Kinetics of cyclohexene oxidation by p-quinones in aqueous-organic solutions of cationic palladium(II) complexes

Article abstract of DOI:10.1134/S0023158410050101

A kinetic study of cyclohexene oxidation by p-quinones in water-acetonitrile solutions of cationic Pd(II) complexes has been carried out for the first time. The observed kinetic regularities (dependence of the initial rate on the HClO₄ and p-benzoquinone concentrations) and the shape of the kinetic curves differ radically from those for the chloride systems. The unsteady-state process can be described only under the assumption that complexes of reduced palladium species with quinones are catalytically active forms. The role of solvated palladiumcarbenium ions in particular steps of the mechanism is discussed.

Full text of DOI:10.1134/S0023158410050101

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2010, Vol. 51, No. 5, pp. 691–703. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © O.N. Temkin, L.G. Bruk, D.S. Zakharova, K.Yu. Odintsov, E.A. Katsman, I.V. Petrov, O.Yu. Istomina, 2010, published in Kinetika i Kataliz, 2010, Vol. 51,
No. 5, pp. 715–727.
Kinetics of Cyclohexene Oxidation by pꢀQuinones
in Aqueous–Organic Solutions of Cationic Palladium(II) Complexes
O. N. Temkin, L. G. Bruk, D. S. Zakharova, K. Yu. Odintsov†,
E. A. Katsman, I. V. Petrov, and O. Yu. Istomina
Moscow State Academy of Fine Chemical Technology, Moscow, 119571 Russia
eꢀmail: olegtemkin@mail.ru
Received December 7, 2009; in final form, April 27, 2010
Abstract—A kinetic study of cyclohexene oxidation by
Pd(II) complexes has been carried out for the first time. The observed kinetic regularities (dependence of the
initial rate on the HClO4 and ꢀbenzoquinone concentrations) and the shape of the kinetic curves differ radꢀ
pꢀquinones in water–acetonitrile solutions of cationic
p
ically from those for the chloride systems. The unsteadyꢀstate process can be described only under the
assumption that complexes of reduced palladium species with quinones are catalytically active forms. The
role of solvated palladiumcarbenium ions in particular steps of the mechanism is discussed.
DOI: 10.1134/S0023158410050101
†
fins changes sharply. In the sulfate system under the
The discovery of catalytic systems based on palladium
complexes and the oxidation of olefins into aldehydes
and ketones [1, 2] (Wacker process) and into acetals,
ketals, and enol esters [3, 4] in solutions of PdCl2 and
CuCl2 stimulated studies aimed at modifying the cataꢀ
lytic systems and oxidation conditions in order to
increase the rate and selectivity of the reactions, espeꢀ
same conditions, the oxidation rate of propylene
exceeds that of ethylene by a factor of about 40 [15],
whereas in the chloride system the ratio of these rates
is 0.56 (Cl/Pd 20, [HClO4] = 1.0 mol/l, 298 K) [20].
≈
In the Pd(OAc)2–DMF–water system, benzoquinone
was also regenerated: hydroquinone was oxidized elecꢀ
trochemically [16] or via a reaction with oxygen cataꢀ
lyzed by the iron phthalocyaninate complex (PcFe)
[21].
The following interesting hypotheses, which
undoubtedly need experimental verification, have
been suggested [10–15].
cially those of the syntheses of ketones from higher ꢀ
α
olefins, internal olefins, and cycloolefins. Aqueous–
organic systems with dimethylfomamide (DMF), aceꢀ
tonitrile, and alcohols based on PdCl2 with CuCl2
(CuCl2–O2) or pꢀbenzoquinone (BQ) as the oxidant
were used [5, 6]. The reaction of ethylene with pallaꢀ
dium sulfate and nitrate in acidic media was observed
by Smidt et al. [7] as early as 1962. However, the nonꢀ
chloride systems with quinones and heteropoly acids
as oxidants were likely to be proposed and studied for
the first time by Matveev and his colleagues [8–15].
The PdSO4–H2SO4 (HClO4)–H2O–BQ systems are
active in strongly acidic media (0.1–3.0 M HClO4) at
20–30°С in ethylene and propylene oxidation, and
the PdSO4–HPA systems [12, 13] are active in the oxiꢀ
dation of ethylene and butylenes. These systems
(1) The main state of palladium in the Pd2+–H+–
BQ–H2O catalytic system is represented by the
2+
Pd2
reduced palladium species Pd(0) and
stabilized
by quinone and olefins. There is up to 2 mol of olefin
(ethylene, propylene) per mole of palladium in the
complex. The complex Pd2SO4(BQ) · 3H2O was isoꢀ
lated [22], but its structure was not determined.
(2) The increase in the rate of olefin oxidation by
quinones with an increase in the hydrogen ion conꢀ
centration is explained by the fact that the slow step of
this process, unlike the slow step in the chloride sysꢀ
(PdSO4, Pd(OAc)2 with BQ or HPA) in aqueous–
organic homogeneous [16–18] or twoꢀphase [19] tems, is oxidation by quinone involving the proton of
the
plex, yielding a Pd(II)
Pd(BQ)(C2H4)2 + H+
This ꢀcomplex decomposes rapidly under the
π
ꢀcomplex Pd(0)(BQ)(CnH2 )2 or a Pd(I)
ꢀcomplenx, for example,
Pd(C2H4)2(BQH)+
π
ꢀcomꢀ
media turned out to be especially efficient in the case
π
of higher olefins and cycloolefins. For instance, on
−
passing from the Сl– anion to the
anion in an
.
ClO4
acidic medium, the productivity of ꢀdecene oxidaꢀ
α
π
tion (mol l–1 h–1) increases by a factor of about 24 at action of a water molecule and a proton to
[
H+] = 0.1 mol/l and by a factor of ~60 at [H+] =
Pd(0)(C2H4), CH3CHO, and BQH2, probably
through the formation of the hydroxyethyl–palladium
(metallahydrin) intermediate that was proposed by
Moiseev [4] and is now accepted by nearly all
0.3 mol/l [17]. In addition, the reactivity of lower oleꢀ
†
Deceased.
691

692
TEMKIN et al.
researchers. Some of the earlier observations [10–15] nonꢀchloride aqueous–organic and aqueous systems.
were confirmed by the study of aqueous–organic nonꢀ Now the likelihood of catalysis of oxidation processes
chloride systems in the oxidation of higher olefins by by complexes of reduced palladium species with
p
ꢀquinones [16–18, 21].
The main conclusions from these works are as folꢀ
quinones (Q) is doubtless [30]. For instance, Pd(I)
carbonyl complexes catalyze the oxidative carbonylaꢀ
tion of alcohols to dialkyl oxalates [31, 32] and Pd(0)
complexes catalyze the oxidative dehydrogenation of
olefins [33] and the oxidative carbonylation of alkynes
at the C–H bond [34, 35]. Interesting information on
the mechanism of action of palladium dications in
aqueous acidic media was provided by the study of
their reduction by alcohols and Fe(II) ions and their
catalytic effect in alcohol oxidation by oxygen [36–
lows:
(1) Evidently, it is Pd(II) cationic complexes that
are the precursors of the catalytically active species in
the process. It was shown that the reaction rate in these
systems is independent of the nature of the anion in
2−
−
the palladium salt ( ,
ence of strong acids. Undoubtedly, the Pd3(OAc)6 triꢀ
mer, reacting with a strong acid and acetonitrile, is
,
OAc– [17]) in the presꢀ
SO4 NO3
39]. ꢀAllyl complexes and products of the allylic oxiꢀ
π
2+
dation of olefins [40] were observed in the oxidation of
olefins and cycloolefins substituted at the double bond
by palladium(II) cations in water.
The purpose of the present work is to carry out a
kinetic analysis of olefin oxidation by quinones under
conditions of the existence of palladium cationic
complexes using cyclohexene oxidation in aqueous–
organic media as an example and to find approaches
to the elucidation of the mechanism of this reaction
and of the chemical and kinetic functions of
converted into Pd(CH3CN)4 –
·
complexes.
(H2O)x
х
(2) In all of the “cationic” systems, the oxidation
rate or the product yield increases over the same time
as [H+] increases up to a constant (maximum) value.
The acceleration of the reaction by strong acids due to
the formation of palladium cationic complexes is also
known for the oxidation of arenes by palladium aceꢀ
tate into biaryls in acetic acid [23–25].
It should be mentioned that the dependence of the
olefin oxidation rate (
systems is not always described by the classical equaꢀ
tion
/[H+] [4]. The zero order with respect to the
acid concentration was observed for the oxidation of
cyclohexene by ꢀbenzoquinone in the PdCl2–LiCl–
HClO4–H2O system [20, 26]. The independence of
the ethylene oxidation rate of [HCl] in the 0.5– 10 ml in volume in a temperatureꢀcontrolled (23°С
2.0 mol/l range ([HCl + LiCl] = 5 mol/l) was observed sealed reactor equipped with a magnetic stirrer. The
in the PdCl2–HgCl2–FeCl3 system at 90°С (
reactor was charged with a palladium salt (palladium
/[H+]) [27]. Dependences on [H+] with an extreꢀ acetate),
ꢀbenzoquinone (or another quinone), and
r
) on [H+] in aqueous chloride
p
ꢀquinones [30].
r
= a
EXPERIMENTAL
Kinetic experiments were carried out in a solution
p
)
r
= b +
a
p
mum were observed at Cl/Pd < 20 in the oxidation of 6 ml of an acetonitrile–water (7 : 1 vol/vol) mixture as
transꢀhexꢀ2ꢀene [20], and the firstꢀorder reaction rate the solvent. Next, the necessary amount of perchloric
increased only in the [H+] range from 0.02 to acid in the solvent (2 ml) was introduced. The reactor
0.1 mol/l.
was blown with nitrogen (500 ml, measured with a
burette) and sealed, and the solution was stirred magꢀ
netically for 20 min. The necessary amounts of the
substrate (cyclohexene) and the reference substance
(methyl ethyl ketone) were dissolved in the solvent
(2 ml), and the resulting solution was analyzed by
GLC. After that, the solution was syringed into the
reactor. The moment of injection of the solution of the
substrate was taken to be the onset of the reaction. The
first sample for GLC analysis was taken with a
microsyringe after 1 min. Thereafter, the reaction
mixture was sampled at 6ꢀmin intervals during the
experiment.
Estimation of the degree of hydrolysis of Pd2+ in
water from the equilibrium constants of hydrolysis
[28] shows that, for the largest known equilibrium
constants (pK = 1.6), the degree of hydrolysis does not
exceed 20% at [HClO4] = 0.1 mol/l and [Pd] =
0.004 mol/l and decreases to 9% on going to an acid
concentration of 0.2 mol/l. According to the preꢀ
sented estimates, if they are valid for the acetonitrile–
water system, the influence of the acid concentration
on the increase in the concentration of the palladium
cation does not explain the approximately twofold
increase in the rate of decꢀ1ꢀene oxidation in the
[
HClO4] = 0.1–0.3 mol/l range [17]. The absence of
GLC analysis was carried out on an LKhMꢀ8MD
chromatograph (thermalꢀconductivity detector, colꢀ
umn length of 1 m, Porapak Q as the adsorbent,
helium as the carrier gas (1.7 l/h), injection port temꢀ
perature of 250°С, oven temperature of 200°С, detecꢀ
an influence of the anion nature on the rate of olefin
oxidation, mentioned above [17], may also indicate an
insignificant contribution of the hydroxylꢀcontaining
palladium(II) complexes to the oxidation rate.
All these observations [6, 10–18, 21] and especially tor temperature of 180°С, detector current of 150 mA)
the change in the relative reactivity of the ethylꢀ for determination of cyclohexene, another LKhMꢀ
ene/propylene pair [15] suggest that oxidation via 8MD chromatograph (thermalꢀconductivity detector,
mechanisms different from the classical mechanism column length of 3 m, Apiezon as the adsorbent,
substantiated by Moiseev [4, 29] takes place in the helium as the carrier gas (1.35 l/h), injection port temꢀ
KINETICS AND CATALYSIS Vol. 51
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2010

KINETICS OF CYCLOHEXENE OXIDATION BY
p
ꢀQUINONES
693
perature of 240°С, oven temperature of 150°С, detecꢀ
[S], mol/l
0.16
tor temperature of 220°С, detector current of 140 mA
for the determination of cyclohexanone, and a Tsvet
chromatograph (flameꢀionization detector, column
length of 3 m, 10% PPMSꢀ4/Polikhrom I phase, air
(0.33 l/h), hydrogen (0.03 l/h), helium (1.2 l/h),
injection port temperature of 300°С, oven temperaꢀ
ture of 100°С). The composition of the liquid phase
after the run was determined by chromatography couꢀ
pled with mass spectrometry on an Agilent Technoloꢀ
gies 6890 N gas chromatograph (United States)
equipped with an Agilent Technologies 5973 mass
detector (capillary column HPꢀ1; polymethylsiloxane
active phase; column length of 50 m; inner diameter of
0.12
0.08
0.04
A
B
0
20
40
60
min
t,
0.32 m; 0.51ꢀ mꢀthick phase layer; helium as the carꢀ
μ
Fig. 1. Time dependence of the cyclohexene concentration
[S]. The data reproducibility in four experiments is demꢀ
onstrated. [Pd] = 0.004 mol/l, [ВQ] = 0.2 mol/l,
rier gas (1 ml/min); flow ratio of 100 : 1; temperatureꢀ
programmed heating of the columns: isotherm at
50°С (5 min), heating rate 15 K/min (12 min), isoꢀ
0
0
[
HClO ] = 0.2 mol/l.
4
therm at 230°С (5 min); sample size of 0.2 l).
μ
Specimens for highꢀresolution transmission elecꢀ
tron microscopy (HRTEM) on a JEOL JEM 2100
instrument were prepared as follows. Magnesium
oxide was deposited onto the copper grid of the detecꢀ
tor. For this purpose, the grid was held for 2–5 s in the
white smoke of burning magnesium. Next, the soluꢀ
tion to be examined was sprayed as a mist onto the grid
using an ultrasonic disperser or a sprayer. The analyzed
samples were compared with those obtained by sprayꢀ
strate concentration calculated using the optimum
model from the experimental concentration was
somewhat larger than 4 mmol/l, which is close to the
experimental error (Fig. 1).
RESULTS AND DISCUSSION
The study of reaction (I) involving BQ and other
ing solutions of nanoclusters (giant palladium clusters quinones (Q),
[41] kindly presented by I.P. Stolyarov) in acetonitrile.
C6H10 + H2O + Q
C6H10O + QH2
(I)
Distilled cyclohexene (reagent grade, Acrosorganꢀ
ics), acetonitrile (reagent grade), freshly sublimated
in the Pd(OAс)2–HClO4–СH3СN–H2O system
1
pꢀbenzoquinone, and palladium acetate synthesized
showed [45] that, in the ranges of Pd(II) concentraꢀ
according to a known procedure [42] were used.
tions from 0.001 to 0.005 mol/l and HClO4 concentraꢀ
tions from 0.1–0.5 mol/l at an acetonitrile to water
molar ratio of 2.4 (used earlier [16–18]) and temperaꢀ
tures of 20–25°С, the substrate disappearance kinetics
is well reproducible (Fig. 1). Cyclohexene (S) is oxiꢀ
dized to cyclohexanone (P) with high selectivity (95–
100%). Among the byꢀproducts, we identified 2ꢀ
cyclohexenꢀ1ꢀol and 2ꢀcyclohexenꢀ1ꢀone. When the
quinone is absent or deficient, palladium black or palꢀ
ladium mirror forms and benzene and cyclohexane
appear among the products. At a Pd(OAc)2 concentraꢀ
tion of 0.05 mol/l, the proportion of 2ꢀcyclohexenꢀ1ꢀ
one reaches 13%. The study of the Pd(OAс)2
(0.05 mol/l)–HClO4 (0.2 mol/l)–BQ (0.2 mol/l)–S
(0.06 mol/l)–CH3СN–H2O mixture showed that the
Pd(OAс)2–СH3СN–H2O system is quite stable both
in the absence and in the presence of the acid. The
Pd(OAс)2–BQ system without the acid and substrate
does not yield palladium black. In the Pd(OAс)2–
HClO4–S system, the solution rapidly turns colorless
and palladium black, cyclohexanone, and benzene
form. Cyclohexanone is almost nonoxidizable in the
Pd(OAс)2–HClO4 system.
The program for solving the direct problem of
chemical kinetics in a closed wellꢀstirred reactor [43]
uses the STIFF subprogram [44] recompiled for an
ATX computer (Fortran 95 language) for numerical
integration of systems of ordinary differential equaꢀ
tions. The program for numerical estimation of the
parameters of a given kinetic model [43] uses the
method of configurations in the minimization of the
deviation of calculated responses from the measured
values. This method is among the direct extremum
seeking methods; that is, it requires no calculation of
derivatives of the objective function of the sought
parameters. Since the values of the sought parameters
differ strongly in their order of magnitude, they were
varied in the logarithmic field. The increment was
changed automatically during the search in such a way
that the maximum convergence rate was ensured along
with the required accuracy. The objective function, a
measure of the deviation of a model from experimenꢀ
tal data, was the squared difference between the calcuꢀ
lated and measured responses (current concentrations
of the substrate in the reaction mixture) averaged over
the entire data array. Particular runs were compared in
terms of the sum of the squares of these deviations.
Note that the rootꢀmeanꢀsquare deviation of the subꢀ
1
E.A. Sudachenko took part in the preliminary experiments.
KINETICS AND CATALYSIS Vol. 51
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2010

694
[S], mol/l
TEMKIN et al.
(in terms of the sum of squared deviations between
the experimental and calculated [S] values (SS)) of
region using Eq. (1) is attained at = 2.
In the case of the model described by law (2), the
best combination of the and values is = 2 and
= 0.2. The introduction of the quinone concentraꢀ
tion ([Q] = + [S], where = [Q]0 – [S]0) into Eqs. (1)
and (2), which yielded
0.16
0.12
0.08
0.04
A
n
n
m
n
m
3
a
а
2
–d[S]/d
[S]n[Q]m =
[S]n(
–d[S]/d + [S])/(1 + k
t
=
k
k
[S]n(
a
+ [S])m,
(3)
(4)
t
=
k
a
1[S]m)
1, 1'
improved the description of the curves by Eq. (3) for
the experiments presented in Fig. 1 at = 1 and = 1
= 0.05). However, the best description (in terms of
SS) of experiments at = 0.25 ( = 2. = 1) is at variꢀ
ance with the run of the kinetic curves in Fig. 2. This
situation is not improved by passing to Eq. (4). It folꢀ
lows from these results that several forms of active speꢀ
cies are involved in the process.
0
10
20
30
40
50
60
min
n
m
t,
(a
а
n
m
Fig. 2. Kinetics of cyclohexene disappearance at different
[Pd] values of ( ) 0.004 ( is a replica of ), ) 0.002, and
) 0.0015 mol/l; [ВQ] = 0.4 mol/l, [HClO ] = 0.2 mol/l.
1
1
'
1 (2
0
(
3
0
4
A kinetic analysis showed that the kinetic curves
have two characteristic parts (Figs. 1, 2) under nearly
all conditions. The rapid decrease in the reaction rate
The next stage of the work was to determine the
dependences of the initial rates (region A) on the iniꢀ
tial concentrations of Н+ and BQ. The dependence of
r0 on [H+]0 (Fig. 4) was obtained at a constant ionic
strength of [HClO4] + [LiClO4] = 0.7 mol/l. In full
agreement with the literature [17], the initial reaction
rate r0 is zero at [H+] = 0 and increases with an
increase in [H+]0, passing through a weak maximum at
with time, depending on [Pd]0, [S]0, [H+]0, and [Q]0
,
is observed in the region A. In the region B, the rate
changes insignificantly with time as the substrate conꢀ
centration ([S]) decreases, being almost independent
(in a certain concentration range) of the initial conꢀ
centrations of the reactants and catalyst (Fig. 3).
Under conditions of a 2.6ꢀfold excess of quinone
over the substrate and a constant acid concentration,
the dependences are not described by simple kinetic
laws (1) and (2) with respect to [S]:
[
H+] = 0.4–0.5 mol/l. In the region
B, the rate rB
remains almost unchanged with time and is indepenꢀ
dent of [H+]0. The initial oxidation rate increases with
an increase in the benzoquinone concentration to
approximately [BQ]0 = 0.4 mol/l and then passes
through a maximum (Fig. 5). As in the experimental
–d[S]/d
t
=
k
[S]n,
1[S]m),
(1)
(2)
series with [H+]0, the rate
rB in the region
B is almost
–d[S]/d k
t
=
k
[S]n/(1 +
independent of [BQ]0. Since the initial rate depends
which are characteristic of processes involving a single explicitly on the benzoquinone concentration, we
form of the active species. The best description studied the influence of the nature of the ꢀquinone on
p
1
1
[S], mol/l
0.16
r
0, mol l− min−
0.06
0.04
0.02
0.12
0.08
0.04
4, 4'
3
2
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7
[HClO4], mol/l
1, 1'
0
10 20 30 40 50 60
min
t,
Fig. 3. Kinetics of cyclohexene disappearance in the
region at different [Pd] value of ( ) 0.004 ( ' is a replica
of ), ( ) 0.001 ( ' is a replica of
mol/l; [BQ] = 0.4 mol/l, [HClO ] = 0.2 mol/l.
Fig. 4. Dependence of the initial oxidation rate (r ) on
0
+
B
1
1
[H ] at a constant ionic strength of [HClO ] + [LiClO ] =
0
0 4 4
1
2
) 0.002, (
3
) 0.0015, (
4
4
4)
0.7 mol/l, [Pd] = 0.004 mol/l, [ВQ] = 0.2 mol/l, and
0 0
[S] = 0.15 mol/l.
0
4
0
KINETICS AND CATALYSIS Vol. 51
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2010

KINETICS OF CYCLOHEXENE OXIDATION BY
p
ꢀQUINONES
695
1
1
0, mol l− min−
[S], mol/l
r
0.16
0.12
0.08
0.04
0.08
0.06
0.04
0.02
1
2
3
0
0.1 0.2 0.3 0.4 0.5 0.6
[BQ], mol/l
0
10 20 30 40 50 60 70
, min
t
Fig. 5. Dependence of the initial oxidation rate on the
p
ꢀ
Fig. 6. Influence of the nature of
p
ꢀquinone on the cycloꢀ
) 2,6ꢀdi(tertꢀbutyl)ꢀp
ꢀbenzoquinone, and ( pꢀ
benzoquinone. [Pd] = 0.004 mol/l, [HClO ] = 0.2 mol/l,
benzoquinone concentration; [Pd] 0.004 mol/l,
=
hexene disappearance kinetics: (
1
ꢀ
0
[
HClO ] = 0.2 mol/l, [S] = 0.15 mol/l.
benzoquinone, (2) trimethylꢀ
p
3)
4
0
0
4
[Q] = 0.2 mol/l.
0
the rate of reaction (I). When trimethylꢀ
pꢀbenzoꢀ
quinone and 2,6ꢀdi(tertꢀbutyl)ꢀ ꢀbenzoquinone were
p
with respect to [S]0 is below unity. An analysis of the
unsteadyꢀstate model (against eight experiments)
showed the high inaccuracy of the description, and
reaction scheme (III) does not describe the run of the
used, the catalytic oxidation process was observed, but
the rate of oxidation by quinones containing electronꢀ
donating substituents was considerably lower (Fig. 6).
The other quinones (anthraquinoneꢀ2,6ꢀdisulfonic
kinetic curves either: the regions
A and B are absent,
acid,
2,3ꢀdichloroꢀ1,4ꢀnaphthaquinone,
ꢀbenzoquinone) are
and the dependences on the palladium concentration
are described inadequately. An analysis of reaction
scheme (III) also confirms the assumption that several
forms of palladium, namely, Pd(II) and Pd(0), Pd(II)
and Pd(I), Pd(I) and Pd(0), or all the three forms of
the complexes contribute to the rate of substrate disapꢀ
pearance in the multiroute reaction.
anthraquinone, and tetrachloroꢀ
p
very poorly soluble in the acetonitrile–water mixture.
The shape of the kinetic curves, an analysis of the
reaction order with respect to [S], and the depenꢀ
dences of the initial rates on [H+]0, [BQ]0, and the
nature of pꢀquinones are in accord with the concluꢀ
sions from the works on ethylene and propylene oxidaꢀ
tion in aqueous solutions of cationic palladium(II)
complexes [14, 15] and suggest that the mechanism of
reaction (I) in the system examined is not defined by
the classical sequence of steps of the soꢀcalled Wacker
process [4]:
Under conditions of the formation of several active
species from the precursors and catalysis by the most
active species, the initial reaction rates in the region
A
cannot correspond to the kinetics of a quasiꢀsteadyꢀ
state process. Note that the condition for applicability
+
S, H O
2
–P
BQ, H
Pd(0) –BQH Pd(II).
Pd(II)
(II)
2
1
1
r
0, mol l− min−
If this oneꢀroute mechanism is modified by the
introduction of BQ into the substrate conversion step,
0.10
0.08
0.06
0.04
0.02
+
H
S
2
Pd(II)
Pd(II)S BQ, H O Pd(0)BQ –BQH Pd(II),(III)
P
2
the kinetic equation obtained under the quasiꢀsteadyꢀ
state conditions with respect to the Pd(0)BQ intermeꢀ
diate with allowance made for the participation of all
palladium species in the material balance will not
agree with the partial dependences [S]–
order with respect to the substrate decreases with a
0–[BQ]0
t, rt–[S] (the
0
0.002
0.004 0.006
[Pd]0, mol/l
decrease in its concentration),
r
, and
r0⎯[H+]0. The independence of r0 on [BQ]0 is observed
at the zero order with respect to the substrate, whereas
the first order with respect to [BQ]0 is obtained at the
Fig. 7. Dependence of the initial oxidation rate on [Pd] at
[BQ] = 0.004 mol/l, [HClO ] = 0.2 mol/l, and [S] =
0.15 mol/l.
0
0
0
4
zero order with respect to [H+] and when the order
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TEMKIN et al.
of quasiꢀsteadyꢀstate treatment to homogeneous proꢀ ined contains nanoclusters that are inactive in catalyꢀ
cesses ([S]0/[Pd]0 15 [30]) in the concentration sis, are stabilized by BQ and the substrate, and are in
≥
ranges examined is not satisfied at conversions higher equilibrium with the active monomeric and dimeric
2+
than 60–80%.
molecular complexes of Pd(0) or
, respectively. In
Pd2
It follows from the analysis of the kinetic curves for
various [Pd]0 values (Fig. 2) that the initial rate is a
complicated function of [Pd]0 represented by an
Sꢀshaped curve in the concentration range from 0.001
to 0.006 mol/l (Fig. 7), secondꢀorder with respect to
[Pd]0 in the 0–0.002 mol/l range. The reaction rate for
this case, the concentration of the active complexes
can change little with the initial metal concentration
[M]0) [30]. A TEM study of the reaction solutions
showed, however, that the solutions do not contain
particles larger than 1 nm. The particles of the “giant
cluster” of Pd [41] 26.0 3.5 Å in size are clearly seen
in the TEM images.
(
the oxidation of the substrate S in the region
almost independent of [Pd]0 (Fig. 3).
B is
An analysis of very simple unsteadyꢀstate models
with one active catalyst species, М•, and the precursor
M (one catalytic route), in which the rates of catalytic
cycles on various catalyst species were defined as the
rate of the first, irreversible step, ignoring the fast conꢀ
version of the intermediates, for example,
The weak dependence of r0 on [Pd]0 in the region B,
some darkening of the solution during the reaction,
and the known facts of catalysis by nanoparticles and
colloids in homogeneous systems [30], in combination
with published data [36–39] on the appearance of colꢀ
loidal palladium in aqueous cationic systems with such
reducing agents as alcohols and olefins and on the
catalysis of oxidation reactions of these substrates by
M• + S
M• + P,
–1.50
colloidal palladium, suggest that the solution examꢀ showed that models 1.30
M + S
M• + P, 2M + S
M• + P, M• + S
M• + P, M + S
M• + P, M• + S
2M• + P,
M• + P
M• + S
1.30
1.40
1.50
do not fit the experimental data for [BQ] = 0.4 mol/l ments with the number of measurements in each entry
(Fig. 2). The symbol M in model 1.50 designates the taken into account), models 1.3 and 1.5 being preferꢀ
Pd22+
from Pd2+
The introduction of a catalytic cycle for the
species M,
able: 1.3 (0.236
1.4 (0.86
10–4). In all models, there is fast converꢀ
sion of M into М• at the beginning of the region
Modification of models 1.3 1.5 by the introduction
of the reversible step of intermediate formation,
×
10–4)
≈
1.5 (0.23
×
10–4) <
ꢀtype dimeric complex forming immediately
×
.
А.
–
M + S
M + P,
M• + S MS•,
into these models (models 1.3
ance of the second catalytic route, substantially
improves the description of the curves presented in
–1.5), i.e., the appearꢀ
provided a very insignificant improvement to the
description of the experiment. Models 1.3 1.5 can be
–
Fig. 2 (in terms of the average sum of the squared deviꢀ clearly visualized by representing the catalytic cycles
ations SS (given in parentheses) for particular experiꢀ as a loop in a graph, for instance,
S
Pd2+, S
(2)
S
Pd2+
Pd0
Pd2+
Pd22+
Pd22+
Pd0
2
(2)
(2)
(1)
(3)
(1)
(3)
(1)
(3)
1.3
1.4
1.5
To take into account the variation of the quinone rates of several steps of kinetic models 1.3
–
1.5
,
concentration with time, the quinone concentraꢀ and the model series 1.3М2
–1.5М2 was thus
tion [Q] was introduced into the equations for the obtained:
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697
Q
S
S
Pd2+
X1
Pd0Q
X2
–P
,
1.3M2
P
Pd2+
Q
P
Q
Pd2+, Q
–P
S
S
X1
Pd2Q2+
X2
,
1.4M2
P
Q
P
Q
Q
S
S
Pd2Q2+
X1
Pd0Q
X2
2
–P
,
1.5M2
1.5M3
P
Q
P
Q
S, Q
–P
Q
S
2 Pd2+
S
Pd2Q2+
X1
Pd0Q
P
X2
2
–P
.
P
Q
Q
Model 1.5М3 includes irreversible [Pd2Q]2+ forꢀ observations, the steps in which M• is converted into
mation from Pd2+. Passing to these models noticeably M, i.e., the processes of oxidation by quinones and a
improved the description of the experimental curves proton of the complexes in the lower oxidation states
for different values of [Pd]0 and [Q]0. According to the to the complexes in the higher oxidation state (e.g.,
SS values (in parentheses), the models are arranged in Pd2+), should be introduced into the next generations
the following order:
1.5М3 (0.05
1.5М2 (0.66
Almost the same order of models,
1.5М3 (0.67
10–4) < 1.3М2 (1.43
1.5М2 (1.44
of the kinetic models. The introduction of these steps
will result in the appearance of threeꢀroute mechaꢀ
nisms; for instance, model 1.5М3 will transform into
×
10–4) < 1.3М2 (0.12
10–4) < 1.4М2 (1.0
×
×
10–4)
10–4).
<
×
model 3.1.5М3
:
(7)
2 Pd2+
×
×
10–4)
≈
×
10–4) < 1.4М2 (1.79 10–4)
×
S, Q
(1)
–P
was obtained in terms of the sums of squared deviaꢀ
tions calculated for all experiments with one set of rate
constants, nonlinear model 1.5М3 being preferable.
3.1.5M3
Q
S
S
(2)
Pd2Q2+
X1
2 Pd0Q
X2
–P
(4)
(5)
An analysis of simple models 1.3–1.5 and 1.3М2
–
1.5М3 shows that, if in the region
А
the species M is
rapidly converted into the species М•, then, after the
consumption of the substrate and the addition of a new
portion of cyclohexene, it is to be expected that the
process will be repeated at the rate corresponding to
the activity of the species М•, for which the rate conꢀ
stants are 10–100 times lower than those for the speꢀ
cies M. It turned out that the addition of the substrate
at the end of the reaction results in an almost complete
reproduction of the initial curve, whereas the addition
of the substrate in the 8th or 20th minute changes the
shape of the curve, reproducing the shape of some porꢀ
tion of the initial curve. It follows from these data that,
during the process, the species М• is converted back
into the species M, and both species coexist in differꢀ
ent ratios throughout the process. In view of these
(6)
(3)
P
Q
P
Q
Model 3.1.5М3 describes the effect of substrate
addition but does not completely reproduce the shape
of the kinetic curve in the region
upon the introduction of a new portion of the substrate
after the complete consumption of the initial portion.
For this reason, the oxidation steps Pd(I)
and Pd(0) Pd(I) were additionally introduced into
model 3.1.5М3
B and its character
Pd(II)
,
(8) Pd2Q2+ + 2H+
(9) PdQ0 + Pd2+
2Pd2+ + QH2,
Pd2Q2+,
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698
TEMKIN et al.
[S], mol/l
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
4
3
2
1
0
10
20
30
40
50
60
70
, min
t
Fig. 8. Kinetics of cyclohexene disappearance at different initial concentrations of palladium and
0.004, ( ) 0.002, and ( ) 0.0015 mol/l; [BQ] = 0.4 and (
p
ꢀbenzoquinone: [Pd] = (1)
0
3
4
2
) 0.004 mol/l; [BQ] = 0.2 mol/l, [HClO ] = 0.2 mol/l.
0
0
4
taking into account that step (9) (Pd(I) disproportionꢀ in this part of the curve have the same order, but rP2 is
ation) is reversible, and model 4.1.5М3 was obtained. higher than rP3 by a factor of 2–3.
The fiveꢀroute mechanism of cyclohexene oxidation
(model 4.1.5М3) provides a satisfactory description of
both regions of the curves (А and B) for different conꢀ
(4) After the substrate is consumed (73rd minute),
more than a half of the palladium still remains in the
form of Pd(0), and it is on these Pd(0) and Pd(I) forms
that the process resumes upon substrate addition. If
the substrate is not added in the 73rd minute, then,
according to the kinetic model, by the 90th minute
nearly all of the Pd will have been convert into Pd(II),
and in the 76th minute it will exist mainly in the form
of Pd(I).
ditions (Fig. 8) and for experiments with the initial and
added substrate at high benzoquinone concentrations
(Fig. 9). The SS value is 0.17
×
10–4, i.e., 4.1 mmol/l,
which is close to the experimental error. Step (9)
turned out to be close to equilibrium at all reaction
×
times, with an equilibrium constant of 3.6
104 l/mol
(in the computation variant presented). A similar disꢀ
proportionation reaction, but without a revere proꢀ
cess, was proposed for the reduction of palladium(II)
complexes with hydrogen [46].
The results of determination of the rate constants
of the reaction steps, the rates of these steps, and the
concentrations of the intermediates in the framework
of model 4.1.5М3 and the performed computation
suggest the following:
(5) The region
B of the kinetic curve for the subꢀ
strate indicates a nearly constant reaction rate in spite
of the substrate concentration [S] decreasing with
time (Figs. 2, 3). This is due to the fact that the
increase in the univalent palladium concentration is
approximately in inverse proportion to [S], owing to
which the rate of the main route, including steps (2)
and (3), remains almost unchanged. The contribution
from this route is great (more than 80%);
(1) Step (8) nearly does not contribute to the
change in the Pd(I) concentration, and the rate of the
reaction via route P1 (steps (1) and (8)) is the lowest.
(6) An increase in the total catalyst concentration
from 0.0015 to 0.004 mol/l does not result in a proporꢀ
tional increase in the reaction rate (Fig. 3). However,
the main reason for this behavior is that, in the
(2) In the region A, at the first moments of the reacꢀ
tion, the entire Pd(II) is rapidly converted into Pd(0)
through Pd(I), and the high rate of substrate disapꢀ
pearance in the first 5 min is mainly due to the rate of
routes P2 (steps (2) and (3)) and P3 (steps (5) and (6)),
the contribution of route P2 being strongly predomiꢀ
nant. The maximum contribution to the reaction rate
is made by routes P4 (steps (1), (2), (4), and (7)) and
region
B of the kinetic curve, the low substrate conꢀ
centration corresponds to the high catalyst concentraꢀ
tion and vice versa.
Thus, in the framework of the model considered,
the initial palladium(II) participates mainly in the forꢀ
mation of the Pd(0) and Pd(I) complexes, including
those resulting from step (9). It should be noted that
simpler models do not fit the observed kinetics.
P5 (steps (1), (7), and (
(3) In the region , the greater part of the pallaꢀ
dium exists in the form of PdQ0 and a smaller part
⎯9)).
B
For further discrimination of the kinetic models,
exists in the form of Х1. The rates of routes P2 and P3 it is necessary to analyze hypotheses about the
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KINETICS OF CYCLOHEXENE OXIDATION BY
p
ꢀQUINONES
699
mechanisms of olefin oxidation involving possible
[S], mol/l
0.15
2+
Pd2+,
, and Pd0 complexes in order to establish
Pd2
theoretically possible ways of kinetic coupling
between the intermediates and routes with the
quinone, acid, substrate, and palladium concentraꢀ
tions taken into account.
In the catalytic system under study, the main
ligands stabilizing the palladium complexes in differꢀ
ent oxidation states are acetonitrile, quinones, and
olefins. The cationic complexes [Pd(CH3CN)4]2+ and
[Pd2(CH3CN)6]2+ are known [47]. The complex
Pd2SO4(BQ) was described [22]. The following comꢀ
plexes were synthesized and characterized: LPdQ [48]
0.12
0.09
0.06
0.03
2
1
0
25
50
75
100
125
, min
t
Fig. 9. Comparison of the kinetic curves recorded before
and after the addition of the substrate: [Pd] = 0.004 mol/l,
[BQ] = (1) 1.05 and (2) 0.85 mol/l, [HClO ] = 0.2 mol/l.
0
(L = 1,5ꢀcyclooctadiene, Q = various
[L2Pd(
2ꢀQ)]2(QH2) · MeOH, and L2Pd2(
Q)2 · MeOH [34, 35] (L = PPh3, Q =
p
ꢀquinones),
2ꢀ, 2ꢀ
ꢀbenzoꢀ
0
4
η
μ η η
ꢀ
p
quinone). Thus, palladium can be in the oxidation
states (II), (I), and (0) during the oxidation of cycloꢀ
hexene (and other olefins) in the CH3CN–H2O–H+–
us consider, in greater detail, the probable mechaꢀ
nisms of oxidation (using ethylene as an example) in
2+
Pd2
solutions of Pd2+,
, and Pd(0) complexes, taking
Q
system.
into account some earlier hypotheses [10–15].
Various functions of pꢀquinones in the oxidation
reactions catalyzed by Pd(II) and Pd(I) complexes
were analyzed [49]. In the framework of the classical
mechanism [4], but involving the intermediate resultꢀ
ing from the transꢀaddition of a water molecule to the
Active Species Pd2+(S–C2H4)
π
ꢀcomplex of olefin with Pd2+ (protonated olefin palꢀ
+
The two routes leading to [Pd2Q]2+ from the Pd2+
ladiumhydrin compound, e.g., ~PdCH CH
2OH2
), let
precursor are shown in reaction scheme (IV).
2
–QH2 2H+
–P
–QH2
Q
Pd2+
Q
S
H2O
Pd2+
PdS2+
+PdCH2CH2O+H2
Pd22+
Pd2Q2+
–P
(IV)
–2H+
–P
Q
–2H+
Pd2+
2H+
–QH2
PdQ0
Active Species [Pd2Q]2+
Two routes seem likely to involve the active comꢀ
plex [Pd2Q]2 +
Active Species Pd0Q
H+
S
H2O
Pd0Q
Pd0Q(S)
HQPdS+
HQPdR
. (VI)
–H+
:
H+
HQPd2Q+
Q
–QH2
–P, –QH2
–P Q
QPd2R+
–P
S
H2O
–H+
Pd2Q2+
Pd2Q(S)2+
Reaction schemes (IV)–(VI) show that indepenꢀ
dent catalytic cycles are possible on the Pd0Q and
[Pd2Q]2+ complexes and three of the four linearly
independent routes associated with Pd2+ according to
scheme (IV) are related to the formation of the Pd0Q
or [Pd2Q]2+ intermediates and differ slightly from the
classical mechanism of the Wacker process. Both the
quadraticꢀlaw dependences with respect to the conꢀ
centration [PdCl4]2– and the formation of palladium
complexes in the oxidation state (I) were established
(V)
H+
–QH2
Q
–H+
H+
HQPd2Q+
Pd2Q2
R = CH2CH2OH.
The dissociation of the Pd2Q2 complex can result in
the Pd0Q complex,
Pd2Q2
2Pd0Q,
forming an additional link between reactions (IV) by the study of the kinetics of olefin oxidation in chloꢀ
and (V). ride systems [4]. Only one route in this scheme, with
KINETICS AND CATALYSIS Vol. 51
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700
TEMKIN et al.
the intermediate compound +PdCH2CH2O+H2, can
proceed through intermediates specific, to a greater
extent, to [PdL4]2+, although the formation of interꢀ
mediates of this type cannot be excluded in the case of
ions
likely. These shifts result in cyclohexanone via the forꢀ
mation of intermediate hydroxycarbenium ion (clasꢀ
sical demetallation mechanism [4]) or Pd2+ carbene
complex , respectively. The mechanism including the
antiꢀaddition of a water molecule to the ꢀcomplex
involving intermediates and is likely to occur in the
oxidation of cyclohexene in solutions of palladium(II)
chloride complexes [20, 26] (see the discussion of the
synꢀaddition of Pd–OH to olefin and the antiꢀaddiꢀ
1, both hydride shifts, namely, 2–1 and 1–2, are
2
3
Pd22+
or protonated Pd0Q
.
π
1
2
Reaction schemes (IV) and (V) contain routes in
which no changes occur in the oxidation state of palꢀ
ladium in the intermediates; i.e., the soꢀcalled conꢀ
certed mechanism (term used in classical oxidation
catalysis) occurs, with electrons transferred from the
substrates (organic ligands) to the oxidant through the
metal atom (G.K. Boreskov, K.B. Yatsimirskii). For
the oxidation of carbon oxide by pꢀquinones catalyzed
by the palladium complexes, this mechanism was sugꢀ
gested for the first time by Gololodov [50, 51].
tion of a water molecule to the
πꢀcomplex in the
review dedicated to the 50th anniversary of the develꢀ
opment of the Wacker process [67]). The deprotonaꢀ
tion of
) compounds, respectively. The conversions of
(probably, through the carbene
2ꢀvinyl derivative)
and (through cyclohexenol ) also can result in
cyclohexanone. Another very probable mechanism of
the formation of is the
ꢀelimination of PdH+ from
positions 1–6 of intermediate . The route involving
the
ꢀelimination of PdH+ from positions 1 and 2 of
intermediate to form the intermediate ꢀcomplex of
PdH+ with the derivative of vinyl alcohol
and the
isomerization of to , which was discussed for the
Wacker process [4, 68], is possible for in the case of
the transꢀaddition of a water molecule to the ꢀcomꢀ
plex of cyclohexene with Pd2+. The 3–2 shift of the
hydride ion in intermediate can yield the 3ꢀcycloꢀ
1 from positions 1 or 3 affords vinyl (4) or allyl
(
5
4
η
5
6
The cationic character of the initial complex
[Pd(CH3CN)4]2+ or [Pd(CH3CN)4 – x(H2O)x]2+ and,
hence, the carbocationic character of the primary
6
β
1
πꢀcomplex make it possible to consider other mechaꢀ
β
nisms of olefin oxidation including solvated metal carꢀ
benium ions (protonated olefin metalhydrin intermeꢀ
diates) and specific, to a greater extent, to [PdL4]2+ in
aqueous solutions. Assumptions about the formation
of such ions were made previously for the reactions
involving the trimer Pd3(OAc)6 in acetic acid, namely,
the oxidative dimerization [52, 53] and the oxidation
esterification of olefins [54–56] (Moiseev reaction).
The formation of products of the allyl and, first of all,
1
π
8
1
9
1
π
1
hexenꢀ1ꢀol homoallyl oxidation products, which have
not been observed yet in aqueous acid solutions. Howꢀ
ever, the mechanism of the positional isomerization of
homoallyl acetoxylation of ꢀolefins and cyclohexene
α
in acetic acid (in the latter case) [54–57] and of allyl
esters in the oxidation of cyclododecatriene in methaꢀ
nol [58] agrees with these assumptions, as well as with
7
, including the conversions of the
with PdH+, can also result in such products. The
decomposition of intermediates , and in the
presence of
ꢀquinones and an acid can yield Pd2+
π
ꢀcomplex of 7
1,
3
,
4
5
the formation of the ꢀallyl complexes and products of
π
p
the allyl oxidation of cyclohexene and other olefins in
aqueous solutions [40].
without the participation of the intermediate complex
Pd0Q or [Pd2Q]2+; i.e., the route involving only the
Pd(II) compounds can appear (see reaction scheme
(IV)).
The possibility of these mechanisms taking place in
olefin oxidation into saturated ketones requires seriꢀ
ous experimental proofs. However, the theoretical
probability of these mechanisms makes it possible to
introduce internal catalytic cycles associated with
Pd2+ into the analyzed hypotheses involving the pallaꢀ
dium(I) and palladium(0) complexes.
To discriminate possible hypotheses as to the reacꢀ
tion mechanism, it is necessary to additionally study
the composition of the isotopomers of the cyclohexene
oxidation products, the kinetic isotope effects, the
dependences of the initial rates on [H+] and the initial
substrate concentration at different benzoquinone
Similar regularities were observed earlier in the
reactions of the Hg2+ ion [59–63], another strong
electrophile, particularly in the selective formation of
acrolein from propylene in acid solutions of merꢀ
cury(II) perchlorate [63]. The mechanisms involving
mercurycarbenium ions and vinyl mercury intermediꢀ
ates formed from olefins in acetonitrile solutions were
also discussed [64–66]. The reversible absorption of
ethylene by mercury(II) perchlorate in aqueous soluꢀ
tions of perchloric acid with an equilibrium constant
of 1.5
×
104 l/mol at 298°С, independent of the Н+
concentration, is likely due to the formation of the solꢀ
vated mercurycarbenium ion +HgCH CH
+ rather
2ОН2
ꢀcomplex, contrary to what was believed preꢀ
viously [66].
2
than a
π
Thus, the conversions of the metalcarbenium ion concentrations, the influence of acetonitrile on the
solvated by the water molecule give rise to a mechaꢀ hydrolysis of Pd2+, the nature of the palladium comꢀ
nism similar to the classical oxidation mechanism [4] plexes in solution (using electronic and NMR specꢀ
(reaction scheme (IV)) and to the
and even carbenic mechanisms of olefin oxidation by water in the binary solvent on the rate and selectivity of
ꢀquinones in acid solutions of cationic palladium(II) the process. It is also required to elucidate the routes of
complexes (Scheme). In solvated metal carbenium the transformation of cyclohexenol and allyl alcohol
πꢀallylic, vinylic, troscopy), and the influence of the mole fraction of
p
KINETICS AND CATALYSIS Vol. 51
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2010

KINETICS OF CYCLOHEXENE OXIDATION BY
p
ꢀQUINONES
701
3
6
1
2
–PdH+
–H3O+
+OH2
+Pd
3
O+H2
1
+PdH
6
7
9
8
2
–H3O+
+Pd
–Pd0
O+H2
1
+OH2
+Pd
–H+
–Pd0
–H+
–H2O
–+PdH
–H+
H2O
3ꢀallyl
–H+
η
H2O
5
OH
6
2
3
4
PdH+
(H+)
OH
+Pd
+OH2
O
Pd2+
–Pd0
–Pd0
–H+
–H3O+
H2O
H2O
–2H+
O
Allyl
mechanism
O
O
O
Classical
mechanism
I.I. Moiseev
2–1 Hꢀshift
Carbene
mechanism
1–2 Hꢀshift
Vinyl
mechanism
(
)
Scheme. Mechanisms of olefin oxidation by pꢀquinones in solutions of cationic palladium(II) complexes.
4. Moiseev, I.I.,
π
ꢀKompleksy v zhidkofaznom okislenii oleꢀ
under the conditions of cyclohexene oxidation,
because the formation of propionaldehyde was
observed upon allyl alcohol oxidation by palladium
chloride [69].
finov ꢀComplexes in LiquidꢀPhase Olefin Oxidaꢀ
(
π
tion), Moscow: Nauka, 1970.
5. Clement, W.H. and Selwitz, C.M., J. Org. Chem., 1964,
vol. 29, p. 241.
6. Takehira, K., Oh, H.J., Martinez, V.C., Chavira, R.S.,
Hayakawa, T., Orita, H., and Shmidzu, M., J. Mol.
Catal., 1987, vol. 42, p. 237.
ACKNOWLEDGMENTS
The authors are grateful to I.P. Stolyarov and
V.V. Istomin for assistance in the TEM studies of soluꢀ
tions and to E.S. Zhizhina for kindly providing benzoꢀ
quinone with tertꢀbutyl substituents synthesized at the
Boreskov Institute of Catalysis (Siberian Branch, Rusꢀ
sian Academy of Sciences, Novosibirsk).
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vol. 10, no. 4, p. 717.
This work was supported by the Russian Foundaꢀ
tion for Basic Research, project no. 07ꢀ03ꢀ00315.
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