 1#[ Chemical or electrochemical reduction
of the oxidized complexes gives the ðRuIII"acac#"L#1Ł com!
plexes\ identi_ed by their characteristic electronic spectra[
Coulometric reduction of the ðRuIII"acac#"L#1Ł com!
plexes near −9[5 V vs SCE quantitatively "n½0# a}orded
pink "blue for Lꢁan!H# solutions of ðRuII
"acac#"L#1Ł−[ The reduced solutions show identical cyclic
voltammograms ðbesides that the ruthenium"III#Ð
ruthenium"II# couple appears as an oxidative responseŁ
as their respective precursors[ The same pink "or blue#
solutions were also generated by chemical reduction of
ðRuIII"acac#"L#1Ł with hydrazine[ Electronic spectra of the
bivalent complexes\ recorded in acetonitrile solution\
show intense transitions in the visible and UV region
"Table 5\ Fig[ 1#[ Coulometric or chemical "by hydrogen
peroxide# oxidation of the pink "or blue# solutions gives
back original solutions of the respective ðRuIII"acac#"L#1Ł
complexes[ The above interconversions indicate that both
the ruthenium"III#Ðruthenium"IV# and ruthenium"III#Ð
ruthenium"II# couples are chemically reversible[
Addition of excess hydrazine to ðRuIII"acac#"L#1Ł\
though initially causes instantaneous reduction to ðRuII
"acac#"L#1Ł−\ slowly brings about a secondary chemical
transformation producing deep pink solutions which
show an intense absorption near 429 nm "e½49\999 M−0
cm−0#[ Though the exact composition of these species is
not clear yet\ it appears that the coordinated acac has
underwent Schi}!base condensation with hydrazine
yielding stable complexes of bivalent ruthenium[ Further

K[ Sui et al[ : Polyhedron 07 "0888# 520Ð539
528
Fig[ 4[ Least!squares plot of E0:1 values of "a# RuII:RuIII couple vs 1s and "b# RuIII:RuIV couple vs 1s[
characterization of these species is currently under
Acknowledgements
progress[
Financial support from the Council of Scienti_c and
Industrial Research\ New Delhi ð90"0397#:85:EMR!IIŁ\
India\ is gratefully acknowledged[ Our sincere thanks go
to Professor Animesh Chakravorty "Indian Association
for the Cultivation of Science\ Calcutta#\ Professor Guru!
nath Mukherjee "Calcutta University# and Dr[ Rup!
endranath Banerjee "Jadavpur University# for their kind
help[
4[ Conclusions
This study shows that in spite of coordination by two
azo nitrogens\ coordination by phenolate oxygen is more
e}ective in stabilising higher oxidation states of
ruthenium[ The possibility of using the ðRu"D1#"L#1Ł com!
plexes as starting materials for the synthesis of complexes
of type ðRu"D1#"L#1Ł\ where D1 is either a bidentate ligand
or two monodentate ligands\ is under scrutiny[ Template
Schi} base condensation at the coordinated acetyl!
acetonate ligand is also under investigation[
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