Synthesis and electrochemical, spectral and catalytic properties of diphosphine-polypyridyl ruthenium complexes

Article abstract of DOI:10.1016/j.poly.2005.10.009

Two new ruthenium aqua complexes containing phosphines and polypyridine as ligands were synthesized: [Ru(L)(totpy)(H₂O)](PF₆) ₂ (L = 1,2-bis(dimethylphosphino) ethane and 1,2- bis(dichlorophosphino)ethane; totpy = 4′-(4-tolyl)-2,2′:6′, 2″-terpyridine). The synthetic routes for both complexes are similar. The complexes were characterized by elemental analysis, cyclic voltammetry and differential pulse voltammetries, UV-Vis and EPR spectra. E_pa versus pH studies are consistent with a one-electron, one-proton transfer; at high pH, potentials become pH independent. Catalytic tests with benzyl alcohol, cyclohexanol, cyclohexene, ethylbenzene, 1,2-butanediol and 1,4-butanediol were carried out using the oxo complexes, which are electrochemically generated from the corresponding aqua complexes, in solution and immobilized in carbon paste and ITO/Nafion electrodes. In the presence of excess substrates, the cyclic voltammograms of the complexes and of the modified electrodes show an enhancement of the oxidation peaks and a decrease of the reduction peaks upon potential reversal, indicating catalytic activities. Plots of the anodic peak currents (I_pa) against organic substrate concentrations showed straight lines, indicating pseudo-first-order kinetics behavior. For the modified electrodes the plots of I_pa versus ν^1/2 showed that the anodic peak currents change linearly with the square root of the scan rate; this is characteristic of surface electrocatalysis with the catalyst confined to the electrode surface with diffusion control of the substrates.

Full text of DOI:10.1016/j.poly.2005.10.009

Polyhedron 25 (2006) 1457–1463
www.elsevier.com/locate/poly
Synthesis and electrochemical, spectral and catalytic properties of
diphosphine–polypyridyl ruthenium complexes
*
´
Eliana M. Sussuchi, Andreia A. de Lima, Wagner F. De Giovani
ˆ
Departamento de Quı´mica, Faculdade de Filosofia, Ciencias e Letras de Ribeira˜o Preto, Universidade de Sa˜o Paulo, Campus USP,
Av. Bandeirantes 3900, 14040-901 Ribeira˜o Preto, Sa˜o Paulo, Brazil
Received 14 March 2005; accepted 17 October 2005
Available online 28 November 2005
Abstract
Two new ruthenium aqua complexes containing phosphines and polypyridine as ligands were synthesized: [Ru(L)(totpy)(H2O)](PF6)2
(L = 1,2-bis(dimethylphosphino) ethane and 1,2-bis(dichlorophosphino)ethane; totpy = 40-(4-tolyl)-2,20:60,200-terpyridine). The synthetic
routes for both complexes are similar. The complexes were characterized by elemental analysis, cyclic voltammetry and differential pulse
voltammetries, UV–Vis and EPR spectra. Epa versus pH studies are consistent with a one-electron, one-proton transfer; at high pH,
potentials become pH independent. Catalytic tests with benzyl alcohol, cyclohexanol, cyclohexene, ethylbenzene, 1,2-butanediol and
1,4-butanediol were carried out using the oxo complexes, which are electrochemically generated from the corresponding aqua complexes,
in solution and immobilized in carbon paste and ITO/Nafionꢀ electrodes. In the presence of excess substrates, the cyclic voltammograms
of the complexes and of the modified electrodes show an enhancement of the oxidation peaks and a decrease of the reduction peaks upon
potential reversal, indicating catalytic activities. Plots of the anodic peak currents (Ipa) against organic substrate concentrations showed
straight lines, indicating pseudo-first-order kinetics behavior. For the modified electrodes the plots of Ipa versus m1/2 showed that the ano-
dic peak currents change linearly with the square root of the scan rate; this is characteristic of surface electrocatalysis with the catalyst
confined to the electrode surface with diffusion control of the substrates.
ꢁ 2005 Elsevier Ltd. All rights reserved.
Keywords: Ruthenium complexes; Oxo complexes; Phosphine ligands; Polypyridyl ligand; Modified electrodes; Electrocatalysis
1. Introduction
several organic substrates [3,6–13]. The orbital and ener-
getic properties of RuIV@O2+ promote complex mecha-
nisms; these complexes are remarkably versatile oxidants
mechanistically in providing pathways for atom transfer,
C–H insertion, and proton-coupled electron transfer
[3,11,14–16]. Some attempts to combine the oxidative
properties of these complexes in solution with the advan-
tage offered by modified electrodes, with respect to their
potential application in electrocatalysis, have been
described [17–19]. In this context, ruthenium(II) complexes
have been immobilized within Nafionꢀ films on electrodes
and in carbon paste electrodes, and catalytic activities in
the presence of alcohols have been studied [7,20–23].
In this work, we have synthesized the com-
plexes [Ru(L)(totpy)(H2O)](PF6)2 (L = 1,2-bis(dimethyl-
phosphino)ethane and 1,2-bis(dichlorophosphino)ethane;
Ruthenium-aqua/oxo complexes containing polypyridyl
and bidentate phosphine ligands have been recently
investigated [1–3]; several complexes containing tertiary
phosphine ligands were previously studied [4,5]. Phosphine
ligands can modify the oxidation chemistry of ruthenium-
aqua/oxo complexes because they provide a broad variety
of well-characterized steric and electronic ligand properties
[4].
The reactivity of high oxidation state ruthenium(IV)
polypyridyl complexes has played an increasingly impor-
tant role in studies involving electrocatalytic processes of
*
Corresponding author. Tel.: +55 16 360 23810; fax: +55 163 633 8151.
E-mail address: wfdgiova@usp.br (W.F. De Giovani).
0277-5387/$ - see front matter ꢁ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2005.10.009

1458
E.M. Sussuchi et al. / Polyhedron 25 (2006) 1457–1463
totpy = 40-(4-tolyl)-2,20:60,200-terpyridine); their spectral,
electrochemical and catalytic properties were studied. The
ability of the complexes to act as catalysts for electrooxida-
tions of benzyl alcohol, cyclohexanol, cyclohexene, ethyl-
benzene, 1,2-butanediol and 1,4-butanediol were studied
in homogeneous solution and with the complexes immobi-
lized in carbon paste electrodes and within Nafionꢀ films
on electrodes.
solid was recrystallized from ethanol to obtain 1.01 g
(54%) of light green crystals, identified as 40-(4-tolyl)-
1
2,20:60,200-terpyridine (totpy) (Fig. 1). H NMR: m, 8.65–
00
00
0
0
00
8.85 (H3,3 ; H6,6 ; H3 ,5 ); m, 7.80–8.00 (2Ho; H4,4 ); m,
00
7.35–7.45 (2Hm; H5,5 ); s, 2.47 (–CH3). ESI MS: m/z
323.20. Anal. Calc. for C22H17N3: C, 81.71; H, 5.30; N,
12.99. Found: C, 82.09; H, 5.28; N, 13.38%.
2.4. Preparation of the complexes
2. Experimental
[RuCl3(totpy)] was prepared according to the literature
procedure [1,2]. Yield: 85%. Anal. Calc. for
C22H17N3Cl3Ru: C, 49.78; H, 3.23; N, 7.92. Found: C,
48.95; H, 3.28; N, 7.67%.
2.1. Materials
Water was doubly distilled from alkaline potassium
permanganate. RuCl3 Æ xH2O, 1,2-bis(dichlorophosphine)-
ethane and 1,2-bis(dimethylphosphine) ethane were pur-
chased from Aldrich Chemical Co. Methylene chloride
and acetonitrile were kept in an alumina column before
use. All other reagents and solvents were of reagent quality
and were used without further purification.
2.4.1. [Ru(L)(totpy)Cl](PF6) (L = 1,2-
bis(dimethylphosphino)ethane)
A mixture of [RuCl3(totpy)] (0.38 g, 0.71 mmol) and 1,2-
bis(dimethylphosphino)ethane (0.15 g, 1.00 mmol) were
heated to reflux, under a N2 atmosphere, in 115 mL of
3:2 ethylene glycol:water in the presence of NEt3
(0.5 mL) and LiCl (0.15 g, 3.55 mmol) for 2.5 h. The hot
solution was filtered and 2.0 mL of a saturated aqueous
solution of NH4PF6 was added to the filtrate. The resulting
red-brown crystals were washed with cold water and dried
under vacuum. The product was recrystallized by dissolu-
tion in acetone and precipitated by adding diethyl ether.
Yield: 0.32 g (60%). The complex is pure enough to be used
in the synthesis of the aqua complex described in Section
2.4.3.
2.2. Instrumentation and measurements
Routine UV–Vis spectra were obtained in 1-cm quartz
cells by using a Hewlett–Packard 8453 spectrophotometer.
Electrochemical experiments were conducted by using a
PAR Model 273A Potentiostat/Galvanostat. Cyclic vol-
tammetric and differential pulse voltammetric experiments
were performed in a one-compartment cell using a carbon
working electrode, a platinum wire auxiliary electrode and
a saturated calomel (SCE) or a Ag/AgCl reference elec-
trode. The E1/2 values for reversible couples were calcu-
lated from half the difference between Ep values for the
cathodic and anodic waves. The catalytic tests were per-
formed at 25 1 ꢂC. The pH values of aqueous solutions
were buffered at an ionic strength of 0.25 mol Lꢀ1 by
HNO3 and NaNO3 (pH 2.0), potassium hydrogen phtha-
late, HNO3 and NaNO3 (pH 3.0), potassium hydrogen
phthalate, NaOH and NaNO3 (pH 4.0–5.5), Na2HPO4
and NaH2PO4 (pH 6.0–8.0), Na2B4O7 Æ 10H2O and NaNO3
(pH 9.0) and Na2B4O7 Æ 10H2O and NaOH (pH 10.0–10.8).
Elemental analyses were performed using a CHNS-O CE
Instruments Model EA 1110 elemental analyzer. Ruthe-
nium analyses were performed with a Model V-85 B Braun
ICP-AES-atomic emission spectrometer. EPR spectra of
the solid complexes were recorded at room temperature
on a Bruker EP 200D spectrometer. 1H NMR spectra were
obtained with a Bruker AC-80 spectrometer. Mass spectra
were obtained with a Hewlett–Packard gas chromato-
graph/MS system 5988A.
2.4.2. [Ru(L0)(totpy)Cl](PF6) (L0 = 1,2-
bis(dichlorophosphino)ethane)
The complex was prepared analogously to that des-
cribed in the above section, except that a 4:1 ratio of aceto-
nitrile:water and N-ethylmorpholine (30 drops) were used.
Yield: 0.40 g (68%). The complex is pure enough to be used
in the synthesis of the aqua complex described in Section
2.4.4.
CH3
m
o
5'
3'
3"
6"
2.3. Preparation of the ligand 40-(4-tolyl)-2,20:60,200-
terpyridine (totpy)
3
4"
5"
N
1'
4
5
N
1"
N
1
The 40-(4-tolyl)-2,20:60,200-terpyridine ligand was pre-
pared analogously to the literature procedure [24,25]; a
2:1 ratio of 2-acetylpyridine:p-tolualdehyde was used. The
6
Fig. 1. 40-(4-Tolyl)-2,20:60,200-terpyridine (totpy).

E.M. Sussuchi et al. / Polyhedron 25 (2006) 1457–1463
1459
2.4.3. [Ru(L)(totpy)(OH2)](PF6)2 (L = 1,2-
bis(dimethylphosphino)ethane) (1)
1.8
1.5
1.2
0.9
0.6
0.3
0.0
One hundred and seventy milligrams (0.23 mmol) of
[Ru(L)(totpy)Cl](PF6) and 0.07 g (0.27 mmol) of silver p-
toluenesulfonate were heated to reflux in 46 mL of 1:1
acetone:water under a N2 atmosphere for 1 h; the hot solu-
tion was filtered and 3.0 mL of a saturated aqueous solu-
tion of NH4PF6 was added to the filtrate. The volume
was reduced to 20 mL in a rotatory evaporator. The brown
crystals were collected by filtration, washed twice with cold
water and dried under vacuum. The complex was reprecip-
itated twice from acetone–diethyl ether. Yield: 0.14 g
(69%). Anal. Calc. for C28H35N3OP4F12Ru: C, 39.11; H,
4.00; Ru, 11.47. Found: C, 39.44; H, 4.29; Ru, 11.46%.
300
400
500
600
700
800
2.4.4. [Ru(L0)(totpy)(OH2)](PF6)2 (L0 = 1,2-
bis(dichlorophosphino)ethane) (2)
λ/nm
The complex was prepared analogously to 1, except that
a 1:2 ratio of acetone:water was used. Yield: 0.10 g (43%).
Anal. Calc. for C24H23ON3Cl4P4F12Ru: C, 29.90; H, 2.40;
Cl, 14.7; Ru, 10.48. Found: C, 31.40; H, 2.39; Cl, 14.2;
Ru, 10.27%.
Fig. 2. UV–Vis spectrum of 1.0 · 10ꢀ4 mol Lꢀ1 [Ru(L)(totpy)-
(H2O)](PF6)2 (1) (L = 1,2-bis(dimethylphosphino)ethane) in CH2Cl2.
10
8
a
2.5. Modified electrodes
2.5.1. Carbon paste electrodes
6
Carbon paste electrodes [7,20] containing 3:2 by weight
of carbon powder: mineral oil were used for cyclic voltam-
metric studies. The modified electrodes contained 3:22 by
weight of complex:paste. The carbon paste holder was con-
structed of a thick-walled Teflon tube with 0.3 cm i.d. A
copper sleeve equipped with a copper wire plunger was
mounted at the top of the Teflon tube. By rotating the
sleeve, the plunger could be made to extrude a used paste
layer which was sliced off to form a fresh paste surface.
4
2
0
-2
-4
-6
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
2.5.2. Nafionꢀ transparent ITO modified electrode [21–23]
Sixty microliters of Nafionꢀ ethanolic solution (2.5%)
was cast on the ITO electrode surface (effective
area = 1.0 cm2); the solvent was air evaporated at room
temperature; the coated electrode was dipped in pure water
for 1 h and then soaked in an aqueous solution of the com-
plex (2.0 mmol Lꢀ1) for 30 min.
E (V vs Ag/AgCl)
b
5
4
3
2
1
3. Results and discussion
3.1. Spectroscopic and redox properties of the aqua
complexes in solution
Fig. 2 shows the absorption spectrum of 1 in methylene
chloride. Cyclic and differential pulse voltammograms of 2
in methylene chloride are shown in Fig. 3. Table 1 shows
spectroscopic and redox data for the complexes. The aqua
complexes have similar spectra. Both aqua complexes
exhibit several very intense bands in the UV region,
assignable to polypyridyl ligand p ! p* excitations; in
the visible region the bands at kmax = 490 nm (e =
0.4
0.6
0.8
1.0
1.2
1.4
1.6
E (V vs Ag/AgCl)
Fig. 3. (a) Cyclic voltammogram and (b) differential pulse voltammogram
of 1.0 · 10ꢀ3mol Lꢀ1 [Ru(L0)(totpy)(H2O)](PF6)2 (2) (L0 = 1,2-bis(di-
chlorophosphino) ethane) in CH2Cl2 + 0.1 mol Lꢀ1 TBAP, glassy carbon
working electrode; (a) m = 100 mV sꢀ1; (b) 10 mV sꢀ1
.

1460
E.M. Sussuchi et al. / Polyhedron 25 (2006) 1457–1463
Table 1
Electrochemical and spectral data for the complexes
Complexes
E1/2 (V)
k )
max/nm (emax · 103/molꢀ1 L cmꢀ1
RuIII/II
RuIV/III
[RuCl3(totpy)]
[Ru(L)(totpy)(H2O)](PF6)2 (1)
+0.05a
+0.85
+0.46
+0.96
+0.57
226 (17), 285 (16), 310 (13), 406 (4.7)b
285 (17), 305 (18), 414 (3.7), 490 (1.9)e
+1.02c
+0.66d
+1.26c
+0.77d
[Ru(L0)(totpy)(H2O)](PF6)2 (2)
285 (39), 310 (41), 475 (7.9)e
a
MeCN + 0.1 mol Lꢀ1 TBAP; glassy carbon working electrode; m = 100 mV sꢀ1
.
b
c
CH3CN.
CH2Cl2 + 0.1 mol Lꢀ1; glassy carbon working electrode; m = 100 mV sꢀ1
.
d
e
In 7:3 phosphate buffer (l = 0.20 mol Lꢀ1): tert-butyl alcohol solution, pH 6.8.
CH2Cl2.
1.9 · 103 mol Lꢀ1 cmꢀ1
)
and 475 nm (e = 7.9 · 103
1 and 2 aqua complexes show no EPR signals, indicating
the oxidation state Ru(II).
mol Lꢀ1 cmꢀ1) for 1 and 2, respectively, can be attributed
to metal-to-ligand dp(Ru) ! p* (diphosphine) transitions.
One additional band is observed for 1 at 414 nm
(e = 3.7 · 103 mol Lꢀ1 cmꢀ1) and can be attributed to
metal-to-ligand dp(Ru) ! p* (polypyridyl) transitions
[26]. For 1, the MLCT band appears at higher wavelength;
this reflects the electron-donation effect of the methyl sub-
stituents on the phosphine ligand, which destabilize the
dp(Ru) electrons, causing the dp(Ru) ! p* (diphosphine)
transitions to occur at lower energy. On the other hand,
for 2, the MLCT band appears at higher energy, due to
the electron-withdrawing effect of the chloro substituents
on the phosphine ligand; similar effects were observed for
a substituted bipyridine ligand [10]. It can be observed that
the MLCT bands are shifted to higher energies upon sub-
stitution of the anionic chloride ligand for the neutral aqua
ligand.
3.2. E1/2 versus pH of the aqua complexes
The potentials of the RuIV/III couple of 1 and the poten-
tials of the RuIII/II couple of 2 vary with the pH of the solu-
tion, resulting in the E1/2 versus pH variations shown in
Fig. 4 for 2. Within the 8.3 > pH > 2.0 region, the plot
for 1 is a straight line, with slope ꢀ0.062; for 2, within
the 9.6 > pH > 2.0 region, the straight line has slope
ꢀ0.059; the slopes are very close and coincident with the
Nernstian prediction of ꢀ0.059 V/pH unit, indicative of
one-electron oxidation accompanied by the dissociation
of one proton [7,28] (Schemes 2 and 3).
For 1, the potentials become pH independent at
pH > 8.3, indicating that, in this region, [RuIII(L)-
(totpy)(O)]+ is the dominant species (Scheme 4).
The same behavior is shown for 2 at pH > 9.6; in this
case [RuII(L0)(totpy)(OH)]+ is the dominant species
(Scheme 5):
The cyclic voltammograms for both 1 and 2 in meth-
ylene chloride exhibit two reversible redox waves corre-
sponding to the RuIII/II and RuIV/III couples (0.85/1.02
and 0.96/1.26 V versus Ag/AgCl, respectively). The oxo-
aqua ligands allow rapid proton transfer to be concomi-
tant with electron transfer, permitting the accessibility of
several oxidation states via gain or loss of protons
(Scheme 1). The redox potentials of 1 are less positive,
reflecting the electron-donation effect of the methyl sub-
stituents on the phosphine ligand. The observed poten-
tials in methylene chloride are more positive than those
obtained in aqueous solution (Table 1); for 2, the separa-
tion between the two couples (mV) is larger than in
water. This behavior can be due to a very slow deproto-
nation of the first oxidized species; the deprotonation of
the aqua ligand to form RuIII–OH is facilitated in aque-
ous solution but not in methylene chloride (low protic
and polar solvent).
0.9
0.8
0.7
0.6
0.5
0.4
0.3
EPR spectra of ruthenium(III) complexes show g tensor
values [27] that can be used to derive ground states. The
2
4
6
8
10
12
pH
-e, -H+
IV
II
-e, -H+
III
[Ru=O]2+
[Ru-OH]2+
[Ru-OH2]2+
Fig. 4. Plot of E1/2 vs. pH for the RuIII/II redox couple of
[Ru(L0)(totpy)(H2O)](PF6)2 (2) (L0 = 1,2-bis(dichlorophosphino)ethane);
glassy carbon working electrode.
+e, +H+
+e, +H+
Scheme 1.

E.M. Sussuchi et al. / Polyhedron 25 (2006) 1457–1463
1461
-e-, -H+
reaction) induces a stabilization by resonance with a conju-
gated unsaturation at the carbon ring. Nevertheless, as
cyclohexene has fewer canonic structures than benzyl
alcohol, the lower positive charge delocalization decreases
the reactivity. The higher reactivity of a primary alcohol
compared to a secondary one was also observed in homo-
geneous catalysis by using [Ru(tpy)(bpy)(H2O)]2+ [8].
A generalization can be made: benzyl alcohol and 1,4-
butanediol are more reactive than cyclohexanol;
cyclohexene has an intermediate reactivity; ethylbenzene
and 1,2-butanediol have the lowest reactivities.
[RuIII(L)(totpy)(OH)]2+
[RuII(L')(totpy)(OH2)]2+
[RuIII (L)(totpy)(O)]+
[RuIV(L)(totpy)(O)]2+
[RuIII(L')(totpy)(OH)]2+
[RuIV(L)(totpy)(O)]2+
+e-, +H+
Scheme 2.
-e-, -H+
+e-, +H+
Scheme 3.
-e-
+e-
3.4. Electrocatalytic oxidation of organic substrates at a
carbon paste electrode (CPE) containing 1 and 2
Scheme 4.
The cyclic voltammetric behavior of the CPE contain-
ing 1 and 2 in 7:3 phosphate buffer (pH 6.8):tert-butyl
alcohol solutions, in the presence of benzyl alcohol,
cyclohexanol, 1,4-butanediol, 1,2-butanediol and ethyl-
benzene shows pronounced enhancement of the anodic
currents. The observed reactivity order for 1 is benzyl
alcohol > cyclohexanol > 1,4-butanediol > 1,2-butanediol
> cyclohexene > ethylbenzene and for 2 is benzyl alco-
hol > ethylbenzene > cyclohexanol > 1,4-butanediol cyclo-
hexene > 1,2-butanediol. The reactivities of CPE
containing 1 and 2 were somewhat different from those
of homogeneous catalysis; surprisingly, high reactivity
in the oxidation of ethylbenzene by 2 was observed.
Higher reactivity of the primary alcohol was also
observed in homogeneous catalysis by using 1 and 2.
The high reactivity of benzyl alcohol can be due to its
higher permeability into the active surface of the carbon
paste electrode.
3.3. Electrocatalytic oxidations of organic substrates in
homogeneous solution
The catalytic activities of 1 and 2 for the oxidation of
organic substrates were investigated by cyclic voltammetric
experiments in 7:3 phosphate buffer (pH 6.8):tert-butyl
alcohol solutions. Mixed solvent systems have been used
by other groups for similar experiments with no adverse
effects [12]. In the presence of a 50-fold excess of substrate,
the cyclic voltammograms of 1 and 2 show an enhancement
of the oxidation peaks and a decrease of the reduction
peaks upon reversal potential scan. This is indicative of cat-
alytic activity [7].
Catalytic tests can give the reactivity order for the elect-
rooxidations: for 1, benzyl alcohol > 1,4-butanediol >
cyclohexanol > cyclohexene > ethylbenzene > 1,2-butaned-
iol; for 2, 1,4-butanediol > benzyl alcohol > cyclohexa-
nol > cyclohexene > 1,2-butanediol > ethylbenzene.
The
For quantitative evaluation of the electrocatalysis sur-
face, the dependence of the peak height on the benzyl alco-
hol concentration and on the potential scan rate was
studied [30]. The peak height was linearly dependent
(Fig. 5) (correlation coefficient 0.998, with plot intercept
near zero) on the concentration of benzyl alcohol, indicat-
ing that the electrocatalysis is pseudo-first-oder in this
oxidation products were identified in bulk electrolyses
experiments using polypyridyl and phosphine ruthenium
complexes described previously [6–10]: benzyl alcohol was
oxidized to benzaldehyde and benzoic acid, 1-4 butanediol
to d-butyrolactone, 1-2-butanediol to 1-hydroxy-2-buta-
none, cyclohexanol to cyclohexanone, cyclohexene ! 2-
cyclohexen-1-one, and ethylbenzene to acetophenone. The
proposed mechanism for oxidations using ruthenium oxo
complexes suggests the formation, in the transition state,
of an electron-deficient carbon [29]. A substituent can
decrease the activation energy, inducing delocalization of
the positive charge by hyperconjugation, inductive or reso-
nance effects. In this way, the benzyl alcohol has a higher
reactivity than cyclohexene: the resonance effect in the
benzyl alcohol aromatic ring is responsible by positive
charge delocalization; in cyclohexene, the allylic carbocat-
ion formed at C-3 before the water molecule attack (SN2
1.5
1.3
1.1
0.9
0.7
0.5
0.3
0.1
0
20
40
60
80
100
120
140
-1
[ ] (mmol.L )
-e-
Fig. 5. Plot of the oxidation peak currents of the CPE containing
[Ru(L)(totpy)(H2O)](PF6)2 (1) (L = 1,2-bis(dimethylphosphino)ethane)
against benzyl alcohol concentrations; in 7:3 phosphate buffer (pH
6.8):tert-butyl alcohol solution, m = 100 mV sꢀ1
.
[RuII(L')(totpy)(OH)]+
[RuIII(L')(totpy)(OH)]2+
+e-
Scheme 5.

1462
E.M. Sussuchi et al. / Polyhedron 25 (2006) 1457–1463
benzyl alcohol concentration range. The anodic peak
currents change linearly with the square root of the scan
rate; this is characteristic of surface electrocatalysis with
the catalyst confined to the electrode surface under diffu-
sion control of the substrates.
100
80
60
40
20
0
50 mV.s-1
100 mV.s-1
200 mV.s-1
300 mV.s-1
500 mV.s-1
a
3.5. Electrocatalytic oxidation of organic substrates at a
ITO/Nafionꢀ/complex electrode
-20
-40
The cyclic voltammogram of 1 in the ITO/Nafionꢀ
transparent electrode shows the RuIV/III redox couple only
(E1/2 = +0.74 V versus SCE). This is probably due to the
sluggishness of the RuIII/II couple observed in modified
electrodes [18b].
0.0
0.2
0.4
0.6
0.8
1.0
1.2
E (V vs SCE)
The reactivity of RuIV@O2+ complexes has been
explored with ITO electrodes [31]. The cyclic voltammo-
gram in the presence of 50 mmol Lꢀ1 of benzyl alcohol
shows a decrease of the anodic current, as a consequence
of the dissolution of the aqua complex adsorbed in the
Nafionꢀ polymer film. The dissolution of the aqua com-
plex is clearly noticed when the solution changes from
colorless to yellow. Therefore, benzyl alcohol being a
polar substrate can dissolve the Nafionꢀ film. When
the substrate is cyclohexene, that which has a low polar-
ity, the catalytic activity is noticed by the significant
increase of the anodic current and the decrease of the
cathodic current (Fig. 6).
105
90
75
60
45
30
15
b
0
3
6
9
12
15
18
21
24
27
1/2
(mV.s-1)1/2
The dependence of the anodic peak current with scan
rate was studied from 50 to 500 mV sꢀ1, in the presence
cyclohexene (Fig. 7(a)). The plot of Ipa versus m1/2
(Fig. 7(b)) shows that the anodic peak current varies line-
arly with the square root of the scan rate (correlation coef-
ficient 0.998), indicating that the reaction is also controlled
by diffusion of the substrate [7].
Fig. 7. (a) Cyclic voltammograms of ITO electrode, modified by Nafionꢀ
containing [Ru(L)(totpy)(OH2)](PF6)2 (1) (L = 1,2-bis(dimethylphosph-
ino)ethane), in 4:1 HNO3/NaNO3 buffer (pH 2.3):tert-butyl alcohol
solution, in the presence of 50 mmol Lꢀ1 of cyclohexene, several scan
rates. (b) Plot Ipa vs. m1/2
.
4. Conclusions
The results obtained in this work show that the com-
plexes [Ru(L)(totpy)(OH2)](PF6)2 (L = 1,2-bis(dimethyl-
phosphino)ethane) (1) and [Ru(L0)(totpy)(OH2)](PF6)2
(L0 = 1,2-bis(dichlorophosphino)ethane) (2) can be used
in electrooxidations of organic substrates containing differ-
ent functional groups. The phosphine–ruthenium(IV)-oxo
complexes have the advantage of being mild oxidants, since
more powerful oxidants usually cause undesirable side
reactions.
35
30
Absence of substrate
......... Presence of substrate
25
20
15
10
5
0
The preparations of carbon paste and ITO/Nafionꢀ
electrodes are simple and straightforward and the charac-
teristics of the mediators are preserved upon transfer from
solution to the immobilized state.
-5
-10
0.0
0.2
0.4
0.6
0.8
1.0
1.2
E (V vs SCE)
Fig. 6. Cyclic voltammograms of ITO electrode, modified by Nafionꢀ
containing [Ru(L)(totpy)(H2O)](PF6)2 (1) (L = 1,2-bis(dimethylphosph-
ino)ethane) in 4:1 HNO3/NaNO3 buffer (pH 2.3):tert-butyl alcohol
solution (——), and in the presence of 50 mmol Lꢀ1 of cyclohexene
Acknowledgements
The authors thank Fundac¸ao de Amparo a Pesquisa do
˜
(nnn), m = 100 mV sꢀ1
.
Estado de Sao Paulo (FAPESP) for financial support.
˜

E.M. Sussuchi et al. / Polyhedron 25 (2006) 1457–1463
1463
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