Ultrasonic-assisted ruthenium-catalyzed oxidation of some organic compounds in aqueous medium

Article abstract of DOI:10.1016/j.molliq.2015.01.023

Ultrasonic generation of ruthenium complexes K₂[RuO₃(OH)₂] and [RuO₄] is reported from K₂[RuCl₅(H₂O)] with excess K₂S₂O₈ in 1 M KOH and/or with excess KIO₄ in biphasic solvent system (H₂O/CH₂Cl₂/CH₃CN) at room temperature. Dehydrogenation of some primary amines to nitriles and primary alcohols to their respective acids by K₂[RuCl₅(H₂O)] with excess K₂S₂O₈ at room temperature was reported. A number of nitriles were hydrated to amides by the same catalyst system at 100 °C. The oxidation of two arenes and four alkenes to their aldehydes, ketones or acids by K₂[RuCl₅(H₂O)] with excess KIO₄ using ultrasonic irradiation technique was also reported. Cyclic voltammetric, electronic and Raman spectroscopic techniques have been used to elucidate the nature of the active species present in these catalytic oxidation reactions.

Full text of DOI:10.1016/j.molliq.2015.01.023

Journal of Molecular Liquids 206 (2015) 68–74
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Ultrasonic-assisted ruthenium-catalyzed oxidation of some organic
compounds in aqueous medium
Abdel Ghany F. Shoair
Department of Chemistry, Faculty of Science, Damietta University, Damietta 34517, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
Ultrasonic generation of ruthenium complexes K2[RuO3(OH)2] and [RuO4] is reported from K2[RuCl5(H2O)] with
excess K2S2O8 in 1 M KOH and/or with excess KIO4 in biphasic solvent system (H2O/CH2Cl2/CH3CN) at room tem-
perature. Dehydrogenation of some primary amines to nitriles and primary alcohols to their respective acids by
K2[RuCl5(H2O)] with excess K2S2O8 at room temperature was reported. A number of nitriles were hydrated to
amides by the same catalyst system at 100 °C. The oxidation of two arenes and four alkenes to their aldehydes,
ketones or acids by K2[RuCl5(H2O)] with excess KIO4 using ultrasonic irradiation technique was also reported. Cy-
clic voltammetric, electronic and Raman spectroscopic techniques have been used to elucidate the nature of the
active species present in these catalytic oxidation reactions.
Received 14 November 2014
Received in revised form 6 January 2015
Accepted 14 January 2015
Available online 17 January 2015
Keywords:
Ultrasonic irradiation
Ruthenium
© 2015 Published by Elsevier B.V.
Catalytic oxidation
Organic compounds
1. Introduction
JASCO 410 spectrophotometer. Raman spectra were measured on a
PerkinElmer 1760X Fourier Transform Raman instrument with 1064 nm
Many organic transformations which involve ruthenium species as
catalyst are known and well documented [1,2]. Ruthenium(III)
complexes are also well known to catalyze a variety of organic transfor-
mations [3–7]. Recently, we have reported the catalytic dehydrogena-
tion of benzylamine, p-methylbenzylamine and p-nitrobenzylamine to
their respective nitriles by using [RuIIICl2(8-hq)2]Cl in DMF as a solvent
and in the presence of N-methyl morpholine-N-oxide (NMO) as co-oxidant
[8]. It has been reported that the reagent trans-[RuVI(OH)2O3]2−/S2O82− in
aqueous base dehydrated primary amines to nitriles at room tempera-
tures and longer reaction time resulted in hydrolysis of produced
nitriles to amides [9].
Herein, we report the ultrasonic generation of K2[RuO3(OH)2] and
[RuO4] from K2[RuCl5(H2O)] with excess K2S2O8 in 1 M KOH and/or
with excess KIO4 in biphasic solvent system (H2O/CH2Cl2/CH3CN) at
room temperature, respectively. We found that ultrasonic irradiation
accelerates the catalytic oxidation of some organic compounds (primary
amines, nitriles, primary alcohols, arenes and alkenes) by the in situ
generated ruthenium complexes. Different spectroscopic techniques
were used to establish the nature of the active catalytic intermediates
formed in these catalytic oxidation reactions.
Nd–YAG excitation with a power of 2 W, and on a Dilor LabRam Infinity in-
strument with 632 nm. He–Ne excitation and 532 nm frequency doubled
Nd–YAG excitations, as appropriate, each at 5 mW. Ultrasonication was
performed in thermostated ultrasonic cleaner with a frequency of 28 kHz
and a normal power of 300 W (Delta sonic 920 No 484 (France)). The
reaction flask was located in the water bath of the ultrasonic cleaner.
Melting points were measured on a Buchi Melting Point B-540 instrument.
1H NMR spectra in d6-DMSO were recorded on a Varian Unit plus
300 MHz model using TMS as an internal standard. A computerized
voltammetric analyzer CHI610C Electrochemical Analyzer controlled by
CHI Version 9.09 software (CH Instruments, USA) was used for the
voltammetric measurements. Home-made three printed electrodes: a
working screen-printed carbon electrode (3.1 mm diameter) printed
from a carbon-based ink (Electrodag 421, Acheson); a silver–silver chlo-
ride pseudo-reference electrode made from a silver-based ink (Electrodag
477, Acheson) and the auxiliary electrode from a carbon ink, were used.
The UV spectra were performed by a PerkinElmer UV–VIS double beam
spectrophotometer equipped with a PC for data processing UV WinLab-
version 2.80.03 (PerkinElmer, USA). The spectra were recorded over the
wavelength range from 200 to 650 nm at a scan speed of 240 nm/min.
A quartz cell with a 1.0 cm path length was used.
2. Experimental
2.2. Preparation of ruthenium complexes
2.1. Chemicals and physical measurements
2.2.1. Ultrasonic-assisted generation of trans-[RuO3(OH)2]2−/S2O82−
ruthenate reagent
All chemicals and solvents were of analytical grade and used without
further purification and purchased from Sigma-Aldrich Chemicals Compa-
ny (USA). The IR spectra were recorded as KBr discs (4000–400 cm−1) on a
A mixture of 50 cm3 of 1 M KOH containing K2[RuCl5(H2O)]
(0.0374 g, 0.1 mmol) and excess K2S2O8 (1.4 g, 5 mmol) was irradiated
http://dx.doi.org/10.1016/j.molliq.2015.01.023
0167-7322/© 2015 Published by Elsevier B.V.

A.G.F. Shoair / Journal of Molecular Liquids 206 (2015) 68–74
69
Table 2
Optimization of the catalytic oxidation of benzylamine and benzyl alcohol to benzonitrile
and benzoic acid by K2[RuCl5(H2O)]/K2S2O8, respectively.
Entry
Benzonitrile (amide) yield %
Entry
Benzoic acid yield %
1a
2a
3a
4a
5a
6a
7a
8a
–
95 (5)
75
67
70
30
80 (19)
0
0
–
1b
2b
3b
4b
5b
6b
7b
8b
9b
95
95
20
95
50
95
83
75
44
Scheme 1. Ultrasonic-assisted catalytic dehydrogenation of primary amines to nitriles.
by ultrasonic radiation for 2 min at room temperature after which the
orange solution appeared. The orange solution was identified by its
electronic spectrum and concentrated under vacuum to give red
crystals of K2[RuO3(OH)2] which sealed in melting point tube.
For preparation of the solid Ba[RuO3(OH)2], an aqueous solution of
BaCl2·2H2O (0.0244 g, 0.1 mmol) was added to the orange solution of
K2[RuO3(OH)2] with stirring until a red precipitate was obtained. It
was centrifuged and the residue was washed three times with water
and dried in a vacuum. Yield: 70%.
Reaction conditions: 50 cm3 of 1 M KOH, K2S2O8 (1.4 g, 5 mmol), K2[RuCl5(H2O)] (0.0374
g, 0.1 mmol) and substrate (2 mmol).
pH 2 and extracted with diethyl ether. The extract was dried over anhy-
drous MgSO4 then filtered and evaporated to give the product.
2.2.2. Ultrasonic-assisted generation of [RuO4]
A solution of 50 cm3 of distilled H2O containing K2[RuCl5(H2O)]
(0.0374 g, 0.1 mmol) and KIO4 (1.1 g, 5 mmol) was irradiated by
ultrasonic radiation for 2 min producing the yellow solution in water.
The clear yellow solution of [RuO4] was freshly identified by its electronic
spectrum.
3. Results and discussion
3.1. Ultrasonic-assisted catalytic dehydrogenation of primary amines to
nitriles
2.2.3. Ultrasonic-assisted catalytic dehydrogenation of primary amines to
nitriles by trans-[RuO3(OH)2]2−/S2O82−
Typical results of the ultrasonic-assisted catalytic dehydrogenation
of primary amines to nitriles (Scheme 1) are shown in Table 1.
Oxidation of benzylamine was performed as a model reaction with
K2S2O8 in 1 M KOH and in the presence of catalytic amount of
K2[RuCl5(H2O)]. In the catalyst system, K2[RuCl5(H2O)]/K2S2O8 catalyst
(pH = 14) showed good catalytic activity and selectivity for the trans-
formation of benzylamine to benzonitrile in 95% under ultrasound-
assisted irradiation technique at room temperature.
To 50 cm3 of trans-[RuO3(OH)2]2−/S2O28− reagent, primary amine
(2 mmol) was added, the reaction mixture turned dark green. The reac-
tion mixture was further irradiated for half an hour, after which time,
the reaction is completed since the original orange color of ruthenate
reappears. The mixture was then extracted with diethyl ether
(3 × 25 cm3). The ether extracts were dried over anhydrous MgSO4
then filtered and evaporated to give the product. Similarly, nitriles
were hydrated to their respective amides at 100 °C.
A mixture of K2S2O8 (1.4 g, 5 mmol) in 50 cm3 of 1 M KOH and
catalytic amounts of K2[RuCl5(H2O)] (5 mmol%) was ultrasonically
irradiated at room temperature until the appearance of the orange
color, expected to be trans-[RuO3(OH)2]2− ion in less than 2 min.
Also, primary alcohols were oxidized to their respective carboxylic
acids. The alkaline aqueous layer was acidified with 2 M H2SO4 to
Table 1
Ultrasonic-assisted catalytic dehydrogenation of primary amines to nitriles.
Entry
1
Amine
Nitrile
Yield % (TOF, h−1
95 (38)
)
2
3
94 (37.6)
93 (37.2)
4
5
90 (36.0)
80 (32.0)
6
7
85 (34.0)
83 (33.2)
Reaction conditions: 50 cm3 of 1 M KOH, K2S2O8 (1.4 g, 5 mmol), K2[RuCl5(H2O)] (0.0374 g, 0.1 mmol) and primary amine (2 mmol). Irradiation at room temperature for 0.5 h. TOF =
moles of product/mol of catalyst × time.

70
A.G.F. Shoair / Journal of Molecular Liquids 206 (2015) 68–74
Scheme 3. Ultrasonic-assisted catalytic dehydrogenation of primary alcohols to carboxylic
acids.
Scheme 2. Ultrasonic-assisted catalytic hydration of nitriles to amides.
3.2. Ultrasonic-assisted catalytic hydration of nitriles to amides
Once primary amine (2 mmol) was added, the reaction mixture gave a
transient green nitrile complex. When the reaction was completed, the
original orange color returns (i.e. self-indicating). Other co-oxidants
BrO3−, [Fe(CN)6]3−, NaOCl and HCO4− were used instead of K2S2O8,
they gave benzonitrile in 75, 67, 70 and 30% yields, respectively
(Table 2, entries 2a–5a). Therefore, K2S2O8 is considered the best
co-oxidant and HCO−4 is the worst one, since it is formed with
t1/2 ≈ 5 min [10,11]. However, when the reaction is also conducted
at 60 °C, benzonitrile was obtained in 80% yield and benzamide in 19%
yield (Table 2, entry 6a). This indicates that at high temperature selec-
tivity of benzonitrile decreased and that of benzamide increased.
Benzonitrile was not formed in the absence of K2[RuCl5(H2O)] or in
the presence of K2S2O8 only (Table 2, entries 7a and 8a). To examine
the scope of benzylamine dehydrogenation reaction with
K2[RuCl5(H2O)]/K2S8O8, we extended our studies to various para-
substituted benzylamine derivatives. We noticed that benzylamine
derivatives containing electron-donating groups (Table 1, entries
1–4) were smoothly converted to their respective nitriles in better
yields than those containing electron-withdrawing groups (Table 1,
entries 5–7). In all reactions, yield and turnover frequency (TOF) were
calculated and the produced nitriles were identified by measuring
their melting point and recording their IR and 1H NMR spectra where
appropriate. As can be seen, the catalyst system, K2[RuCl5(H2O)]/
S2O28− can be used efficiently and simply for direct dehydrogenation
of these primary amines to their respective nitriles at room temperature
by using ultrasonic irradiation technique in a short reaction time (0.5 h).
Typical results of the ultrasonic-assisted catalytic hydration of
nitriles to amides (Scheme 2) are shown in Table 3.
Hydration of nitriles was investigated at 100 °C since we noticed the
formation of amides as side-product during the dehydrogenation of
benzylamine at 60 °C (Table 2, entry 6a). Griffith et al. [9] reported that dur-
ing the dehydrogenation of benzylamine and p-methoxybenzylamine to
nitriles, benzamide and p-methoxybenzamide were produced as side
products in relatively small yields (ca. 10%), this does not occur in the
absence of S2O28−
.
Recently, Kathó et al. [12] reported the catalytic hydration of
benzonitrile to benzamide in 99% yield by the ruthenium(II) complex,
[RuCl2(DMSO)4], with the various water-soluble phosphines at reflux
temperature. This really encourages us to investigate our catalyst
system for hydration of some nitriles to their respective amides. The
composition of the catalytic system here is exactly the same as that
used for dehydrogenation of primary amines and alcohols, but reactions
were performed at 100 °C.
When benzonitrile (2 mmol) was added to the solution of
K2[RuCl5(H2O)] in 1 M KOH containing 10-fold excess of S2O28−, an
immediate dark red color is formed, probably due to the coordination
of benzonitrile to ruthenium center. Attempts to isolate this
complex were unsuccessful. Benzamide was obtained in 95% yield
from benzonitrile (Table 3, entry 1). No formation of by-products
(e.g., benzoic acid) was observed. Other benzonitrile derivatives gave
their respective amides in 90–93% yields. All reactions were also
conducted in the absence of ultrasonic irradiation, in this case, vigorous
Table 3
Ultrasonic-assisted catalytic hydration of nitriles to amides.
Entry
1
Nitrile
Amide
Yield % (TOF, h−1
95 (38)
)
2
3
4
90 (36.0)
91 (36.4)
90 (36.0)
5
90 (36.0)
Reaction conditions: 50 cm3 of 1 M KOH, K2S2O8 (1.4 g, 5 mmol), K2[RuCl5(H2O)] (0.0374 g, 0.1 mmol) and nitrile (2 mmol). Irradiation at 100 °C for 0.5 h. TOF = moles of product/mol of
catalyst × time.

A.G.F. Shoair / Journal of Molecular Liquids 206 (2015) 68–74
71
Table 4
Ultrasonic-assisted catalytic oxidation of primary alcohols to acids.
Entry
1
Alcohol
Carboxylic acid
Yield % (TOF, h−1
95 (38)
)
2
3
93 (37.2)
94 (37.6)
4
5
6
92 (36.8)
85 (34)
80 (32)
7
94 (37.6)
Reaction conditions: 50 cm3 of 1 M KOH, K2S2O8 (1.4 g, 5 mmol), K2[RuCl5(H2O)] (0.0374 g, 0.1 mmol) and primary alcohols (2 mmol). Irradiation at room temperature for 0.5 h. TOF =
moles of product/mol of catalyst × time.
stirring of the reaction mixture for 24 h yielded similar results with only
minor changes in the isolated yields.
3.3. Ultrasonic-assisted catalytic dehydrogenation of primary alcohols to
acids
In all reactions, yield and turnover frequency (TOF) were calculated
and the produced amides were identified by measuring their melting
point and recording their IR and 1H NMR spectra where appropriate.
Typical results of the ultrasonic-assisted catalytic dehydrogenation
of primary alcohols to carboxylic acids (Scheme 3) are shown in Table 4.
Table 5
Ultrasonic-assisted catalytic oxidation of alkenes and arenes to aldehydes, ketones or acids.
Entry
1
Substrate
Product
Yield % (TOF, h−1
75 (7.5)
)
2
80 (8)
3
4
94 (9.4)
92 (9.2)
5
6
94 (9.4)
95 (9.5)
Reaction conditions: alkene or arene (2 mmol), KIO4 (2.3 g, 10 mmol), K2[RuCl5(H2O)] (0.0374 g, 0.1 mmol) and solvents (5 cm3 of CH2Cl2, 10 cm3 of H2O and 5 cm3 of CH3CN). Irradiation
at room temperature for 2 h. TOF = moles of product/mol of catalyst × time.

72
A.G.F. Shoair / Journal of Molecular Liquids 206 (2015) 68–74
3.4. Ultrasonic-assisted catalytic oxidation of alkenes and arenes
Typical results of the ultrasonic-assisted catalytic oxidation of
alkenes and arenes are shown in Table 5. Here we investigated the use
of K2[RuCl5(H2O)] with excess of KIO4 in a biphasic solvent system for
oxidation of two arenes and four alkenes.
A mixture of substrate (2 mmol), KIO4 (10 mmol) in a biphasic
solvent system consisting of H2O, CH2Cl2 and CH3CN (ratio 2:1:1) and
catalytic amount of K2[RuCl5(H2O)] (0.0374 g., 0.1 mmol) was irradiated
by ultrasonic radiation for 2 h at room temperature. Naphthalene gave
phthalaldehyde in 75% yield as the main product (Table 5, entry 1).
This is in contrast with the other reported methods where formation of
1,4-naphthoquinone or naphthols were the predominant products [13,
14].
Scheme 4. Mechanistic catalytic cycle for ultrasonic oxidation of arenes and alkenes.
Treatment of anthracene under the same conditions gave the corre-
sponding 1,4-anthraquinones in 80% yield (Table 5, entry 2). Benzoic
acid is obtained in 94 and 92% yields as the main product in the oxida-
tion of styrene and trans-stilbene, respectively (Table 5, entries 3 and
4). For comparison with Sharpless results [15], this protocol was applied
to the oxidation of cyclopentene and cyclohexene. Glutaric acid and
adipic acid were obtained in 94 and 95% yields, respectively (Table 5,
entries 5 and 6).
Although the precise mechanism of the reaction awaits further
studies, by analogy with the previous works on ruthenium-catalyzed
oxidation of olefins [16], a proposed mechanistic catalytic cycle for the
oxidation of these substrates which rationalizes the formation of
[RuO4] is presented in Scheme 4. It is noteworthy that the addition of
acetonitrile improves the yields of the products as it has long been
known in RuCl3·nH2O/NaIO4 system [15], and we have reported a
similar effect in cis-[RuCl2(bipy)2]·2H2O/IO(OH)5 system for alkene
cleavage [16].
The action of CH3CN is not clear; Sharpless and co-workers have
suggested that in the case of ruthenium complexes, CH3CN is itself
resistant to oxidation and may stabilize lower intermediate oxidation
states of ruthenium involved in the catalytic cycle [15]. Yields and
turnover frequencies (TOF) were calculated and the products were
identified by measuring their melting point and recording their IR and
1H NMR spectra where appropriate.
We also studied the use of the same catalyst system prepared as
mentioned in Section 3.1 and under the same conditions for direct
dehydrogenation of some primary alcohols to their respective carboxylic
acids. To optimize the reaction conditions, the catalytic oxidation of
benzyl alcohol was selected as model reaction. Concerning the amount
of catalyst, it was found that increasing the catalyst amount to
0.2 mmol did not affect the yield (Table 2, entry 2b), but by decreasing
the catalyst amount to 0.01 mmol the yield was reduced to 20%
(Table 2, entry 3b), therefore, the best yield was obtained with
0.1 mmol of catalyst (Table 3, entry 1). We noticed that longer reaction
time than 0.5 h had no obvious effect on the yield (Table 2, entry 4b)
while, shorter reaction time than 0.5 h, the yield decreased significantly
(Table 2, entry 5b).
We found that when the reaction conducted at 60 °C, the yield was
not effected (Table 2, entry 6b). Finally, when we used KBrO3,
K3[Fe(CN)6] and NaOCl as co-oxidants instead of K2S2O8, the yield was
83, 75 and 44%, respectively (Table 2, entries 7b–9b). We found that
benzoic acid was not detected when the reaction was conducted in
the absence of K2[RuCl5(H2O)] or in the presence of K2S2O8 only.
To check the ability of this catalytic system to cleave the double
bond, the oxidation of cinnamyl alcohol was carried out and cinnamic
acid was produced in high yield (Table 4, entry 7). We noticed that
the para-substituted benzyl alcohol derivatives containing electron-
donating groups (Table 4, entries 2–4) were smoothly converted to
their respective acids in better yields than those containing electron-
withdrawing groups (Table 4, entries 5–6). In all reactions, yield and
turnover frequency (TOF) were calculated and the produced acids
were identified by measuring their melting point and recording their
IR and 1H NMR spectra where appropriate.
3.5. Synthesis and characterization of the complexes
We describe here the preparation of K2[RuO3(OH)2] and [RuO4] from
K2[RuCl5(H2O)] as a water soluble ruthenium precursor using ultrasonic
irradiation technique. It has been reported that the trans-
[RuO3(OH)2]2−/S2O82− reagent can be made from RuCl3·nH2O and
excess K2S2O8 in aqueous 1 M KOH [9]. The single x-ray crystal
structures of trans-K2[RuO3(OH)2] [17,18] and of trans-Ba[RuO3(OH)2]
[19] showed that these complexes contain the trigonal bipyramidal
1.4
1.3
O
O
O
O
O
O
r
r
r
r
r
r
i
i
i
i
i
i
g
g
g
g
g
g
i
i
i
i
i
i
n
n
n
n
n
n
P
P
P
P
P
P
r
r
r
r
r
r
o
o
o
o
o
o
8
8
8
8
8
8
E
E
E
E
E
E
v
v
v
v
v
v
a
a
a
a
a
a
l
l
l
l
l
l
u
u
u
u
u
u
a
a
a
a
a
a
t
t
t
t
t
t
i
i
i
i
i
i
o
o
o
o
o
o
n
n
n
n
n
n
O
O
O
O
O
O
r
r
r
r
r
r
i
i
i
i
i
i
g
g
g
g
g
g
i
i
i
i
i
i
n P r o 8 E v a l u a t i o n
____ K [RuCl (H O)]
2
5
2
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
(D3h
)
trans-[RuO3(OH)2]2− anion. During our attempt to use
K [RuO (OH) [
a l2a
3
2
K2[RuCl5(H2O)] as oxidation catalyst for some organic compounds, we
found that irradiation of K2[RuCl5(H2O)] with excess K2S2O8 in 1 M
KOH produces K2[RuO3(OH)2] in less than 2 min. Since a number of
ruthenium(VI) complexes were made by oxidation of ruthenium(III)
complexes using H2O2 or peracids [20], we used NaOCl in place of
K2S2O8 for in situ generation of K2[RuO3(OH)2]. In the latter preparation,
the solutions obtained were all unstable and did not show any advan-
tages over K2S2O8. Connick and Hurley [21] reported that [RuO4] is usu-
ally generated in situ from either RuCl3·nH2O or RuO2·nH2O followed
by extraction in CCl4. We found that [RuO4] is easily generated from
K2[RuCl5(H2O)] with excess KIO4 in H2O in less than 1 min using
ultrasonic irradiation technique. Electronic spectroscopy was used to
characterize the ultrasonically generated complexes K2[RuO3(OH)2]
and [RuO4] and also to establish the nature of intermediate catalytic
species in these catalytic oxidation reactions. Comparisons between
the electronic spectra of K2[RuCl5(H2O)] with the electronic spectra of
the ultrasonically generated complexes K2[RuO3(OH)2] and [RuO4]
n
n
n
n
n
P
P
P
P
P
r
r
r
r
r
o
o
o
o
o
8
8
8
8
8
E
E
E
E
E
v
u
t i o n
[RuO ]
4
v
v
v
v
a
a
a
a
l
l
l
l
u
u
u
u
a
a
a
a
t
t
t
t
i
i
i
i
o
o
o
o
n
n
n
n
200
250
300
350
400
450
500
550
600
650
Wavelength (nm)
Fig. 1. Electronic spectra of [RuO4], K2[RuO3(OH)2] and K2[RuCl5(H2O)] complexes.

A.G.F. Shoair / Journal of Molecular Liquids 206 (2015) 68–74
73
Fig. 2. Raman spectra of (A) K2[RuO3(OH)2] and (B) Ba[RuO3(OH)2].
showed that the complex K2[RuCl5(H2O)] displayed two bands at 224
3.6. Electrochemical properties of K2[RuCl5(H2O)]
and 314 nm, these bands are similar to those reported by Viljoen [22].
While the orange solution showed maxima at 369 and 468 nm due to
trans-[RuO3(OH)2]2− anion (Fig. 1), these data are similar to the data
reported by Griffith [23,24]. We also recorded the electronic spectrum
for the reaction mixture used in oxidation of alkenes and arenes and
found that the electronic spectra of the aqueous layer showed the
characteristic peaks of [RuO4] with attendant vibrational fine structure
at 374, 384 and 394 nm (Fig. 1). We have also evaporated and concen-
trated the orange solution to give dark red solution of K2[RuVI(OH)2O3]
from which Ba[RuVI(OH)2O3] is isolated (see Experimental section).
Raman spectrum of these two complexes showed the symmetric
Ru_O stretch in the range of 815–820 cm−1 (Fig. 2). It thus seems
clear that K2[RuCl5(H2O)] was oxidized by K2S2O8 in 1 M KOH to
generate K2[RuO3(OH)2] and by KIO4 in water/CCl4/CH3CN mixture to
generate [RuO4]. We also conclude that these are the effective catalytic
species in these catalytic oxidation reactions. Similar results were
reported for the oxoruthenate complexes containing [RuO4] [21–23].
The electrochemical behavior of K2[RuCl5(H2O)] was investigated by
cyclic voltammetry (Fig. 3). The investigation was carried out in
phosphate buffer (pH = 4.5) to confirm the possibility for generation
of oxidation states higher than three from K2[RuCl5(H2O)]. The complex
showed one reduction wave (peaks A and E), one oxidation wave
(peaks B and D) and one oxidation peak (C). The reduction wave is
quasireversible, characterized by peak-to-peak separation of 240 mV
and is assigned to Ru(III)/Ru(II) reduction. The oxidation wave is
quasireversible (ΔE = 214 mV) versus Ag+/AgCl electrode, and assign-
able to RuIII/RuIV oxidation. An oxidation peak found at 940 mV, the
height of the peak current for this peak is nearly twice that of the height
of peak current of the reduction wave. For this reason this peak is
assigned to oxidation of Ru(III) to higher oxidation states probably,
Ru(VI) or Ru(VIII), respectively. These results are similar to that reported
for the perruthenate anion, [RuO4]− [25].
4. Conclusions
We concluded that the complex K2[RuCl5(H2O)] is ultrasonically
oxidized by K2S2O8 in 1 M KOH to generate K2[RuO3(OH)2] in situ and
by KIO4 in H2O/CCl4/CH3CN mixture to generate [RuO4] at room temper-
ature in less than 2 min. The complex K2[RuCl5(H2O)] dehydrogenated
primary amines to nitriles and primary alcohols to acids at room
temperature. It also hydrated some nitriles to amides at 100 °C by
using ultrasound-assisted irradiation technique and in the presence of
K2S2O8 as co-oxidant. The structures of the generated active species in
these catalytic oxidation reactions were elucidated by using cyclic
voltammetric, electronic and Raman spectroscopic techniques and
found to be potassium ruthenate, K2[RuO3(OH)2] with K2S2O8, and ru-
thenium tetroxide, [RuO4] with KIO4.
0.0003
0.0002
0.0001
0.0000
-0.0001
-0.0002
B
C
A
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