Effect of the cis- and trans-[1,2-bis(diphenylphosphino)ethylene] ligands in the properties of diphosphine-polypyridyl complexes of ruthenium(II). Application to electrocatalytic oxidations of organic compounds

Article abstract of DOI:10.1016/j.molcata.2006.05.059

The ruthenium(II) complexes [Ru(cis-L)(totpy)(H₂O)](PF₆)₂ (1) and [Ru(trans-L)₂(totpy)(H₂O)](PF₆)₂ (2) (L = 1,2-bis(diphenylphosphino)ethylene; totpy = 4′-(4-tolyl)-2,2′:6′,2″-terpyridine) were synthesized and characterized by elemental analysis, cyclic and differential pulse voltammetries and UV-vis, IR and ³¹P NMR spectra. The redox potentials of 2 are less anodic than those of 1. The redox potentials are a result of the different environments created by the phosphine ligands in the trans-complex, associated with their electron donating effect. Electrocatalytic oxidations of benzyl alcohol, cyclohexanol, 1-pentanol, 1,2-butanediol, 1,4-butanediol and cyclohexene were performed using complexes 1 and 2 both in solution and immobilized in carbon paste electrode.

Full text of DOI:10.1016/j.molcata.2006.05.059

Journal of Molecular Catalysis A: Chemical 259 (2006) 302–308
Effect of the cis- and trans-[1,2-bis(diphenylphosphino)ethylene] ligands in
the properties of diphosphine–polypyridyl complexes of ruthenium(II)
Application to electrocatalytic oxidations of organic compounds
∗
´
Eliana M. Sussuchi, Andreia A. de Lima, Wagner F. De Giovani
Departamento de Qu´ımica, Faculdade de Filosofia, Cieˆncias e Letras de Ribeira˜o Preto, Universidade de Sa˜o Paulo,
Av. Bandeirantes 3900, 14040-901 Ribeira˜o Preto, Sa˜o Paulo, Brazil
Received 11 January 2006; received in revised form 24 May 2006; accepted 25 May 2006
Available online 30 August 2006
Abstract
The ruthenium(II) complexes [Ru(cis-L)(totpy)(H2O)](PF6)2 (1) and [Ru(trans-L)2(totpy)(H2O)](PF6)2 (2) (L = 1,2-bis(diphenylphos-
phino)ethylene; totpy = 4ꢀ-(4-tolyl)-2,2ꢀ:6ꢀ,2ꢀꢀ-terpyridine) were synthesized and characterized by elemental analysis, cyclic and differential pulse
voltammetries and UV–vis, IR and 31P NMR spectra. The redox potentials of 2 are less anodic than those of 1. The redox potentials are a result of
the different environments created by the phosphine ligands in the trans-complex, associated with their electron donating effect. Electrocatalytic
oxidations of benzyl alcohol, cyclohexanol, 1-pentanol, 1,2-butanediol, 1,4-butanediol and cyclohexene were performed using complexes 1 and 2
both in solution and immobilized in carbon paste electrode.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Ruthenium aquacomplexes; Phosphine ligands; Polypyridyl ligands; Carbon paste electrodes; Electrocatalytic oxidation
1. Introduction
of ruthenium oxo complexes is partly due to the stabilization of
the high oxidation states easily achieved by the metal (IV–VI)
in processes involving rapid proton transfer concomitant with
electron transfer [6,20]. Oxo ruthenium complexes are known
to be useful stoichiometric and electrocatalytic oxidants for the
transformation of a variety of inorganic and organic substrates
[4–10,15].
The potential catalytic activity of ruthenium(II) complexes
containing chiral diphosphine ligands has been previously
described [3,13,14].
In this work, we describe the syntheses of [Ru(cis-
L)(totpy)(H2O)](PF6)2 (1) and [Ru(trans-L)2(totpy)(H2O)]
(PF6)2 (2) (L = 1,2-bis(diphenylphosphino)ethylene; totpy = 4ꢀ-
(4-tolyl)-2,2ꢀ:6ꢀ,2ꢀꢀ-terpyridine), as well as their spectral, elec-
trochemical, and catalytic properties. We aimed at investigating
how a bulky donating group in the 4ꢀ-position of terpyridine
could influence the chelating properties of this well-known
ligand. Whereas the cis-diphosphine ligand leads to a chelat-
ing coordination mode, the trans-diphosphine ligand is a non-
chelating species due to the rigid double bond. The ability of
the complexes to act as catalysts in the electro-oxidation of
organic compounds was studied in both homogeneous solution
and immobilized in carbon paste electrode.
In recent years, many ruthenium complexes have been pre-
pared and characterized for application in electrocatalysis, pho-
tolysis, bioinorganic chemistry, asymmetrical catalytic hydro-
genation, among other fields [1–9].
New synthetic procedures for the obtention of terpyridine-
and bipyridine-substituted ligands have been developed to
improve the performance and the chelating ability of the lig-
ands, resulting in adequate changes in the electrochemisty and
spectroscopic states of transition metal complexes contain-
ing these ligands [10,11]. The electrochemical behaviour of
ruthenium(II)–polypyridyl complexes has been intensely inves-
tigated [4–9,12–14].
Aqua polypyridyl complexes are an important class of com-
pounds, and their electrochemical oxidation can produce high
oxidation states in ruthenium complexes containing oxo ligands,
which are exceptionally reactive sites for multi-electronic oxida-
tionofsubstrates[4–19]. Thehighinterestintheredoxchemistry
∗
Corresponding author. Tel.: +55 16 3602 3810; fax: +55 16 3602 4838.
E-mail address: wfdgiova@usp.br (W.F. De Giovani).
1381-1169/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2006.05.059

E.M. Sussuchi et al. / Journal of Molecular Catalysis A: Chemical 259 (2006) 302–308
303
2. Experimental
tallizedfromethanolandwereobtainedlightgreencrystals(54%
yield) identified as 4ꢀ-(4-tolyl)-2,2ꢀ:6ꢀ,2ꢀꢀ-terpyridine (totpy). 1H
ꢀꢀ ꢀꢀ ꢀ ꢀ
2.1. Materials
NMR: m, 8.65–8.85 (H3,3 ; H6,6 ; H3 ,5 ); m, 7.80–8.00 (2Ho;
ꢀꢀ
ꢀꢀ
H4,4 ); m, 7.35–7.45 (2Hm; H5,5 ); s, 2.47 (–CH3). EI 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%.
Water was doubly distilled from alkaline potassium per-
manganate. RuCl3·xH2O, cis- and trans-1,2-bis(diphenyl-
phosphino)ethylene were purchased from Aldrich Chemical Co.
Methylene chloride and acetonitrile were kept in an alumina col-
umnbeforebeingused. Allotherreagentsandsolventswereused
without further purification.
2.4. Synthesis of the complexes
[RuCl3(totpy)] was prepared according to a procedure
reported in the literature [21,22] (85% yield).
2.2. Instrumentation and measurements
2.4.1. [Ru(cis-L)(totpy)Cl]Cl
Routine UV–vis spectra were obtained in 1 cm quartz cells
by using a Hewlett-Packard 8453 spectrophotometer. Electro-
chemical experiments were carried out in a PAR model 273A
Potentiostat/Galvanostat. E1/2 values for reversible couples were
calculated from half the difference between Ep values for the
cathodic and anodic waves. Cyclic voltammetric and differ-
ential pulse voltammetric experiments were performed in a
one-compartment cell using a glassy carbon working electrode,
a platinum wire auxiliary electrode, and a saturated calomel
(SCE) or Ag/AgCl reference electrode. Bulky electrolyses were
performed in 7:3 phosphate buffer:tert-butyl alcohol solutions,
in a 50 mL two-compartment cylindrical cell, using a platinum
gauze working electrode (164 cm2), a platinum plate auxiliary
electrode (1 cm2), and an SCE. Electrolyses were carried out at
25 1 ◦C, at fixed applied potentials of +1.18 and +1.10 V when
1and2wereusedascatalysts, respectively. Thepotentialapplied
in each case was sufficient to generate RuIV = O2+ oxidizing
A mixture of [RuCl3(totpy)] (0.20 g, 0.38 mmol) and cis-
1,2-bis(diphenylphosphino)ethylene (0.18 g, 0.45 mmol) was
heated to reflux, under N2 atmosphere, in 60 mL of a 3:1
dichloroethane:ethanol solution, in the presence of NEt3
(0.25 mL) and LiCl (0.08 g, 1.89 mmol), for 3.0 h. The hot solu-
tion was filtered and its volume was evaporated to dryness. The
product was recrystallized by dissolving it in acetone and pre-
cipitating by addition of diethyl ether. Yield: 0.27 g (80%).
2.4.2. [Ru(trans-L)2(totpy)Cl](PF6)
This complex was prepared using a procedure similar to that
employed for the obtention of [Ru(cis-L)(totpy)Cl]Cl. The dif-
ference was that a 2:1:1 ethanol:ethylene glycol:water ratio and
a two-fold quantity (0.36 g, 0.90 mmol) of trans-L ligand were
used. The hot solution was filtered and 2.0 mL of an NH4PF6
saturated aqueous solution were added to the filtrate. Red brown
crystals were obtained. Yield: 0.32 g (60%).
2+
species from the corresponding RuII–OH2 complexes. Elec-
trolyses were allowed to continue until the current fell to residual
values or until the desired number of coloumbs was reached. The
electrolyses products were extracted with diethyl ether and ana-
lyzed by gas chromatography (GC); the chromatograms were
recorded in an Intralab 3000 gas chromatograph. The pH val-
ues of aqueous solutions were buffered at an ionic strength of
0.25 mol L−1 by using HNO3 and NaNO3 (pH 2.0), potassium
hydrogen phthalate, 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), Na2B4O7·10H2O and NaNO3 (pH
9.0), Na2B4O7·10H2O and NaOH (pH 10.0–10.8). Elemen-
tal analyses were performed using a CHNS-O CE Instruments
model EA 1110 elemental analyzer. Ruthenium analyses were
performed in a V-85 B Braun ICP-AES atomic emission spec-
trometer. EPR spectra of the solid complexes were recorded at
room temperature on a Bruker EP 200D spectrometer. 1H NMR
and 31P NMR spectra were obtained in a Brucker AC-300 spec-
trometer. Mass spectra were obtained in a Hewlett-Packard gas
chromatograph/MS system 5988A.
2.4.3. [Ru(cis-L)(totpy)(H2O)](PF6)2 (1)
[Ru(cis-L)(totpy)Cl]Cl (0.10 g; 0.11 mmol) and silver p-
toluenesulfonate (0.08 mg; 0.26 mmol) were heated to reflux in
40 mL of 1:1 ethanol:water, under N2 atmosphere, for 5 h. The
hot solution was filtered and 2.0 mL of an NH4PF6 saturated
aqueous solution were added to the filtrate. The volume was
reduced to 20 mL in a rotary evaporator. The brown crystals
were collected by filtration, washed with cold water and diethyl
ether, and dried under vacuum. The complex was reprecipitated
from acetone–diethyl ether. Yield: 0.08 g (65%).
2.4.4. [Ru(trans-L)2(totpy)(H2O)](PF6)2 (2)
This complex was prepared using a procedure similar to that
employed for the obtention of 1. The difference was that the solu-
tion was refluxed for 1.5 h under N2 atmosphere. Yield: 0.11 g
(69%).
2.5. Modified electrodes
2.5.1. Carbon paste electrodes
2.3. Preparation of the ligand
Carbon paste electrodes [8,23] containing 3:22 by weight
of complex:paste were used for cyclic voltammetric studies.
The paste was prepared by mixing 3:2 by weight of carbon
powder:mineral oil. The carbon paste holder consisted 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
4ꢀ-(4-tolyl)-2,2ꢀ:6ꢀ,2ꢀꢀ-terpyridine (totpy)
The ligand 4ꢀ-(4-tolyl)-2,2ꢀ:6ꢀ,2ꢀꢀ-terpyridine was prepared
analogously to a literature procedure [14]. A 2:1 ratio of 2-
acetylpyridine:p-tolualdehyde was used. The solid was recrys-

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E.M. Sussuchi et al. / Journal of Molecular Catalysis A: Chemical 259 (2006) 302–308
Table 1
Elemental analysis and 31P NMR data for the complexes
Complexes
31P NMR δ (ppm)a
Calc./exp.
C (%)
H (%)
Ru (%)
[Ru(totpy)Cl3]
[Ru(cis-L)(totpy)Cl]Cl
[Ru(trans-L)2(totpy)Cl](PF6)
[Ru(cis-L)(totpy)(H2O)](PF6)2 (1)
[Ru(trans-L)2(totpy)(H2O)](PF6)2 (2)
–
49.82/48.95
64.64/63.70
63.59/65.20
51.07/51.91
58.27/56.56
3.23/3.28
4.41/4.12
4.39/4.15
3.66/3.97
4.16/4.07
–
–
–
69.5 (d), 63.5 (d)
152.2 (s)
73.5 (d), 65.7 (d)
153.4 (s)
8.95/9.20
6.63/6.85
a
CH2Cl2 (H3PO4/85%).
tube. By rotating the sleeve, it was possible to make the plunger
extrude an used past layer which was sliced off to form a fresh
paste surface.
also contains the metal–ligand d␲ (Ru) → ␲* (totpy) transitions
[12]. The transitions of the trans-complex are of lower energy
than those of cis-complex. This fact can be attributed to the
effect of the two phosphine ligands in trans. In this case, when
␲-acceptor ligands such as phosphines find themselves in trans-
positions to each other, they compete for the electrons of the
metallic center with the same intensity, leaving it rich in elec-
tronic density. Such competition favors a high delocalization of
the electronic density in the metal in favor of the ␲-acceptor
phosphine ligands, explaining the lower energy of the MLCT
transition of complex 2.
Both aqua complexes have similar spectra. As observed in
the case of other aqua complexes, the MLCT energy shifts to
higher values upon substitution of the anionic chloride ligand
for the neutral aqua ligand [8,10].
The correlation between MLCT energies of various com-
plexes and their respective electrochemical redox potentials
(E1/2) has been reported [7a]. It has been observed that com-
plexes with lower transition energies have lower redox poten-
tials.
3. Results and discussion
3.1. Synthesis
The reaction between [RuCl3(totpy)] and the diphosphine
ligands (cis- and trans-Ph2PCH CHPPh2), produced dis-
tinct complexes. The product containing cis-1,2-bis(diphe-
nylphosphino)ethylene favors the chelating coordination mode,
so compound [Ru(cis-L)(totpy)Cl]Cl is preferentially formed.
However, in the case of the trans-Ph2PCH CHPPh2 lig-
and, which is non-chelating [24–26] due to its rigid back-
bone, the dominant product was [Ru(trans-L)2(totpy)Cl](PF6).
The chloride species are converted in good yield to the
corresponding aqua complexes, by using Ag(I) ion under
reflux in ethanol/water. Elemental analysis data are shown in
Table 1.
3.2. EPR and 31P NMR spectroscopies
The 31P NMR spectrum of 1 displays a double doublet of
similar intensity in the 73.5–65.7 ppm range (Fig. 1a), show-
ing that the phosphorus atoms are not equivalent. On the other
hand, the spectrum of complex 2 displays only a single singlet
at 153.4 ppm (Fig. 1b), indicating that all phosphorus atoms are
equivalent and that the two ligands are in trans-position. The
31P NMR data are summarized in Table 1 and correspond to the
structures shown in Fig. 2.
EPR spectra of ruthenium(III) complexes give g tensor values
[27] that can be used to derive ground states. The 1 and 2 aqua
complexes show no EPR signals, indicating the oxidation state
Ru(II).
3.3. Electronic absorption spectra
The spectral data are summarized in Table 2. The intense
bands in the UV region are assignable to ligands ␲ → ␲* tran-
sitions. The band in the visible region, at λmax = 418 nm for 1
can be attributed to metal–ligand d␲ (Ru) → ␲* (totpy) tran-
sitions; the wide band at λmax = 475 nm for 2 can be attributed
to metal–ligand d␲ (Ru) → ␲* (diphosphine) transitions and it
Fig. 1. 31P NMR spectrum of (a) 1 and (b) 2 in CH2Cl2 (H3PO4/85%).

E.M. Sussuchi et al. / Journal of Molecular Catalysis A: Chemical 259 (2006) 302–308
305
Fig. 2. Structure of: (a) [Ru(cis-L)(totpy)(H2O)](PF6)2 (1) and (b) [Ru(trans-
L)2(totpy)(H2O)](PF6)2 (2) (L = 1,2-bis(diphenylphosphino)ethylene; totpy =
4ꢀ-(4-tolyl)-2,2ꢀ:6ꢀ,2ꢀꢀ-terpyridine).
3.4. Electrochemistry
Fig. 3. (a) Cyclic voltammogram and (b) differential pulse voltammo-
gram of 1.0 × 10−3 mol L−1 [Ru(cis-L)(totpy)(H2O)](PF6)2 (1) (L = 1,2-
bis(diphenylphosphino)ethylene) in CH2Cl2 + 0.1 mol L−1 TBAP, glassy carbon
The cyclic voltammogram data obtained for the complexes
are summarized in Table 2. The chloro complexes exhibit
reversible one-electron oxidations; the Ru(III/II) redox couple
is +1.21 V (versus Ag/AgCl) for the cis-complex and +1.03 V
for the trans-complex. This decrease in potential ongoing from
the cis- to the trans-complex is well established in Ru(II) chem-
istry [13]. The eletronic nature of the ligands are very similar,
but the peculiar oxidation potentials are probably a result of
the different environments created by the two phosphine lig-
working electrode; (a) ν = 100 mV s−1; (b) 10 mV s−1
.
ands in the trans-complex. The corresponding aqua complexes
show two reversible redox waves corresponding to the Ru(III/II)
andRu(IV/III)redoxcouples(+1.22/+1.54 Vand+1.08/+1.32 V
versus Ag/AgCl, for the cis- and trans-complex, respectively,
in CH2Cl2). Fig. 3a shows the cyclic voltammogram and b
Table 2
Electrochemical and spectral data for the complexes
Complexes
E1/2 (V)
λmax (nm) (εmax × 103, mol−1 L cm−1
)
Ru(III/II)
Ru(IV/III)
[RuCl3(totpy)]
[Ru(cis-L)(totpy)Cl]Cl
[Ru(trans-L)2(totpy)Cl](PF6)
[Ru(cis-L)(totpy)(H2O)](PF6)2 (1)
[Ru(trans-L)2(totpy)(H2O)](PF6)2 (2)
+0.05a
–
–
–
226 (17), 285 (16), 310 (13), 406 (4.7)b
290 (32), 319 (31), 432 (7.5)d
290 (43), 308 (36), 487 (6.8)d
290 (25), 320 (24), 418 (7.0)d
289 (42), 306 (46), 475 (8.4)d
+1.21c
+1.03c
+1.22, +0.99
+1.08, +0.87
+1.54c, +1.11e
+1.32c, +1.05e
a
MeCN + 0.1 mol L−1 TBAP; glassy carbon working electrode; ν = 100 mV s−1
.
b
c
d
e
CH3CN.
CH2Cl2 + 0.1 mol L−1; glassy carbon working electrode; ν = 100 mV s−1
.
CH2Cl2.
7:3 phosphate buffer (μ = 0.20 mol L−1):tert-butyl alcohol solution, pH 6.8.

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E.M. Sussuchi et al. / Journal of Molecular Catalysis A: Chemical 259 (2006) 302–308
Scheme 2.
Scheme 3.
The Ru(IV/III) redox couple is not pH-dependent. Therefore,
the species present at any pH is Ru(III)–O+ and the determinant
step does not involve protons [8,28] (Scheme 3).
3.6. Electrocatalysis
Fig. 4. Plots of E1/2 vs. pH for the Ru(III/II) (ꢀ) and Ru(IV/III) (᭹) redox
couples of [Ru(cis-L)(totpy)(H2O)](PF6)2 (1); glassy carbon working electrode.
3.6.1. Electrocatalytic oxidations of organic substrates in
homogeneous solution
The catalytic activities of 1 and 2 toward a variety of
organic substrates (benzyl alcohol, cyclohexanol, cyclohexene,
1-pentanol, 1,2-butanediol, and 1,4-butanediol) were studied in
7:3 phosphate buffer:tert-butyl alcohol solutions, pH 6.8; tert-
butyl alcohol was added to improve the solubility of the aqua
complexes and substrates. Mixed solvent systems have been
used by other groups in similar experiments, with no adverse
effects [10,15].
In the presence of excess benzyl alcohol (50 times), the cyclic
voltammograms of the complexes show great enhancement in
the oxidation current peaks and decrease in the reduction peaks
reversal potential scans (Fig. 5) shows the process for 1, which is
typical of electrocatalysis [8]. For the quantitative evaluation of
the electrocatalytic behaviour of 1, the dependence of the current
peak height on the concentration of benzyl alcohol and on the
potential scan rate was studied [29]. The peak height is linearly
dependent (Fig. 6, correlation coefficient = 0.996, plot intercept
near zero) on the concentration of benzyl alcohol, indicating that
the electrocatalysis is pseudo-first-order in this benzyl alcohol
concentration range.
shows the pulse differential voltammogram of the cis-complex.
In general, the cyclic voltammograms of the complexes in aque-
ous solutions have less well-behaved redox waves than those
obtained in methylene chloride. The redox potentials of 2 are
alsolessanodic, reflectingtheelectron-donationeffectofthetwo
phosphine ligands. The potentials observed in methylene chlo-
ride are higher than those obtained in aqueous solution (Table 2),
and the separation between the two couples (mV) is larger than in
water. This behaviour can be due to a very slow deprotonation
kinetics of the first oxidized species; the deprotonation of the
aqua ligand to form RuIII–OH is facilitated in aqueous solution,
but not in methylene chloride (a low protic and little polar sol-
vent). These data are practically the same as those reported for
the analogous [Ru(tpy)(dppene)(H2O)](PF6)2 (tpy = 2,2ꢀ:6ꢀ,2ꢀꢀ-
terpyridine; dppene = cis-1,2-bis(diphenylphosphino)ethylene)
complex [12]: E1/2 = +1.17 and +1.53 V. These values are close
to those found for 1 (E1/2 = +1.22 and +1.54 V), indicating that
the bulky donating tolyl group in position 4ꢀ of the terpyridine
ligand practically has no influence on the redox behaviour of the
complex.
3.5. E1/2 versus pH of the aqua complex 1
The Pourbaix diagram for aqua complex 1 reveals two differ-
ent pH regimes for the Ru(III/II) and Ru(IV/III) redox couple.
The potentials of the Ru(III/II) redox couples vary with the
pH of the solution, resulting in the E1/2 versus pH variations
shown in Fig. 4. Within the 8.0 > pH > 2.0 region, the poten-
tials form a straight line with slope −0.051, which is very
close to the Nernstian prediction of −0.059 V/pH unit, indica-
tive of a one-electron oxidation accompanied by the dissociation
of one proton (Scheme 1). At pH > 8.0, the potentials become
pH-independent indicating that [RuII(cis-L)(totpy)(OH)]+ is the
dominant species in this region (Scheme 2) [8,28].
Fig. 5. Cyclic voltammograms of 1 (1.0 mmol L−1) in the absence (—), and in
the presence (. . .) of benzyl alcohol, in 7:3 phosphate buffer:tert-butyl alcohol
Scheme 1.
solutions, pH 6.8; ν = 100 mV s−1; [substrate] = 50 mmol L−1
.

E.M. Sussuchi et al. / Journal of Molecular Catalysis A: Chemical 259 (2006) 302–308
307
hexene. In the case of benzyl alcohol, the resonance effect
in its aromatic ring is responsible by positive charge delocal-
ization. In cyclohexene, the allylic carbocation formed at C-3
before the attack of the water molecule (SN2 reaction) induces
a stabilization by resonance with a conjugated unsaturation at
the carbon ring. Nevertheless, because cyclohexene has fewer
canonic structures than benzyl alcohol, the lower positive charge
delocalization decreases the reactivity. A higher reactivity of the
primary alcohol when compared to that of the secondary one was
also observed in homogeneous catalysis [8]. Benzaldehyde and
benzoic acid are the products generated from the oxidation of
benzyl alcohol; when the phosphine complex is used, though,
benzaldehyde is the only oxidation product.
The following generalization is possible for the systems stud-
ied in this work: secondary alcohols and primary aromatic alco-
hols are more reactive than olefins, the latter being more reactive
than primary aliphatic alcohols and diols. Complex 2, which has
the lowest Ru(IV/III) couple redox potential, is less reactive than
complex 1, indicating a direct relation between redox potential
and reactivity of the complexes: the higher the E1/2, the higher
the reactivity [7].
Fig. 6. Plot of the oxidation peak currents of 1 against benzyl alcohol
concentrations; in 7:3 phosphate buffer:tert-butyl alcohol solutions, pH 6.8;
ν = 100 mV s−1
.
Bulky electrolyses were conducted at controlled potentials
of +1.18 V (versus SCE) for complex 1, and +1.10 V (versus
SCE) for complex 2. The reactivities of the different substrates
with respect to electro-oxidation could be obtained by the rela-
tion between product yields and electrolysis time. In this case, it
is assumed that the rate-determining step is substrate oxidation,
consideringthattheheterogeneousre-oxidationratesofthecom-
plexes at the electrode surface are much higher than the chemi-
cal reaction rates. Results of the electrocatalytic oxidations are
summarized in Table 3. The relationship current efficiency by
time unit gives the following reactivity order for the electro-
oxidation for 1: benzyl alcohol > cyclohexanol > cyclohexene
> 1,4-butanediol > 1,2-butanediol > 1-pentanol; and for 2: ben-
zyl alcohol > cyclohexanol > cyclohexene > 1,4-butanediol >
1-pentanol > 1,2-butanediol. The proposed mechanism for oxi-
dations using ruthenium oxo complexes suggests the formation
of an electron-deficient carbon in the transition state [30]. A
substituent group can decrease the reaction activation energy,
inducing delocalization of the positive charge via hyperconju-
gation, inductive, or resonance effects. The secondary alcohol
and the primary aromatic alcohol are more reactive than cyclo-
3.6.2. Electrocatalysis of organic substrates with carbon
paste electrode containing complexes 1 and 2 (CPE)
The cyclic voltammetric behaviour of CPE, in presence of
organic substrates, in 7:3 phosphate buffer:tert-butyl alcohol
solutions, pH 6.8, shows pronounced enhancement in the anodic
currents corresponding to the Ru(IV/III) couple. There were no
reduction peaks upon reversal scan direction, which is typical
of electrocatalytic processes. The reactivity order is similar to
that observed in homogeneous catalysis. The higher reactivity
of benzyl alcohol can be due to its higher permeability into the
active surface of the carbon paste electrode.
The dependence of the peak height on the bulk concentration
of benzyl alcohol and on the potential scan rate indicates that the
electrocatalysis is pseudo-first-order in the studied benzyl alco-
hol concentration range, as observed in homogeneous catalysis.
The anodic peak currents change linearly with the square root
of the scan rate (Fig. 7), which is typical of surface electrocatal-
ysis with the catalyst confined to the electrode surface under
diffusion rate control of the substrates from solution. This same
Table 3
Electrocatalytic oxidations of organic substrates by [Ru(cis-L)(totpy)(H2O)](PF6)2 (1) and [Ru(trans-L)2(totpy)(H2O)](PF6)2 (2)a
Substrateb
Oxidative coulombs passed
Reaction time (min)
Products
Yields (%)
1
2
1
2
1
2
Benzyl alcohol
Cyclohexene
1-Pentanol
Cyclohexanol
1,2-Butanediol
1,4-Butanediol
324.8
580.0
196.8
295.7
348.1
289.5
319.7
520.6
232.1
302.5
315.2
299.3
78
162
144
300
138
390
294
Benzaldehyde
2-Cyclohexen-1-one
1-Pentanal
Cyclohexanone
1-Hydroxy-2-butanone
␥-Butyrolactone
64c
59d
33c
48c
62c
56c
66c
42d
31c
34c
55c
47c
126
252
72
258
240
a
7:3 phosphate buffer:tert-butyl alcohol solution, pH 6.8. Catalyst concentration 1.0 mmol L−1. Applied potential (vs. SCE): +1.18 V for 1, +1.10 V for 2.
Substrate concentration 50.0 mmol L−1
.
b
c
d
Based on total coulombs passed taking into account a 2 e/mol reaction.
Based on total coulombs passed taking into account a 4 e/mol reaction.

308
E.M. Sussuchi et al. / Journal of Molecular Catalysis A: Chemical 259 (2006) 302–308
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¨
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¨
The same products were obtained in the oxidation of various
substrates in the presence of complexes 1 and 2, but with differ-
ent yields. Best results were obtained with the complex [Ru(cis-
L)(totpy)(H2O)](PF6)2 (1), which has a higher redox potential
than the complex [Ru(trans-L)2(totpy)(H2O)](PF6) (2). There
is a direct relation between redox potential and reactivity of the
complexes: the higher the E1/2, the higher the reactivity.
Electrocatalysis of organic substrates with carbon paste elec-
trodes containing complexes 1 or 2 (CPE) presents some advan-
tages: the preparation of the carbon paste electrode is simple
and straight forward, and the characteristics of the mediators are
maintained upon their transfer from the solution to the immobi-
lized state.
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These complexes can also be useful in the oxidation of other
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Acknowledgement
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˜
The authors thank Fundac¸ao de Amparo a Pesquisa do Estado
˜
de Sao Paulo (FAPESP) for financial support.
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