Mononuclear and dinuclear ruthenium(II)/(III) salicylates incorporating azoimine functionalities as ancillary ligands. Synthesis, spectroscopic and electron-transfer properties

Article abstract of DOI:10.1039/b100163i

Ruthenium-salicylate complexes incorporating azoimine based azopyridine ligands, L, of the type [Ru^II(L)₂-(salicylate)] 1-5 [L = NC₅H₄N=NC₆H₄(R), R = H, m-Me/Cl or p-Me/Cl] have been synthesized and their spectroelectrochemical aspects investigated. The complexes systematically exhibit two 1e^- and two 2e^- oxidation processes and four successive one-electron reductions. The stepwise electrochemical oxidations were followed by electronic and EPR spectral studies on each oxidation step which indicate that the initial one-electron oxidation process corresponds to stereoretentive oxidation of the ruthenium(II) centre to ruthenium(III), [Ru^III(L)₂(salicylate)]⁺ 1⁺-5⁺. The second oxidation step corresponds to oxidation of the coordinated salicylate moiety in 1⁺-5⁺ to the ruthenium(III)-salicylate semiquinone cationic radical, [Ru^III(L)₂(salicylate)]²⁺ E. The electrogenerated ruthenium(III) congeners (1⁺-5⁺) exhibit rhombic EPR spectra corresponding to distorted octahedral complexes. The electrogenerated semiquinone salicylate radical [Ru^III(L)₂(salicylate)]²⁺ E undergoes a radical recombination process which leads to formation of antiferromagnetically coupled dimeric species, [(L)₂Ru^III(X)Ru^III(L)₂] ⁴⁺ F [(X = ^-O₂C(O)C₆H₄C₆H ₄(O)CO₂^-]. The next two 2e^- oxidation processes are associated with oxidation of the bridging moiety of the dimeric species [(L)₂Ru^III(X′)Ru^III (L)₂]⁴⁺ G [(X′ = ^-O₂C(O)C₆H₃=C₆H ₃(O)CO₂^-] followed by oxidation of the metal centres, [(L)₂Ru^IV(X′)Ru^IV(L)₂] ⁶⁺ H. The chemical oxidation of the complexes 1-5 by HNO₃ leads to formation of dimeric complexes, G, straightaway. The complexes display intense charge-transfer bands in the UV-visible region which have been found to be reasonably blue shifted while moving from 1-5 to 1⁺-5⁺ to F to G.

Full text of DOI:10.1039/b100163i

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Mononuclear and dinuclear ruthenium(II)/(III) salicylates
incorporating azoimine functionalities as ancillary ligands.
Synthesis, spectroscopic and electron-transfer properties†
Ramapati Samanta, Biplab Mondal, Pradip Munshi and Goutam Kumar Lahiri*
Department of Chemistry, Indian Institute of Technology, Bombay, Powai, Mumbai-400076,
India
Received 3rd January 2001, Accepted 12th April 2001
First published as an Advance Article on the web 25th May 2001
Ruthenium–salicylate complexes incorporating azoimine based azopyridine ligands, L, of the type [RuII(L)2-
(salicylate)] 1–5 [L = NC H N᎐NC H (R), R = H, m-Me/Cl or p-Me/Cl] have been synthesized and their
᎐
5
4
6
4
spectroelectrochemical aspects investigated. The complexes systematically exhibit two 1eϪ and two 2eϪ oxidation
processes and four successive one-electron reductions. The stepwise electrochemical oxidations were followed by
electronic and EPR spectral studies on each oxidation step which indicate that the initial one-electron oxidation
process corresponds to stereoretentive oxidation of the ruthenium() centre to ruthenium(), [RuIII(L)2(salicylate)]ϩ
1ϩ–5ϩ. The second oxidation step corresponds to oxidation of the coordinated salicylate moiety in 1ϩ–5ϩ to the
ruthenium()–salicylate semiquinone cationic radical, [RuIII(L)2(salicylate)]2ϩ E. The electrogenerated ruthenium()
congeners (1ϩ–5ϩ) exhibit rhombic EPR spectra corresponding to distorted octahedral complexes. The electro-
generated semiquinone salicylate radical [RuIII(L)2(salicylate)]2ϩ E undergoes a radical recombination process
which leads to formation of antiferromagnetically coupled dimeric species, [(L)2RuIII(X)RuIII(L)2]4ϩ F [(X = ϪO2C(O)-
C6H4C6H4(O)CO2Ϫ]. The next two 2eϪ oxidation processes are associated with oxidation of the bridging moiety of
the dimeric species [(L)2RuIII(XЈ)RuIII(L)2]4ϩ G [(XЈ = ϪO C(O)C H ᎐C H (O)CO Ϫ] followed by oxidation of the
᎐
2
6
3
6
3
2
metal centres, [(L)2RuIV(XЈ)RuIV(L)2]6ϩ H. The chemical oxidation of the complexes 1–5 by HNO3 leads to
formation of dimeric complexes, G, straightaway. The complexes display intense charge-transfer bands in the
UV-visible region which have been found to be reasonably blue shifted while moving from 1–5 to 1؉–5؉ to F to G.
transfer processes.3 The observed electro-non-innocent
behaviour of the salicylate function on coordination to
Ru(bpy)2 has prompted us to examine its reactivity particularly
in combination with bipyridine like, but stronger π-acidic
Introduction
The relevance of salicylic acid in a wide range of biological
applications1 besides its potential use as a chelating function2
makes it a molecule of profound interest. Moreover, it has
been observed recently that the salicylate function undergoes
fascinating electron-transfer properties on coordination to the
Ru(bpy)2 core, [Ru(bpy)2(salicylate)] 1 (bpy = 2,2Ј-bipyridine).
This leads to hydroxylation of the electrogenerated coordinated
salicylate semiquinone intermediate (E) via successive electron
arylazopyridine ligands L, [Ru(L) (salicylate)] [L = NC H N᎐
᎐
2
5
4
NC6H4(R), R = H, m-Me/Cl or p-Me/Cl]. The present study
shows that in the presence of L the electrogenerated salicylate
semiquinone radical (E) preferentially follows
a radical
recombination process leading to the formation of a dimeric
species F as a reactive intermediate. This dimeric species is
found to undergo further successive proton transfer and
electron transfer processes.
Herein we report the synthesis and detailed spectroelectro-
chemical properties of the complexes [Ru(L)2(salicylate)]
† Electronic supplementary information (ESI) available: electrospray
mass spectra of complexes 1 and 1G. See http://www.rsc.org/suppdata/
dt/b1/b100163i/
DOI: 10.1039/b100163i
J. Chem. Soc., Dalton Trans., 2001, 1827–1833
This journal is © The Royal Society of Chemistry 2001
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and monomer to dimer conversion as a function of electron-
transfer processes at the salicylate moiety.
reported complexes incorporating the Ru(L)2 moiety,4 it may be
assumed that the reactions in Scheme 1 also follow the stereo-
retentive pathway (structure A).
The oxidation of the complexes 1–5 by using concentrated
HNO3 results in a stable red dimeric product (G, Scheme 2).
Although all the complexes (1–5) behave similarly on HNO3
Results
Synthesis and characterisation
Ruthenium() salicylate complexes, [RuII(L)2(salicylate)] 1–5
oxidation; the oxidised complex [(L1) RuIII{ϪO C(O)C H ᎐
᎐
2
2
6
3
C6H3(O)CO2Ϫ}RuIII(L1)2][ClO4]4ؒ2H2O 1G has been isolated in
the solid state as its dihydrated perchlorate salt. It may be noted
that HNO3 was also used earlier for the selective oxidation of
[RuII(L)2Cl2] → [RuIII(L)2Cl2]ϩ.5 The microanalytical data
of the oxidised species (Experimental section) match well with
the dimeric form, 1G (Scheme 2). The isolated diamagnetic
complex 1G exhibits 1 : 4 conductivity in acetonitrile solvent
(ΛM/ΩϪ1 cm2 molϪ1, 420).
[L = NC H N᎐NC H (R), R = H, m-Me/Cl or p-Me/Cl]
᎐
5
4
6
4
incorporating azoimine based azopyridine (L1–5) ancillary
ligands have been synthesized from the starting ctc-RuII(L)2Cl2
species (ctc = cis-trans-cis with respect to chlorides, pyridine
and azo nitrogens respectively) (Scheme 1). The salicylate group
binds to the ruthenium ion as the usual binegative function,
resulting in neutral and diamagnetic ruthenium() hetero-
chelates (1–5).
1
The H NMR spectra of the complexes 1–5 were recorded
The microanalytical data of the complexes match well with
the calculated values (Experimental section). The unsymmetric
nature of L and the salicylate functions lead to the possibility
of coexistence of four geometrical isomers (A–D). However, the
in CDCl3 solvent and the spectrum of 2 is shown in Fig. 1(a).
The presence of the unsymmetric salicylate group in
1–5 makes all the five aromatic rings non-equivalent, the
complexes 1 and 2–5 thus display twentytwo and twenty non-
equivalent aromatic protons (Fig. 1a) respectively.5 The
partial overlapping of the signals makes the assignment of
the individual signals difficult, however a careful examination
of the spectra clearly indicates the presence of the estimated
number of singlet, doublets and triplets as well as the iso-
meric purity of the products. The complexes 2 and 4 display
two closely spaced Me signals each having 1 : 1 intensity ratio
in the upfield region (δ 2.143 and 2.131 for 2 and 2.232 and
2.221 for 4). The isomer B should exhibit one Me signal,
on the other hand all other isomers (A, C and D) should
display two methyl signals having equal intensity. The observed
closely spaced two methyl signals presumably provides indirect
support in favour of the isomeric structure A as this is the
only form where both the methyl groups are in the best
possible symmetric environment [both are cis to pyridine
nitrogens (Np) and trans to oxygens (phenolato or carboxyl-
ato)].6
The 1H NMR spectrum of 1G in CDCl3 displays signals
corresponding to half of the molecule (1G) as each half is
equivalent due to the localised symmetry (Fig. 1b). The coup-
ling pattern of the resonances particularly in the upfield part of
the aromatic region of 1 differs from that of the dimeric com-
plex 1G. This is due to the fact that the salicylate function in the
starting monomeric complexes (1–5) exists as a four spin system
while the same in the oxidised dimeric complex 1G corresponds
to a three spin system.
complexes (1–5) constantly develop into one particular blue-
violet isomer. Since the tc (trans and cis with respect to pyridine
and azo nitrogens respectively) configuration of the starting
Ru(L)2 fragment is known thermodynamically to be most stable
and has been observed to retain the tc identity in all other
The formation of 1 and 1G has been confirmed by electro-
spray mass spectra (ESI supplementary material). Maximum
molecular ion peaks, at m/z 604.4 (formula weight, 603.60) and
1206.02 (formula weight, 1205.19) are exhibited for 1 and 1G
respectively.
Scheme 1 (i) Dry EtOH, AgNO3, ∆; (ii) HO1C6H4C(᎐O)O2H, NaOH, ∆.
᎐
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Table 1 Electrochemical data in acetonitrile at 298 Ka
[EЊ298/V (∆Ep/mV)]
Salicylate based
L based
RuIII–RuII
Couple I
RuIV–RuIII
Couple IV
Compound
nb
Couple II
Couple III
Couple V
Couple VI
Couple VII
Couple VIII
1
2
3
4
5
0.70(90)
0.67(100)
0.72(100)
0.65(80)
0.76(80)
0.96
0.95
1.05
1.07
1.05
1.03(100)
1.05(100)
1.05(80)
0.98(100)
1.10(80)
1.55(110)
1.54(120)
1.56(100)
1.50(100)
1.58(90)
1.97(110)
1.94(120)
2.00(100)
1.89(100)
2.09(100)
Ϫ0.60(70)
Ϫ0.66(80)
Ϫ0.56(70)
Ϫ0.72(80)
Ϫ0.52(70)
Ϫ1.15(80)
Ϫ1.22(80)
Ϫ1.10(80)
Ϫ1.27(80)
Ϫ1.02(80)
Ϫ1.93(100)
Ϫ1.99(90)
Ϫ1.80(100)
Ϫ2.09(90)
Ϫ1.71(90)
Ϫ2.33(120)
Ϫ2.40(110)
Ϫ1.89(120)
Ϫ2.44(110)
Ϫ1.83(120)
a Solvent, acetonitrile; supporting electrolyte, [NEt4][ClO4] (concentration ≈10Ϫ1 mol dmϪ3); reference electrode, SCE; solute concentration, ≈10Ϫ3
mol dmϪ3; working electrode, platinum wire. Cyclic voltammetric data: scan rate, 50 mV sϪ1; EЊ298 = 0.5 (Epa ϩ Epc) where Epa and Epc are the anodic
and cathodic peak potentials, respectively. b n = Q/QЈ, where QЈ is the calculated coulomb count for 1eϪ transfer and Q is the coulomb count found
after exhaustive electrolysis of a ≈10Ϫ2 mol dmϪ3 solutions of the complexes.
Fig.1 1HNMRspectraof(a)[RuII(L2)2(salicylate)]2and(b)[(L1)2RuIII
{ϪO2C(O)C6H3᎐᎐C6H3(O)CO2Ϫ}RuIII(L1)2][ClO4]4ؒ2H2O 1G in CDCl3.
-
electrode. Representative voltammograms are shown in Fig. 2
and the data are listed in Table 1.
The complexes display one quasi-reversible oxidative
response in the range 0.65–0.76 V which is assigned to be
Ru()–Ru() couple (I, Fig. 2). The one-electron nature of the
couple is confirmed by constant potential coulometry (Table 1).
The presence of trivalent ruthenium in the oxidised solution is
confirmed by the characteristic rhombic EPR spectra of the
ruthenium() species in a distorted octahedral arrangement
(see later). The potential of couple I varies systematically
depending on the electronic nature and location of the ‘R’
groups present in the framework of L. Coulometric oxidations
of the blue-violet starting ruthenium() complexes (1–5) in
acetonitrile solvent at a potential ≈100 mV greater than that
of the corresponding Epa(couple I) result in a violet oxidised
solution. The oxidised complexes (1؉–5ϩ) show voltammo-
grams which are superimposable on those of the parent
bivalent species, implying the stereoretentive nature of the
oxidation process. Coulometric reductions of the oxidised
violet ruthenium() complexes at a potential less than that
of the corresponding Epc of couple I produce the parent blue-
Scheme 2
Electron-transfer properties
Electron-transfer properties of the complexes 1–5 have been
studied in acetonitrile solvent using a platinum working
J. Chem. Soc., Dalton Trans., 2001, 1827–1833
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Table 2 EPR g valuesa of coulometricallyb generated trivalent ruthenium() species, distortion parametersc and NIR transitionsd
Compound
gx
gy
gz
k
∆/λ
V/λ
ν1/λ
ν2/λ
ν2/λobs
1؉
2؉
3؉
4؉
5؉
Ϫ2.378
Ϫ2.382
Ϫ2.389
Ϫ2.388
Ϫ2.385
Ϫ2.105
Ϫ2.115
Ϫ2.111
Ϫ2.109
Ϫ2.119
1.839
1.840
1.840
1.832
1.835
0.645
0.661
0.666
0.652
0.661
5.072
5.046
5.108
4.956
4.939
Ϫ3.696
Ϫ3.552
Ϫ3.730
Ϫ3.579
Ϫ3.401
3.375
3.417
3.393
3.321
3.386
7.135
7.039
7.187
6.966
6.862
7.715
7.756
6.721
7.522
7.589
a Measurements were made in dichloromethane at 77 K. b Solvent, dichloromethane; supporting electrolyte, [NEt4][ClO4] (concentration ≈10Ϫ1 mol
dmϪ3); reference electrode, SCE; solute concentration, ≈10Ϫ3 mol dmϪ3; working electrode, platinum wire gauze. c Symbols have the same meaning as
in the text. d In dichloromethane.
Fig. 2 Cyclic voltammograms and differential pulse voltammograms
of the complex [RuII(L3)2(salicylate)] 3 in acetonitrile at 298 K. The
dashed line indicates the irreversible nature of couple II.
Fig. 3 X-Band EPR spectrum and computed t2 splitting of the
coulometrically oxidised complex [RuIII(L4)2(salicylate)]ϩ 4؉ in dichloro-
methane at 77 K.
violet complexes quantitatively, thus implying the reversibility
of the process.
The complexes (1–5) exhibit a second oxidative process near
1.0 V (Fig. 2, couple II; Table 1). The one-electron nature of
the couple has been confirmed by comparing its differential
pulse voltammogram current height with that of the previous
ruthenium()–ruthenium() couple. The irreversible nature of
this process has been confirmed by reversing the scan direction
immediately after the couple II response (Fig. 2, dashed line).
The observed cathodic part (ipc) of couple II during the first full
scan (Fig. 2, solid line) is possibly developed due to contribu-
tions from the other two irreversible responses (couples III and
IV). Coulometric oxidations of 1–5 at a potential greater than
the Epa of couple II develop a light pink solution. After the
second step the oxidised solution (12؉–52؉) is found to be EPR
inactive even at 77 K.
the ruthenium ion, four successive one electron reductions are
expected for each case and in practice all the responses have
been nicely observed experimentally (Fig. 2, Table 1).
The coulometrically produced reduced light red solution of
1؊–5؊ [obtained by reducing the complexes 1–5 at a potential
100 mV more negative than the corresponding Epc of couple V
(Fig. 2)] have been found to be unstable. However, the EPR
spectrum of the reduced complex 4؊ has been recorded by
performing the electrolysis at 258 K [observed coulomb count
corresponds to one-electron transfer (n = 1.11)] and quickly
freezing the electrolysed solution at 77 K (liquid nitrogen)
(see below).
The two-electron nature of the third (couple III, Fig. 2)
and fourth (couple IV, Fig. 2) oxidation processes has been
established by comparing their differential pulse voltammetric
current heights with those of the previous couples (I and II)
(Fig. 2). The irreversible nature of this response has been
confirmed further by sweeping the scan beyond couple III but
without moving up to couple IV. Coulometric oxidations of
the complexes at a potential higher than the Epa of couple III
generate a red solution (13؉–53؉) which is also EPR silent at
77 K. Coulometric oxidations of the complexes at a potential
greater than the Epa of couple IV provide a very small coulomb
count, implying the unstable nature of the oxidised species
(14؉–54؉) even on the coulometric timescale.
The complexes display four successive one-electron reduc-
tions at negative side potentials with respect to the SCE
(couples V–VIII). The one-electron nature of the couples
has been identified by comparison with the ruthenium()–
ruthenium() couple (I) with the help of the cyclic voltam-
metric as well as differential pulse voltammetric current height.
The ligand L is known to act as a potential electron-transfer
centre and each can accept two electrons in the electro-
chemically accessible LUMO which is predominantly azo in
character.7 As two electroactive azo groups are present around
EPR spectra of the oxidised (1؉–5؉) and reduced (4؊) complexes
The coulometrically oxidised complexes (1؉–5؉ at 258 K)
exhibit rhombic EPR spectra (Fig. 3, Table 2) in dichloro-
methane at 77 K which suggests the correspondence of couple I
with the ruthenium()–ruthenium() process as opposed to
ligand based oxidation like the ruthenium–dioxolene system.8
The EPR spectra of the oxidised complexes have been
analysed in terms of g-tensor theory of low-spin d5 ions.9 The
axial (∆) and rhombic (V) distortions (Table 2) combined with
spin–orbit coupling [λ for RuIII is 1000 cmϪ1 10] transform the t2
shell into three Kramers doublets. The ligand field transitions ν1
and ν2 among them are predicted to lie in the ranges 3321–3417
and 6862–7187 cmϪ1 respectively (Table 2). A relatively weak
transition is indeed observed close to the predicted range 6721–
7756 cmϪ1 and is assigned to be the ν2 transition (Table 2). The
ν1 transition lies outside the range of our instrument. The com-
puted distortion parameters (∆ and V, Table 2) indicate the
presence of a high degree of rhombicity in the complexes.11
The first step reduced complex 4Ϫ in dichloromethane shows
an intense and sharp EPR signal at 77 K with a ‘g’ value at
2.003, implying that the unpaired electron in the reduced
product is localised in the orbital which has predominantly
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Electronic spectra of the electrogenerated 1؉–5ϩ in the visible
Table 3 Electronic spectral data for the complexes [Ru(L)2(salicylate)]
1–5 and their stepwise electrochemically oxidised products in
acetonitrile
region display a strong band in the range (560–581 nm)
followed by a weak shoulder near 460 nm (Fig. 4, Table 3).
The bands are assigned to be the LMCT transitions.14 Thus on
metal oxidation the lowest energy charge-transfer transition has
reasonably been blue shifted (Table 3). The band maxima are
observed to be dependent on the nature of L present in 1؉–
5؉ (Table 3). The intra ligand π–π* and n–π* transitions have
appeared in the UV region as expected (Table 3).
Compound
UV/vis λ/nm(ε/dm3 molϪ1 cmϪ1
)
1
597(10200), 509(3900), 326(24650), 224(47600)
560, 460, 365, 316
549, 452, 358, 309, 282
505, 470, 361, 300, 277
598(9500), 507(3560), 328(23460), 218(53430)
562, 455, 378, 322
542, 510, 358, 312, 287
503, 466, 372, 310, 280
600(8900), 499(3360), 315(24500), 200(66000)
571, 460, 370, 314
549, 455, 370, 302, 285
510, 478, 351, 314, 283
599(10470), 500(4190), 335(23660), 218(50920)
581, 465, 370, 335
548, 455, 380, 312, 289
504, 465, 382, 312, 282
602(9370), 500(3700), 331(25000), 223(43640)
575, 465, 370, 331
552, 445, 365, 334, 305
1؉
1F
1G
2
The second- (12؉–52؉) and the third-step oxidised species
(1؉–53؉) display the lowest energy LMCT band in the ranges
542–552 and 503–510 nm respectively (Table 3, Fig. 4). Thus
the LMCT bands have progressively been blue shifted while
moving from 1؉–5ϩ → 12؉–52؉ → 13؉–53؉ due to oxidation
of salicylate ligand. In addition in the UV region a greater
number of bands have appeared in the cases of 13؉–53؉
(Table 3).
2؉
2F
2G
3
3؉
3F
3G
4
4؉
4F
4G
5
Discussion
5؉
5F
5G
The ruthenium()–ruthenium() couple for the complexes 1–5
(I, Fig. 2, Table 1) appears in the range 0.65–0.76 V versus SCE
whereas the same couple for the similar bipyridine complex
[Ru(bpy)2(salicylate)] has been observed at 0.22 V.3 Therefore, a
positive shift of ≈0.4–0.5 V has taken place while moving from
the bpy to L1–5 environment. This indicates that the stronger
π-acidic property of L compared to bpy provides an additional
stability to the t2(RuII) state in 1–5.15 It may be noted that the
ruthenium()–dioxolene complexes incorporating L or bpy
ancillary ligands, [RuII(L or bpy)2(dioxolene)], display oxid-
ation of the dioxolene moiety prior to electron transfer at the
metal centre.16
505, 475, 376, 290, 280
The difference in potential between the two successive
observed couples (II, I, Fig. 2) is approximately 0.3 V. Since the
separation between the two consecutive metal based processes
[RuIV/RuIII–RuIII/RuII] is known to lie in the range of 1.3–1.5
V,17 the origin of the second couple (II, Fig. 2) as the next metal
based process, ruthenium()–ruthenium() oxidation, appears
to be most unlikely. On the other hand in the similar bipyridine
complex [RuII(bpy)2(salicylate)] the salicylate ligand itself takes
part in an one-electron oxidation to the semiquinone radical
form immediately after the metal oxidation (RuII–RuIII) pro-
cess.3 Thus, by comparison with the bpy system, we ascribe
couple II for the present system (1–5) to oxidation of the
coordinated salicylate to semiquinone cation radical, structure
E in Scheme 2.
Fig. 4 UV-visible spectra of the complex [RuII(L3)2(salicylate)] 3 (—)
and its stepwise electrochemically oxidised products in acetonitrile at
298 K: (ؒؒؒؒؒ) 3؉, (-ؒ-ؒ-) 3F, (---) 3G. The inset shows the electronic
spectrum of the chemically isolated complex, [(L1)2RuIII{ϪO2C(O)-
C6H3᎐᎐C6H3(O)CO2Ϫ}RuIII(L1)2][ClO4]4ؒ2H2O 1G.
In the case of the bipyridine analogue [RuIII(bpy)(salicyl-
ate)]2ϩ the semiquinone salicylate cation radical form (E) can
afford to lose one more electron from the salicylate unit, gener-
ating a reactive cationic species which then undergoes hydroxy-
lation at the cationic centre of the salicylate moiety.3 However,
in the present case the electrogenerated cation radical (structure
E) is found to be reactive enough and instead of losing one
more electron like the bipyridine complex it follows the natural
radical recombination pathway and this leads to the formation
of a dimeric complex (structure F, Scheme 2). This assumption
of dimeric complex (F) formation (Scheme 2) through the
radical recombination pathway finds appropriate justification
based on the redox steps which are observed subsequently (see
later).
The difference in behaviour of the coordinated salicylate
semiquinone radical moiety (E) depending on the nature of the
ancillary functions (bpy and L) may be rationalised based on
their inherent difference in π-acidic property. Here the cationic
radical is stabilised by L and that is why it is difficult to oxidise
again and instead dimerises. The irreversible nature of couple II
(Fig. 2, dashed line) also indicates that the electrochemically
generated semiquinone radical (E) dimerises rapidly, therefore
on scan reversal the reduced form of E does not reappear.
The two-electron nature of couple III (Fig. 2, Table 1) provides
ligand character.12 This supports the successive addition of
electrons to the azo functions as considered above. Coulometric
reductions of the other couples (VI–VIII) (Fig. 2) generated the
unstable reduced species even at 258 K.
Electronic spectra
The solution electronic spectra of the complex 3 and its
oxidation products are displayed in Fig. 4 and the data are
shown in Table 3. The spectral profiles are similar for all the
complexes. In the visible region the complexes exhibit one
strong band near 600 nm followed by a shoulder at a higher
energy region near 500 nm. The band profiles are similar to
those of other reported complexes having the tc-RuII(L)2
moiety, therefore the observed bands are believed primarily
to be originating from the dπ(RuII) → L(π*) MLCT transi-
tions, where L(π*) is dominated by the LUMO of the
azoimine chromophore.7 In the UV region the complexes
exhibit two strong transitions due to the π–π* and n–π*
transitions of the coordinated L.13
J. Chem. Soc., Dalton Trans., 2001, 1827–1833
1831

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strong support in favour of our earlier consideration of
dimerisation (F) of the semiquinone radical (E). We believe
that this couple involves a simultaneous 2eϪ/2Hϩ-transfer
process associated with the bridging part of the dimeric
species F (Scheme 2) which eventually provides additional
conjugation to the bridging unit (structure G).5 The irre-
versible nature of couple III implies that the proton loss takes
place because electrochemical oxidation of F → G cannot
occur during electrochemical reduction of G, particularly in
the non-protic acetonitrile solvent. The diamagnetic nature
of the oxidised 12؉–52؉ species indicates that the two
ruthenium() (t2g5, S = ¹) centres in F are antiferromagnetically
Experimental
Materials
Commercial ruthenium trichloride (S.D. Fine Chemicals,
Bombay, India) was converted into RuCl3ؒ3H2O by repeated
evaporation to dryness with concentrated hydrochloric acid.
The starting complexes ctc-Ru(L)2Cl2 were prepared according
to the reported procedure.20 Salicylic acid was purchased from
Aldrich, USA. Other chemicals and solvents were reagent grade
used as received. Silica gel (60–120 mesh) and alumina (neutral)
were used for chromatographic purifications. For spectroscopic
and electrochemical studies HPLC grade solvents were used.
Commercial tetraethylammonium bromide was converted into
pure tetraethylammonium perchlorate by following an available
procedure.21
¯
²
coupled.8,18
The electronic spectra of the third-step oxidised species (13؉
–
53؉) display the lowest energy LMCT band near 500 nm which
has considerably been blue shifted as compared to the previous
step oxidised species, 12؉–52؉ (550 nm) (Table 3, Fig. 4). More-
over, the UV region of 13؉–53؉ has become more prominent
(Fig. 4) due to the introduction of additional conjugation in the
structure G. Thus spectroelectrochemical studies can provide
suitable justifications in favour of the conversion of F → G
on oxidation.
Physical measurements
UV-visible spectra were recorded by using a Shimadzu-2100
spectrophotometer, FT-IR spectra on a Nicolet spectro-
photometer with samples prepared as KBr pellets. Solution
electrical conductivity was checked using a Systronic 305 con-
ductivity bridge. Magnetic susceptibility was measured with a
PAR vibrating sample magnetometer. Cyclic voltammetric,
differential pulse voltammetric and coulometric measurements
were carried out using a PAR model 273A electrochemistry
system. Platinum wire working and auxiliary electrodes and an
aqueous saturated calomel reference electrode (SCE) were used
in a three electrode configuration. The supporting electrolyte
was [NEt4][ClO4] and the solute concentration was ≈10Ϫ3 mol
The assignment of the observed fourth-step, simultaneous
two-electron oxidation process (couple IV, Fig. 2) correspond-
ing to the ruthenium()–ruthenium() oxidation process
(structure H, Scheme 2) is made based on the fact that the
organic fragments associated with the complex moiety (G)
are in their possible oxidised form.19 The potential difference
(∆E) between the two successive oxidation processes of the
ruthenium centre (RuII/III–RuIII/IV) is observed to be ≈1.3 V
(Table 2) which matches well with earlier reported values
(≈1.3–1.5 V).17
The UV-visible spectrum of the chemically oxidised species
is identical with that of the red solution obtained separately
from the third-step (couple III) electrochemical oxidation
(Fig. 4, inset). Hence, this confirms that the HNO3 oxidation
product essentially represents the dimeric form ‘G’ as shown
in Scheme 2. The isolated oxidised complex (1G) exhibits a
ruthenium()–ruthenium(IV) couple at the same potential
(2.0 V versus SCE) as observed in the case of the correspond-
ing red solution obtained via the third-step electrochemical
oxidation.
dmϪ3
. The half-wave potential EЊ298 was set equal to
0.5 (Epa ϩ Epc), where Epa and Epc are the anodic and cathodic
cyclic voltammetric peak potentials respectively. A platinum
wire-gauze working electrode was used in coulometric experi-
ments. All experiments were carried out under a dinitrogen
atmosphere and were uncorrected for junction potentials. The
EPR measurements were made with a Varian model 109C
E-line X-band spectrometer fitted with a quartz Dewar for
measurements at 77 K (liquid nitrogen). The spectra were
calibrated by using tetracyanoethylene (tcne, g = 2.0023). The
electrospray mass spectra were recorded on a Micromass
Quattro II triple quadrupole mass spectrometer. The elemental
analyses were carried out with a Carlo Erba (Italy) elemental
analyser.
Although we were unable to grow suitable single crystals for
X-ray characterisation, the mass, microanalytical, conductivity,
1H NMR and spectroelectrochemical studies collectively
establish the composition, stereochemistry and the reactivity
pattern of the complexes.
Preparation of complexes
The complexes [RuII(L1–5)2(salicylate)] 1–5 were synthesized
from the corresponding starting [RuII(L1–5)2(EtOH)2]2ϩ species
by following a general procedure. Yields vary in the range
70–75%. Details are given for one representative complex (1).
Conclusion
We have observed that the reactivity pattern of ruthenium
salicylates incorporating strongly π-acidic azopyridine ancil-
lary ligands (L) is markedly different from that of the bipyrid-
ine analogue. The reactivity of the electrogenerated coordinated
semiquinone salicylate cation radical (E) is primarily guided by
the electronic aspects of the ancillary functions (L and bpy).
Hence, in the presence of bipyridine the salicylate semiquinone
cationic radical (E) undergoes hydroxylation via the formation
of a cationic species. On the contrary the presence of the
stronger π-acidic ‘L’ facilitates the radical recombination path-
way leading to dimerisation followed by successive electron
transfer and proton transfer at the metal and the bridging
ligand sites. To the best of our knowledge the present work
demonstrates for the first time the electro-non-innocent
behaviour of the coordinated salicylate function leading to the
formation of a new class of coordinated semiquinone salicylate
based bridging ligand. Further work is in progress in order to
scrutinise the reactivity features of ruthenium salicylates in
combination with other ancillary ligands having different
electronic natures.
[Ru(L1)2(salicylate)] 1. Initially a solution of [RuII(L1)2-
(EtOH)2]2ϩ was prepared by stirring under reflux condition a
mixture of Ru(L1)2Cl2 (100 mg, 0.186 mmol) and AgNO3 (79
mg, 0.465 mmol) in dry ethanol (20 cm3) for 1 h and removing
the AgCl precipitate. To the filtrate were added an excess of
salicylic acid (65 mg, 0.471 mmol) and NaOH (38 mg, 0.95
mmol) and the solution was heated to reflux for 12 h under a
dinitrogen atmosphere, during which time it changed from
purple to blue-violet. The solvent was then removed under
reduced pressure and the solid mass thus obtained purified by
using a alumina (neutral) column. A blue-violet band corre-
sponding to 1 was eluted with acetonitrile. On evaporation the
solid complex was obtained in 72% yield (81 mg). Finally the
product was recrystallised from 1 : 4 v/v dichloromethane–light
petroleum (bp 40–60 ЊC).
Calc. for 1: C, 57.71; H, 3.67; N, 13.92. Found: C, 57.62; H,
3.48; N, 13.75. Calc. for 2: C, 58.95; H, 4.15; N, 13.30. Found:
C, 58. 61; H, 4.40; N, 13.49. Calc. for 3: C, 51.79; H, 2.99; N,
1832
J. Chem. Soc., Dalton Trans., 2001, 1827–1833

View Article Online
5 G. K. Lahiri, S. Bhattacharya, S. Goswami and A. Chakravorty,
12.50. Found: C, 52.11; H, 2.88; N, 12.76. Calc. for 4: C, 58.95;
H, 4.15; N, 13.30. Found: C, 58.72; H, 4.44; N, 13.67. Calc. for 5:
C, 51.79; H, 2.99; N, 12.50. Found: C, 52.17; H, 3.26; N,
12.82%.
J. Chem. Soc., Dalton Trans., 1990, 561; T. Mizoguchi and R. N.
Adams, J. Am. Chem. Soc., 1962, 84, 2058; Z. Galus and R. N.
Adams, J. Am. Chem. Soc., 1962, 84, 2061; R. N. Adams,
Electrochemistry at Solid Electrodes, Marcel Dekker, New York,
1969.
6 S. Goswami, R. N. Mukherjee and A. Chakravorty, Inorg. Chem.,
1983, 22, 2825.
7 S. Goswami, A. R. Chakravarty and A. Chakravorty, Inorg. Chem.,
1981, 20, 2246; R. A. Krause and K. Krause, Inorg. Chem., 1982, 21,
1714.
8 B. Mondal, S. Chakraborty, P. Munshi, M. G. Walawalkar and
G. K. Lahiri, J. Chem. Soc., Dalton Trans., 2000, 2327; S.
Chakraborty, M. G. Walawalkar and G. K. Lahiri, J. Chem. Soc.,
Dalton Trans., 2000, 2875; B. M. Holligan, J. C. Jeffery, M. K.
Norgett, E. Schatz and M. D. Ward, J. Chem. Soc., Dalton Trans.,
1992, 3345.
9 B. K. Santra, M. Menon, C. K. Pal and G. K. Lahiri, J. Chem. Soc.,
Dalton Trans., 1997, 1387; P. Munshi, R. Samanta and G. K. Lahiri,
J. Organomet. Chem., 1999, 586 176; R. Hariram, B. K. Santra and
G. K. Lahiri, J. Organomet. Chem., 1997, 540, 155; G. K. Lahiri,
S. Bhattacharya, B. K. Ghosh and A. Chakravorty, Inorg. Chem.,
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92, 2343.
The complex 1G was prepared from 1 by using concentrated
HNO3 as follows. The starting complex [Ru(L1)2(salicylate)]
(100 mg) was taken in distilled water (20 cm3) and heated to
reflux for 15 min. Concentrated HNO3 (0.5 cm3) was added
dropwise and the mixture heated to reflux for another 1 h,
during which time it changed from blue-violet to red. The
volume was then reduced to 5 cm3 on a water bath and satur-
ated aqueous NaClO4 solution added. The mixture was kept in
a refrigerator overnight. The dark crystalline solid deposited
was collected by filtration, washed with cold water and dried
in vacuo over P4O10. The solid mass was purified by using a silica
gel column. A red band corresponding to 1G was eluted with
dichloromethane–methanol (10 : 0.5). On removal of solvent
the pure solid complex was obtained. Yield 176 mg (65%). The
complex was recrystallised from acetonitrile–benzene (1 : 5
v/v). Calc. for C58H46Cl4N12O24Ru2 1G: C, 42.50; H, 2.83; N,
10.25. Found: C, 42.81; H, 3.02, N, 10.70%.
10 E. M. Kober and T. J. Meyer, Inorg. Chem., 1983, 22, 1614.
11 P. Ghosh, A. Pramanik, N. Bag, G. K. Lahiri and A. Chakravorty,
J. Organomet. Chem., 1993, 454, 237.
Acknowledgements
12 M. D. Ward, Inorg. Chem., 1996, 35, 1712; R. Samanta, P. Munshi,
B. K. Santra and G. K. Lahiri, Polyhedron, 1999, 18, 995; A. K. Deb
and S. Goswami, J. Chem. Soc., Dalton Trans., 1989, 1635.
13 B. K. Santra and G. K. Lahiri, J. Chem. Soc., Dalton Trans., 1998,
139; A. Bharath, B. K. Santra, P. Munshi and G. K. Lahiri, J. Chem.
Soc., Dalton Trans., 1998, 2643.
14 S. Kulkarni, B. K. Santra, P. Munshi and G. K. Lahiri, Polyhedron,
1998, 17, 4365.
15 B. Mondal, M. G. Walawalkar and G. K. Lahiri, J. Chem. Soc.,
Dalton Trans., 2000, 4209; J. L. Robertson, A. Kadziola, R. A.
Krause and S. Larsen, Inorg. Chem., 1989, 28, 2097.
Financial supports received from the Department of Science
and Technology, New Delhi and Council of Scientific
and Industrial Research, New Delhi, India, are gratefully
acknowledged. Special acknowledgement is made to Regional
Sophisticated Instrumental Centre (RSIC), Indian Institute
of Technology, Bombay for providing the NMR and EPR
facilities. The referees comments at the revision stage were very
helpful.
16 C. G. Pierpont and C. W. Lange, Prog. Inorg. Chem., 1994, 41, 331;
R. A. Metcalfe and A. B. P. Lever, Inorg. Chem., 1997, 36, 4762;
F. Hartl, T. L. Snoeck, D. J. Stufkens and A. B. P. Lever, Inorg.
Chem., 1995, 34, 3887; H. Masui, A. B. P. Lever and E. S.
Dodsworth, Inorg. Chem., 1993, 32, 258; H. Masui, A. B. P. Lever
and P. R. Auburn, Inorg. Chem., 1991, 30, 2402; A. M. Barthram,
R. L. Cleary, R. Kowallick and M. D. Ward, Chem. Commun., 1998,
2695; N. Bag, A Pramanik, G. K. Lahiri and A. Chakravorty, Inorg.
Chem., 1992, 31, 40; N. Bag, G. K. Lahiri, P. Basu and A.
Chakravorty, J. Chem. Soc., Dalton Trans., 1992, 113.
17 N. Bag, G. K. Lahiri, S. Bhattacharya, L. R. Falvello and A.
Chakravorty, Inorg. Chem., 1988, 27, 4396; G. K. Lahiri,
S. Bhattacharya, M. Mukherjee, A. K. Mukherjee and A.
Chakravorty, Inorg. Chem., 1987, 26, 3359.
18 C. Trindle and S. N. Dutta, Int. J. Quantum Chem., 1996, 57, 781.
19 E. Ishow, A. Gourdon, J. P. Launay, C. Chiorboli and F. Scandola,
Inorg. Chem., 1999, 38, 1504.
20 P. Munshi, R. Samanta and G. K. Lahiri, Polyhedron, 1998, 17,
1913.
21 D. S. Sawyer, A. Sobkowiak and J. L. Roberts, Jr., Experimental
Electrochemistry for Chemists, Wiley, New York, 1995.
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