Ru(VI)-CATALYZED OXIDATION OF 2-PROPANOL BY HEXACYANOFERRATE(III)
3
905
[hexacyanoferrate(III)] = 1.0 Â 10 M, [Ru(VI)] = 4.6 Â
DISCUSSION
6
10 M, [NaOH = 0.2 M, I = 0.5 M and T = 30°C. After
complete reduction of hexacyanoferrate(III), the organic
products were separated from the reaction mixture by
extraction with diethyl ether. The ether solution was
analyzed by gas chromatography, cyclobutanone being
found as the sole product. This result indicates that the
oxidation of 2-propanol by RuO24 occurs by a two-
electron transfer mechanism.
It is known that the MOx4 species of the 3d transition
metals do not expand their coordination shells in basic
media.23 However, such expansions can occur to a very
small extent in 4d and 5d transition metal oxyanions.24
During a study of the reaction between perruthenate
and manganate ions in aqueous alkaline medium, Luoma
and Brubaker25 suggested that one or more anions could
possibly have hydroxide ions associated with them. This
association with OH ions could be represented by the
equilibrium
Oxidation of 2-propanol by Ru(VI) and catalyst
regeneration by Fe(CN)63
ꢀx1
MOx4 OH ꢀMO4OH
ꢀ7
We also carried out some kinetic experiments to test
whether the oxidation of 2-propanol by catalytic
quantities of RuO24 proceeds at a similar rate to that of
where M is Ru or Mn and x = 1 or 2, depending on the
state of oxidation.
the reoxidation of catalyst by Fe(CN)63
.
Other transition metal oxyions have been postulated to
form hydroxy–oxy complexes: the OsO4(OH)22 species
has been isolated26 and mesoperrhenates ReO4(OH)32
can be isolated, although their concentration in alkaline
solutions of perrhenate must be small.27,28
Similarly, during the kinetic study of the decomposi-
tion of perruthenate ions in alkaline medium, Carrington
and Symons24 suggested that the coordination of OH
ions with RuO4 occurred to a lower extent, because
ruthenium represents an intermediate case between
osmium and rhenium. Therefore, ruthenate ions, Ru(VI),
may coordinate with hydroxide ions to a lower extent.
On the basis of literature data, we propose as the first
step of the reaction the coordination of ruthenate ion to
hydroxide ions, according to the equilibria:
The oxidation of 2-propanol by RuO24 was followed
spectrophotometrically. The initial rate was determined
by monitoring the increase in absorbance at 320 nm. At
this wavelength the product of the reaction [which
appears to be a soluble form of ruthenium(IV) similar to
the species formed during the electrochemical reduction
of ruthenate(VI)] conforms to Beer’s law with ꢀ = 33708
l mol 1 cm 1. Under the conditions [2-propanol] =
3
1 Â 10 M, [NaOH] = 0.5 M, T = 30°C and [RuO24 ] =
4
2 Â 10 M,
the
initial
rate
obtained
was
8
1
v0 = (4.5 Æ 0.02) Â 10 l mol
s
1. Assuming a first-
order dependence with respect to 2-propanol and RuO42
,
the initial rate (under the kinetic experimental conditions
6
[RuO24 ] = 4 Â 10 M,
[2-propanol] = 0.4 M
and
[NaOH] = 0.5 M) is v0 = (3.6 Æ 0.01) Â 10 7 l mol 1 s
.
1
We also determined the initial rate of catalyst
reoxidation in the following way. Ruthenate(VI)
(12.5 Â 10 3 mmol) was allowed to react at 30°C with
2-propanol (12.5 Â 10 3 mmol), both in 25 ml of 0.5 M
NaOH. When the reaction had gone to completion (as
indicated by the complete disappearance of any orange
color), 0.08 ml of the reaction mixture was mixed with an
appropriate volume of a 0.01 M solution Fe(CN)36 in
0.5 M NaOH, so that the final concentrations of Ru(IV)
K1
3
RuO24 OH RuO4ꢀOH
ꢀ8
ꢀ9
K2
3
4
2
RuO4ꢀOH OH RuO4ꢀOH
Moreover, in order to explain the v0 dependence on
[OH ], it is assumed that the active species of catalyst are
RuO24 , as the major species present, and RuO4(OH)3
to a much lower extent.
,
6
3
and Fe(CN)36 were 4 Â 10 and 1.0 Â 10 M respec-
tively. Under these conditions, the initial rate obtained
was v0 = (1.3 Æ 0.01) Â 10 7 l mol 1 s 1. The initial rate
was determined by monitoring the decrease in absor-
bance at 420 nm.
The dependence of the initial rate on [2-propanol]
suggests the formation of an intermediate complex, C12
,
between the catalyst active species and the organic
substrate. Thus, we can write for RuO24 species:
k1
ꢀCH32CHOH RuO24 C12
ꢀ10
k
1
Activation parameters
The results reported by Cundary and Drago29 for an
Under
the
conditions
[2-propanol] = 0.5 M,
[NaOH] = 0.1 M,
MO analysis of the interaction between cis-
3
[K3Fe(CN)6] = 0.4 Â 10 M,
[Ru(HN=CH—HC=
NH)2 (NH3)(O)]2 and methanol
6
[Ru(VI)] = 4 Â 10 M, I = 0.5 M and temperature range
show that the larger size of second-row transition metal
atoms increases the possibility of coordination of the
substrate to the metal via the hydroxylic oxygen. This
coordination implies a large negative DS*, indicating that
26–34°C, the experimental activation parameters deter-
1
mined
were
DH* = 19.02 Æ 0.13 kJ mol
and
DS* = 261.91 Æ 0.45 J mol 1 K
.
1
Copyright 1999 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 13, 901–908 (1999)