Oxidation states and redox behavior of ruthenium ammine complexes with redox-active dioxolene ligands

Article abstract of DOI:10.1016/j.ica.2012.04.002

Synthesis of ruthenium-dioxolene complexes with ammines [Ru(NH ₃)₄(diox-R)]ⁿ⁺, where diox-R is a dioxolene with substituent R = 3,5-di-tert-butyl, H, tetrachloro, and 4-nitro, was carried out. These complexes were characterized by spectroscopy, magnetic susceptibility measurements, and cyclic voltammetry. The oxidation states of ruthenium and the redox-active dioxolene ligand in the complexes varied depending on the nature of the dioxolene substituents. [Ru(NH₃) ₄(diox-R)]ⁿ⁺ (R = 3,5-di-tert-butyl and H) has a divalent ruthenium and dioxolene in the quinone oxidation state with a minor amount of the complex in Ru(III)-semiquinonato oxidation state. The complexes exhibited valence tautomerism depending on the donor number of solvents. However, [Ru(NH₃)₄(diox-R)]ⁿ⁺ (R = tetrachloro and 4-nitro) has a trivalent ruthenium and dioxolene in the catecholate oxidation state. Electrochemical and spectroelectrochemical investigation of these complexes revealed that their electrochemical behavior depended on not only the oxidation state of the prepared complex but also the nature of the dioxolene substituent in the complex. Their redox reaction was found to proceed by the EC mechanism involving valence tautomerism.

Full text of DOI:10.1016/j.ica.2012.04.002

Inorganica Chimica Acta 390 (2012) 47–52
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Oxidation states and redox behavior of ruthenium ammine complexes
with redox-active dioxolene ligands
⇑
Isao Ando , Takemasa Fukuishi, Kikujiro Ujimoto, Hirondo Kurihara
Department of Chemistry, Faculty of Science, Fukuoka University, Nanakuma 8-19-1, Jonan-ku, Fukuoka 814-0180, Japan
a r t i c l e i n f o
a b s t r a c t
Synthesis of ruthenium-dioxolene complexes with ammines [Ru(NH3)4(diox-R)]n+, where diox-R is a
dioxolene with substituent R = 3,5-di-tert-butyl, H, tetrachloro, and 4-nitro, was carried out. These com-
plexes were characterized by spectroscopy, magnetic susceptibility measurements, and cyclic voltamme-
try. The oxidation states of ruthenium and the redox-active dioxolene ligand in the complexes varied
depending on the nature of the dioxolene substituents. [Ru(NH3)4(diox-R)]n+ (R = 3,5-di-tert-butyl and
H) has a divalent ruthenium and dioxolene in the quinone oxidation state with a minor amount of the
complex in Ru(III)-semiquinonato oxidation state. The complexes exhibited valence tautomerism
depending on the donor number of solvents. However, [Ru(NH3)4(diox-R)]n+ (R = tetrachloro and 4-nitro)
has a trivalent ruthenium and dioxolene in the catecholate oxidation state. Electrochemical and spectro-
electrochemical investigation of these complexes revealed that their electrochemical behavior depended
on not only the oxidation state of the prepared complex but also the nature of the dioxolene substituent
in the complex. Their redox reaction was found to proceed by the EC mechanism involving valence
tautomerism.
Article history:
Received 18 July 2011
Received in revised form 22 March 2012
Accepted 1 April 2012
Available online 7 April 2012
Keywords:
Redox-active ligands
Oxidation states
Ruthenium
Valence tautomerism
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
as that with the dioxolene center, because the ammine ligand is
considerably poor
p-acid than the polypyridine ligand. Valence
Transition metal complexes with redox-active ligands often
exhibit unique photochemical and electrochemical behavior. Metal
complexes with dioxolene ligands have been intensively studied,
because dioxolene takes three oxidation states; quinone (bq), semi-
quinonate (sq), and catecholate (cat) [1]. The complexes may take
various oxidation states with different combinations of possible
metal and ligand oxidation states. Thus, the resulting electrochem-
ical properties make these complexes considerably complicated.
Various metal–dioxolene complexes, particularly ruthenium com-
plexes, were prepared by Pierpont and co-workers [2] and Lever
et al. [3]. In ruthenium-dioxolene complexes, the energy level of
the 4d orbital of ruthenium is close to that of p⁄ orbital of dioxolene,
resulting in delocalization between the two orbitals. When consid-
erable delocalization occurs between a redox-active metal center
and coordinated dioxolene, the assignment of oxidation states to
the metal or dioxolene may be ambiguous, particularly for
ruthenium-dioxolene complexes with polypyridines [4].
tautomerism has been observed for transition metal complexes
with dioxolenes [1]; furthermore, ruthenium ammine complexes
show remarkable solvatochromism [5]. Therefore, the oxidation
state of ruthenium-dioxolene complexes with ammines should
be significantly affected by external stimuli including chemical as
well as physical stimuli. We have reported that ruthenium ammine
complexes produced a prominent change in redox potential by
second-sphere coordination with crown ether [6]. Although
ruthenium-ammine complexes are well known, ruthenium-
dioxolene complexes with an ammine ligand have become attrac-
tive complexes for the exploration for the diversity of oxidation
states and redox behavior of the complexes with redox-active
ligands. However, only a few ruthenium-dioxolene complexes with
ammines as an ancillary ligand have been reported to dater [7].
Electron-donating and electron-withdrawing substituents have
opposite influences on the coordination ability and redox ability
of dioxolenes. The oxidation state and electrochemical behavior
of the dioxolene complex may be affected by these dioxolene
derivatives depending on the nature of the dioxolene substituent.
However, such an effect has not been observed.
Ruthenium-dioxolene complexes with polypyridine showed the
redox reaction associated with dioxolene center. In contrast, ruthe-
nium-dioxolene complexes with ammines as an ancillary ligand
may show the redox reaction associated with metal center as well
To examine the effect of the nature of the dioxolene substitu-
ents on the oxidation state of the complex and the possibility of
valence tautomerism, ruthenium-dioxolene complexes with
ammines were newly prepared using dioxolenes with different
⇑
Corresponding author.
E-mail address: isaoando@fukuoka-u.ac.jp (I. Ando).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.04.002

48
I. Ando et al. / Inorganica Chimica Acta 390 (2012) 47–52
substituents in this study. The oxidation states of the prepared
complexes were characterized and their redox behavior was
examined.
PRu: C, 12.47; H, 2.44; N, 9.69; Ru, 17.5. Found: C, 11.83; H, 2.35; N,
9.64; Ru, 18.3%.
2.2.2. cis-[Ru(NH3)4(NO2cat)](PF6)ꢁH2O (2)
The complex was prepared using a procedure similar to that
used for cis-[Ru(NH3)4(TClcat)](PF6) except that NO2catH2 was
used instead of TClcatH2. The pure complex was obtained as
blue-green solid by collecting the blue-green band separated by
column chromatography. Anal. Calc. for C6H17N5F6O5PRu: C,
14.85; H, 3.53; N, 14.43; Ru, 20.88. Found: C, 14.56; H, 3.18; N,
14.45; Ru, 22.3%.
2. Experimental
2.1. Materials and measurements
Tetrachlorocatechol (TClcatH2), 4-nitrocatechol (NO2catH2),
3,5-di-tert-butylcatechol (DBcatH2), and catechol (HcatH2) were
obtained from Aldrich, Tokyo Kasei or Wako and used as received.
The precursor complexes, [Ru(NH3)5Cl]Cl2 and cis-[Ru(NH3)4Cl2]Cl,
were prepared according to the reported procedures [8,9]. Acetoni-
trile d-3 (containing 0.03% tetramethylsilane, Aldrich) was used for
1H NMR spectra measurements. Other chemicals and solvents were
of reagent grade and used as received.
Absorption spectra were recorded on a Hitachi 228A spectro-
photometer. 1H NMR spectra were measured at 400 MHz using a
JEOL JNM/GSX-400 NMR spectrometer and chemical shifts were
obtained using tetramethylsilane as an internal standard. Cyclic
2.2.3. cis-[Ru(NH3)4(DBbq)](PF6)2ꢁH2O (3) (DBbq = 3,5-di-tert-butyl-
1,2-benzoquinone)
The complex was prepared in aerobic condition by a procedure
similar to that used for cis-[Ru(NH3)4(TClcat)](PF6) except that
DBcatH2 was used instead of TClcatH2. The pure complex was ob-
tained as a reddish purple solid by collecting the reddish-purple
band separated by column chromatography. Anal. Calc. for
C14H34N4F12O3P2Ru: C, 24.11; H, 4.62; N, 8.03; Ru, 14.5. Found: C,
24.17; H, 4.74; N, 8.27; Ru, 14.5%.
voltammetry was performed by means of
a BAS 100W/B
2.2.4. cis-[Ru(NH3)4(Hbq)](BPh4)2ꢁH2O (4) (Hbq = 1,2-benzoquinone)
The complex formation reaction was conducted using a proce-
dure similar to that used for cis-[Ru(NH3)4(TClcat)](PF6) except
that HcatH2 was used instead of TClcatH2. However, the crude
complex was isolated as the chloride salt by adding 200 cm3 ace-
tone to the reaction mixture. The chloride salt was purified by alu-
mina column chromatography. The desired blue band was eluted
with H2O and the solvent was removed by evaporation to dryness.
Sodium tetraphenylborate (NaBPh4) was added to the aqueous
solution of the chloride salt of the complex. The resulting mixture
was allowed to stand overnight in contact with air. The precipi-
tated purple complex was collected and was freely washed with
H2O. Anal. Calc. for C54H58N4B2O3Ru: C, 69.46; H, 6.04; N, 6.00;
Ru, 10.8. Found: C, 69.82; H, 6.12; N, 5.84; Ru, 9.7%.
electrochemical workstation. Voltammograms were observed in
acetonitrile solution of 0.1 mol dmꢀ3 tetrabutylammonium hexa-
fluorophosphate using a three-electrode assembly, an Ag/AgNO3
reference electrode, a glassy carbon working electrode, and a
platinum coil auxiliary electrode. Spectroelectrochemical measure-
ments were performed in an optically transparent thin-layer elec-
trolytic (OTTLE) cell [10], using a Hitachi 228A spectrophotometer
combined with a BAS 100W/B electrochemical workstation. The
OTTLE cell uses a gold minigrid (500 mesh) as a working electrode,
the platinum mesh as an auxiliary electrode, and the gold foil as a
pseudo reference electrode. Electron spin resonance (ESR) mea-
surements were carried out with a JES-REIX ESR spectrometer
(X-band). Magnetic susceptibilities were recorded between 2 and
300 K with a Quantum Design MPM-5XL SQUID susceptometer.
Analytical data for elements C, H, and N were obtained from the
Microanalytical Laboratory of Kyushu University. Atomic absorp-
tion analysis of the ruthenium content of the prepared complexes
was conducted according to Rowston’s method [11] by using a
Seiko SAS 7500A spectrophotometer.
3. Results and discussion
Because redox active dioxolene exists in oxidation states such
as cat, sq, and bq, the tetraammineruthenium complexes contain-
ing a dioxolene can exist in several oxidation states as shown in
Scheme 1. Syntheses of tetraammineruthenium-dioxolene com-
plexes were attempted by reactions of cis-tetraammine precur-
sor-complexes in Ru(II) and Ru(III) oxidation states with
dioxolenes of cat and bq forms. However, the complexes were ob-
tained only by the reaction of the precursor complex in the Ru(II)
oxidation state with dioxolene in the cat form, as described in Sec-
tion 2. The analytical data in Section 2 revealed that complexes 1
and 2 were obtained in either the Ru(III)-cat or Ru(II)-sq oxidation
state, and complexes 3 and 4 were obtained in either the Ru(II)-bq
2.2. Synthesis of complexes
2.2.1. cis-[Ru(NH3)4(TClcat)](PF6)ꢁH2O (1)
A suspended solution of cis-[Ru(NH3)4Cl2]Cl (0.29 g, 1 mmol) in
5 cm3 of a 3:2 H2O–EtOH mixture was reduced with Zn-amalgam
under a nitrogen atmosphere until the complex was completely
dissolved. A solution of TClcatH2 (0.265 g, 1 mmol) in 5 cm3 of a
3:2 H2O–EtOH mixture was added to the filtrate after the Zn-amal-
gam was removed. Then concentrated aqueous ammonia (2 mmol)
was slowly added to the resulting solution and the mixture was
heated at 55 °C under a nitrogen atmosphere for 1 h with stirring.
After cooling to room temperature, excess of NH4PF6 (1.63 g,
10 mmol) was added to the solution. The precipitated product
was collected and dried over P2O5 under reduced pressure. The
precipitate was washed three times with CHCl3 and dissolved with
a small amount of CH3CN to remove insoluble impurities. The
filtrate was evaporated to dryness under reduced pressure, and
the solid mass was purified by column chromatography using
neutral alumina. The desired blue band was eluted with EtOH.
After removal of the solvent under reduced pressure, a saturated
aqueous solution of NH4PF6 was added to the aqueous solution
of the solid mass. The precipitated blue complex was collected
and dried over P2O5 under vacuum. Anal. Calc. for C6H14N4Cl4F6O3-
Scheme 1. Schematic diagram of redox pathways for ruthenium-dioxolene com-
plex. Ru(II) and Ru(III) represent a tetraammineruthenium moiety in the bivalent
and trivalent oxidation states, respectively. Cat, sq, and bq represent dioxolene in
catecholate, semiquinonate, and benzoquinone oxidation states, respectively. VT
denotes valence tautomerism.

I. Ando et al. / Inorganica Chimica Acta 390 (2012) 47–52
49
or Ru(III)-sq oxidation state despite similar preparation proce-
dures. The oxidation state of the obtained complexes differs extre-
mely depending on the dioxolene substituent. Thus, the nature of
these substituents significantly affects the stable oxidation state
of the dioxolene complex.
considerably smaller than the theoretical value for S = 1/2. This re-
sult suggests that complexes 1 and 2 were isolated in the Ru(III)-
cat oxidation state, whereas complexes 3 and 4 were isolated in
the Ru(II)-bq oxidation state containing a small amount of the
Ru(III)-sq oxidation states.
Electron spin resonance (ESR) spectra of complexes 1–4 were
measured in the solid state and in acetonitrile solution to confirm
the existence of Ru(III) species and or semiquinonate species in the
complex and are shown in Fig. S5. Complexes 1 and 2 exhibited a
broad ESR signal at g = 2.525 and 2.819, respectively, in the solid
state at 77 K indicating the existence of a Ru(III) center [13]. On
3.1. 1H NMR spectra
1H NMR spectra of the prepared complexes were measured in
deuterated acetonitrile. For complex 3, resonance signals were ob-
served in ordinary region (0–12 ppm); the signals at 1.54 and
1.66 ppm were assigned to methyl protons of tert-butyl group,
those at 5.25 ppm to ammine protons in the cis-position toward
dioxolene, those at 6.44 and 8.21 ppm to aromatic protons, and
those at 11.02 and 11.20 ppm to ammine protons in the trans-
position toward dioxolene according to the sharpness and integral
ratios of the signals (Fig. S1). For complex 4, the signals of aromatic
protons in dioxolene and tetraphenyl borate were observed at
6–8 ppm but those of ammine protons were not observed because
of overlap with the signal of the solvent itself (Fig. S2). For com-
plexes 1 and 2, on the other hand, broad signals were observed
in the extremely low field region: 135 and 151 ppm for complex
1 and 84, 148, and 184 ppm for complex 2 (Figs. S3 and S4,
respectively). For complex 1, the signals at 135 and 151 ppm were
assigned to ammine protons; for complex 2, the signals at 84 ppm
was assigned to aromatic protons, and those at 148 and 184 ppm
were assigned to ammine protons, respectively, according to the
integral ratios [12]. Although the resonance signals were observed
in the ordinary region (0–12 ppm) for complexes 3 and 4, broad
signals were observed for complexes 1 and 2 in the extremely
low field region. These results revealed that ammine ligands bind
to the diamagnetic ruthenium center in complexes 3 and 4 but
to the paramagnetic ruthenium center in complexes 1 and 2.
the other hand, complex
3 showed only a sharp signal at
g = 2.054 in the solid state at 77 K, implying the existence of radical
species. Complex 4 showed a broad signal at g = 2.490 in addition
to a sharp signal at g = 2.063 in the solid state at 77 K, implying
the existence of Ru(III) center and radical species. At room temper-
ature, in solid state, only a sharp signal at g = 2.005 and 2.003 was
observed for complexes 3 and 4, respectively. In acetonitrile solu-
tion at 77 K, complex 1 exhibited a broad signal at g = 2.560 and
complex 3 showed only a sharp signal at g = 2.049.
The ESR spectra supported the results of magnetic susceptibility
mentioned above. Thus, rutheniumtetraammine-dioxolene com-
plexes were isolated in the Ru(III)-cat, Ru(III)-sq, and Ru(II)-bq oxi-
dation states depending on the dioxolene substituent, unlike
tetrakis(pyridine) ruthenium complexes [14].
3.3. Absorption spectra
The dioxolene-tetraammineruthenium complexes were ob-
tained at different oxidation states depending on the dioxolene
substituent. Complexes 3 and 4 were isolated at both oxidation
states simultaneously. This suggests the possibility of valence tau-
tomerism between the Ru(II)-bq and Ru(III)-sq oxidation states.
Absorption spectra of the prepared complexes were measured
in solution. The spectral characteristics are listed in Table 1. The
complexes exhibited bands ascribed to metal-to-ligand charge
transfer (MLCT) or ligand-to-metal charge transfer (LMCT) in the
visible region and the dioxolene center in the ultraviolet region.
The assignment of the charge transfer (CT) bands in the visible re-
gion was determined using solvent dependence as follows.
For rutheniumammine complexes, an MLCT or a LMCT band
shows remarkable solvatochromism; the energies of an MLCT band
decrease linearly with the donor number (DN) of the solvent,
whereas those of an LMCT band increase linearly with the DN of
the solvent [5]. Thus, the energy of the CT band was examined in
solution using solvents with various DN. For complex 2, solvato-
3.2. Magnetic properties
Magnetic susceptibility of the prepared complexes was also
measured at 2–300 K. Fig. 1 shows the temperature dependence
of the magnetic moments of the complexes. Most complexes
exhibited simple paramagnetic behavior. The estimated values of
the effective magnetic moments at 300 K were 2.01 and 1.97 BM
for complexes 1 and 2, respectively, which were close to the
theoretical value of 1.73 BM for S = 1/2. On the other hand, the esti-
mated values of the effective magnetic moments at 300 K were
0.94 and 0.07 BM for complexes 3 and 4, respectively, which were
Table 1
Spectral characteristics of the prepared complexes.
Complexes
kmax, nm (e )
max, molꢀ1 dm3 cmꢀ1
1
300 (6.36 ꢂ 103)
345 sh
in water
in water
658 (2.40 ꢂ 103)
2
3
4
220 (8.68 ꢂ 103)
258 (6.55 ꢂ 103)
330 sh
402 (6.47 ꢂ 103)
650 (1.81 ꢂ 103)
273 (6.36 ꢂ 103)
352 (1.77 ꢂ 103)
370 sh
in water
450 sh
523 (7.82 ꢂ 103)
246 (1.82 ꢂ 104)
in acetonitrile
Fig. 1. Temperature dependence of effective magnetic moment of complexes 1.
440 sh
(blue circles),
2 (green circles), 3 (red circles), and 4 (orange circles). (For
470 sh
interpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.)
516 (7.49 ꢂ 103)

50
I. Ando et al. / Inorganica Chimica Acta 390 (2012) 47–52
Fig. 2. Solvent dependence of the CT band energy for complexes 1 (h), 3 (d), and 4
(s).
Fig. 3. Solvent dependence of the deconvoluted CT band energy in the visible region
for complex 3. Inset shows the CT band in nitromethane, methanol, and
hexamethyphosphoramide.
chromism of the CT band was not successfully observed because of
the overlap of the CT band with absorption by NO2catH2 itself. As
Fig. 2 shows, the CT band of complex 1 showed solvatochromic
behavior typical of LMCT band, although the CT bands of com-
plexes 3 and 4 exhibited rather complicated behavior, i.e., similar
CT bands with a shoulder at shorter wavelength as listed in Table 1.
The CT bands of complex 3 in several solvents are shown in inset
of Fig. 3. The CT band varied with the DN of the solvent; the shoul-
der developed gradually with increasing solvent DN. This fact indi-
cates that the CT band may be composed of two components
attributable to species in different oxidation states. Thus, the CT
band was deconvoluted into two bands and the energies of the
two deconvoluted bands were plotted against the DN of the sol-
vents in Fig. 3. Two components exhibited opposite behavior;
one has a negative slope and the other has a positive slope. This
indicates the coexistence of tetraammineruthenium(II)- and tetra-
ammineruthenium(III)-species. Considering the aforementioned
data, complex 3 exists in both the Ru(II)-bq and Ru(III)-sq oxida-
tion states in solution. The inset in Fig. 3 shows that the species
in Ru(III)-sq oxidation state are predominant in solvents with a
smaller DN, but the species in Ru(II)-bq oxidation state become
predominant with increasing DN for complex 3 in solution. To
the best of our knowledge, this is the first report on ruthenium
complexes demonstrating valence tautomerism induced by
solvents.
valence tautomerism. The oxidation state of the complexes in this
study is significantly affected by the dioxolene substituent on and
other factors. Thus, the effect of dioxolene substituents on the elec-
trochemical behavior of the dioxolene complexes is very interest-
ing. To clarify their redox behavior, electrochemical and
spectroelectrochemical investigation of the tetraamminerutheni-
um-dioxolene complexes were conducted; the results are shown
in Figs. 4 and 5.
The assignment of the observed redox couple is in difficult
because the redox process is either metal centered or dioxolene
centered, as shown in Scheme 1. Ruthenium ammine complexes
reportedly exhibited a change in redox potential toward negative
potential by adduct formation with crown ether [6,15]. The mag-
nitude of the change is large for the redox process associated
with the metal center (larger than ca. 100 mV) but small for that
associated with the aromatic ligand center (less than ca. 50 mV).
The redox couple was judged to involve either the metal-
centered or dioxolene-centered redox process according to the
magnitude of the change in the redox potential on addition of
a 100-fold excess of crown ether to the complex. The spectro-
electrochemical experiments also confirmed whether the redox
processes might involve isomeric equilibrium between valence
tautomers.
Complex 1 showed two reversible redox couples at ꢀ1.039 and
0.290 V versus (Ag+/Ag), as shown in Fig. 4a. These redox couples
are ascribed to the redox process of Eq. (1) or (2) and of Eq. (3)
or (4) considering the oxidation state at a rest potential of ca. 0 V
versus (Ag+/Ag) (the oxidation state of the isolated complex,
Ru(III)-cat).
3.4. Electrochemistry
As shown in Scheme 1, the redox reaction of ruthenium-dioxo-
lene complexes may proceed through versatile pathways including

I. Ando et al. / Inorganica Chimica Acta 390 (2012) 47–52
51
Fig. 5. (a) Cyclic voltammograms of complex 3 in the absence (black line) and
presence (red line) of 18-crown-6 ether in acetonitrile. Scan rate was 50 mV sꢀ1
.
[complex] = 1.0 ꢂ 10ꢀ3 mol dmꢀ3
,
[18-crown-6 ether] = 0 or 0.1 mol dmꢀ3
at (1) 0, (2)
. (b)
Spectral changes during electrochemical reduction of complex
3
ꢀ0.25, (3) ꢀ0.28, (4) ꢀ0.31, and (5) ꢀ0.34 V vs. pseudo Au. (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of
this article.)
Fig. 4. (a) Cyclic voltammograms of complex 1 in the absence (black line) and
presence (red line) of 18-crown-6 ether in acetonitrile. Scan rate was 50 mV sꢀ1
.
[complex] = 1.0 ꢂ 10ꢀ3 mol dmꢀ3, [18-crown-6ether] = 0 or 0.1 mol dmꢀ3. (b) Spec-
tral changes during electrochemical oxidation of complex 1 at (1) 0, (2) 0.30, (3)
0.32, (4) 0.34, (5) 0.36, and (6) 0.40 V vs. pseudo Au. (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of
this article.)
electrochemically oxidized in an OTTLE cell [10] at 0–0.4 V versus
pseudo Au. The band at ca. 650 nm disappeared as oxidation pro-
gressed, whereas the band at ca. 500 nm developed, as shown in
Fig. 4b. This spectral change indicates the oxidation of the
Ru(III)-cat oxidation state to the Ru(II)-bq oxidation state, referring
to the spectral characteristics of the complexes in Table 1. There-
fore, in the redox process at 0.290 V versus (Ag+/Ag), complex 1
was oxidized from the species in the Ru(III)-cat oxidation state to
those in the Ru(III)-sq oxidation state, and the species in the
Ru(III)-sq oxidation state equilibrated with the valence tautomer
in the Ru(II)-bq oxidation state. Similar results were obtained for
complex 2 (Fig. S6).
Fig. 5 shows the results of similar measurements for complex 3.
Complex 3 showed three major redox waves; an irreversible redox
couple at ꢀ1.205 V versus (Ag+/Ag), a reversible redox couple at
ꢀ0.329 V versus (Ag+/Ag), and an oxidation peak at 0.861 V versus
(Ag+/Ag). Furthermore, two minor redox waves were observed in
the region of 0–0.6 V versus (Ag+/Ag); reduction peak at 0.124 V
versus (Ag+/Ag) and a reversible redox couple at 0.468 V versus
(Ag+/Ag). These waves were unobservable at a scan rate of
5 mV sꢀ1 but developed with increasing scan rate.
RuðIIIÞ-cat þ eꢀ ꢀ RuðIIÞ-cat
RuðIIIÞ-cat ꢀ RuðIIÞ-sq
ð1Þ
ð2Þ
RuðIIÞ-sq þ eꢀ ꢀ RuðIIÞ-cat
RuðIIIÞ-sq þ eꢀ ꢀ RuðIIIÞ-cat
RuðIIÞ-bq ꢀ RuðIIIÞ-sq
ð3Þ
ð4Þ
RuðIIÞ-bq þ eꢀ ꢀ RuðIIÞ-sq
RuðIIÞ-sq ꢀ RuðIIIÞ-cat
As shown in Fig. 4a, both couples were shifted toward negative
potentials by the addition of a 100-fold excess of 18-crown-6 ether.
The shift in the redox couple at ꢀ1.039 V versus (Ag+/Ag)
(ꢀ186 mV) was considerably larger, whereas that in the redox
couple at 0.290 V versus (Ag+/Ag) (ꢀ53 mV) is smaller. These
voltammetric results indicate that the couples in the cathodic
and anodic regions correspond to the metal-centered and dioxo-
lene-centered processes, respectively. Thus, the redox couple at
ꢀ1.039 and the redox couple at 0.290 V versus (Ag+/Ag) were
assigned to the processes of Eqs. (1) and (3), respectively.
The major redox waves at ꢀ1.205, ꢀ0.329, and 0.861 V versus
(Ag+/Ag) were shifted by ꢀ232, ꢀ73, and +131 mV, respectively,
by the addition of a 100-fold excess of 18-crown-6 ether (see
Fig. 5a) and were assigned as shown below. Because the valence
tautomeric equilibrium, Eq. (5), holds at the rest potential of ca.
0 V versus (Ag+/Ag), as mentioned in Section 3.3, the oxidation
wave at
To confirm that the redox process at 0.290 V versus (Ag+/Ag)
involves the valence tautomeric equilibrium, the change in the
absorption spectra was measured when complex
1
was

52
I. Ando et al. / Inorganica Chimica Acta 390 (2012) 47–52
RuðIIIÞ-sq ꢀ RuðIIÞ-bq
ð5Þ
of the prepared complexes varied depending on the dioxolene sub-
RuðIIIÞ-bq þ eꢀ RuðIIÞ-bq
RuðIIIÞ-bq þ eꢀ RuððIIIÞ-sq
RuðIIÞ-bq þ eꢀ ꢀ RuðIIÞ-sq
RuðIIIÞ-sq þ eꢀ ꢀ RuðIIÞ-sq
RuðIIIÞ-sq þ eꢀ ꢀ RuðIIIÞ-cat
RuðIIÞ-sq þ eꢀ ꢀ RuðIIÞ-cat
RuðIIIÞ-cat þ eꢀ ꢀ RuðIIÞ-cat
RuðIIÞ-sq ꢀ RuðIIIÞ-cat
ð6Þ
ð7Þ
stituent. For complex 3, both valence tautomers exist in the Ru(III)-
sq and Ru(II)-bq oxidation state in the solid state. In the solution,
the ratio of the tautomers varied with the DN of the solvents.
The reduction and oxidation of the complexes can proceed through
several pathways, as shown in Scheme 1. Complexes 1 and 2 un-
dergo reduction–oxidation via the EC mechanism involving a va-
ð8Þ
ð9Þ
ð10Þ
ð11Þ
ð12Þ
ð13Þ
lence tautomeric equilibrium, whereas complexes
3 and 4
undergo reduction–oxidation via rather complicated pathways
involving two valence tautomeric equilibria. The redox pathway
of a dioxolene complex is significantly affected by the properties
of the dioxolene substituents, and valence tautomerism may play
an important role in changing the redox pathway of dioxolene
complexes of ruthenium. Unfortunately, the effect of temperature
on the valence tautomerism was not observed in this study, and
the investigations of the other factors are in progress.
0.861 V versus (Ag+/Ag) was ascribed to the oxidation process of Eq.
(6) or (7). As the potential shift in positive direction on the addition
of 18-crown-6 ether does not imply the metal-centered process, the
oxidation wave was assigned to the oxidation process of Eq. (7). The
redox couples in the cathodic region were similarly assigned. The
redox couple at ꢀ0.329 V versus (Ag+/Ag) was ascribed to the redox
process of Eq. (8)–(10) and the redox couple at ꢀ1.205 V versus
(Ag+/Ag) is ascribed to that of Eq. (11) or (12). Because the large
negative shift in the potential of both couples on the addition of
18-crown-6 ether is associated with the metal-centered process,
the redox couples at ꢀ0.329 and ꢀ1.205 V versus (Ag+/Ag) were
assigned to the redox processes of Eq. (9) and (12), respectively.
The spectral change during electrochemical reduction at ca ꢀ0.3 V
in Fig. 5b indicates the reduction of the Ru(II)-bq oxidation state
to the Ru(III)-cat oxidation state, as can be seen by comparing the
spectral characteristics of the complexes in Table 1. Therefore, com-
plex 3 was reduced from the species in the Ru(III)-sq oxidation state
equilibrated with the Ru(II)-bq oxidation state to the species in the
Ru(II)-sq oxidation state at -0.329 V versus (Ag+/Ag). Furthermore,
the species in the Ru(II)-sq oxidation state equilibrated with those
in the Ru(III)-cat oxidation state of the valence tautomer formed
by intramolecular electron transfer. Thus, the Ru(III)-sq/Ru(III)-cat
process proceeds through an EC mechanism; the species in the
Ru(III)-sq oxidation state is reduce to the species in the Ru(III)-cat
oxidation state via the species in the Ru(II)-sq oxidation state.
The minor redox waves in the region of 0–0.6 V versus (Ag+/Ag)
may be tentatively assigned to redox associated with the species of
Ru(II)-bq equilibrated with the species in the Ru(III)-sq oxidation
state; the reduction wave at 0.124 V versus (Ag+/Ag) may be as-
signed to the reduction of Ru(II)-bq to Ru(II)-sq, and the redox cou-
ple at 0.468 V versus (Ag+/Ag) may be assigned to the Ru(III)-bq/
Ru(II)-bq redox couple. The latter assignment is consistent with
the shift magnitude of the redox potential on addition of crown
ether (ꢀ140 mV). Similar results were obtained for complex 4
although the oxidation wave ascribed to the oxidation of Ru(III)-
sq to Ru(III)-bq was not observed owing to the oxidation of
tetraphenylborate as a counter ion. (Fig. S7).
Acknowledgments
The authors thank Dr. H. Tanaka of Fukuoka University for ESR
measurements. This work was partly supported by the ’’Nanotech-
nology Support Project’’ of the Ministry of Education, Culture,
Sports, Science and Technology (MEXT), Japan.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ica.2012.04.002.
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4. Conclusions
In this study, dioxolene complexes of ruthenium were synthe-
sized using ammines as an ancillary ligand. The oxidation state