Ethene hydromethoxycarbonylation catalyzed by cis-[Pd(SO 4)(PPh3)2]/H2SO4/PPh 3

Article abstract of DOI:10.1016/j.molcata.2010.10.016

The neutral precursor cis-[Pd(SO₄)(PPh₃)₂] turns into an active catalyst for the hydromethoxycarbonylation of ethene when used in combination with H₂SO₄ and PPh₃. The influence of the following operating conditions on the catalytic activity have been studied: (i) H₂SO₄/Pd ratio; (ii) PPh₃/Pd ratio; (iii) total pressure with CO/ethene = 1/1; (iv) pressure of one gas at constant pressure of the other; (v) H₂O concentration; (vi) temperature. At 100 °C a TOF = 2168 h^-1 has been achieved when the catalytic system is used in the ratios Pd/H₂SO₄/P = 1/107/18 (mol/mol), under 6 bar (CO/E = 1/1), H₂O concentration 0.16% in MeOH by weight. After catalysis and upon addition of LiCl, trans-[Pd(COEt)Cl(PPh₃)₂], which is related to the Pd-H catalytic cycle, has been isolated. Cis-[Pd(SO ₄)(PPh₃)₂] in CD₂Cl₂/MeOH reacts with CO to give a PdCOOMe complex (related to the carbomethoxy mechanism ), which neither inserts ethene, nor gives methyl propanoate (MP). In the presence of H₂O and H₂SO₄ the carbomethoxy complex is unstable giving a Pd-H complex, which yields catalysis to MP in the presence of CO and ethene. The Pd-H and Pd-COOMe catalytic cycles are discussed on the basis of the influence of the operating conditions on the TOF and of NMR evidences.

Full text of DOI:10.1016/j.molcata.2010.10.016

Journal of Molecular Catalysis A: Chemical 333 (2010) 180–185
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Ethene hydromethoxycarbonylation catalyzed by
cis-[Pd(SO4)(PPh3)2]/H2SO4/PPh3
Gianni Cavinatoa, Sarah Facchettib, Luigi Toniolob,∗
a Department of Chemical Sciences, University of Padua, via Marzolo 1, 35100 Padua, Italy
b Department of Chemistry, University of Venice, Dorsoduro 2137, 30123 Venice, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 August 2010
Received in revised form 18 October 2010
Accepted 20 October 2010
Available online 28 October 2010
The neutral precursor cis-[Pd(SO4)(PPh3)2] turns into an active catalyst for the hydromethoxycarbonyla-
tion of ethene when used in combination with H2SO4 and PPh3. The influence of the following operating
conditions on the catalytic activity have been studied: (i) H2SO4/Pd ratio; (ii) PPh3/Pd ratio; (iii) total
pressure with CO/ethene = 1/1; (iv) pressure of one gas at constant pressure of the other; (v) H2O con-
centration; (vi) temperature. At 100 ◦C a TOF = 2168 h−1 has been achieved when the catalytic system is
used in the ratios Pd/H2SO4/P = 1/107/18 (mol/mol), under 6 bar (CO/E = 1/1), H2O concentration 0.16% in
MeOH by weight. After catalysis and upon addition of LiCl, trans-[Pd(COEt)Cl(PPh3)2], which is related
to the “Pd–H” catalytic cycle, has been isolated. Cis-[Pd(SO4)(PPh3)2] in CD2Cl2/MeOH reacts with CO to
give a PdCOOMe complex (related to the “carbomethoxy mechanism”), which neither inserts ethene, nor
gives methyl propanoate (MP). In the presence of H2O and H2SO4 the carbomethoxy complex is unstable
giving a Pd–H complex, which yields catalysis to MP in the presence of CO and ethene. The “Pd–H” and
“Pd–COOMe” catalytic cycles are discussed on the basis of the influence of the operating conditions on
the TOF and of NMR evidences.
Keywords:
Hydrocarbomethoxycarbonylation
Ethene
Palladium(II) catalyst
Mechanisms
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
favours the methanolysis step [11] and impedes further insertions
of E and CO [11,12], which would produce an alternating CO–E
The recent discovery of a highly active and selective cationic
catalyst, [PdH(MeOH)(dtbpx)](TfO) (dtbpx = 1,2-bis[(di-t-butyl)
copolymer, as normally is the case when less hindered disphophine
ligands are used [12–15].
phosphinomethyl]benzene;
TfO = trifluoromethansulphonate),
The use of Pd(II)-monophosphine catalysts for the
hydromethoxycarbonylation of olefins has been known for a longer
time [16–18]. Though less active, they are of interest because they
allow the isolation of Pd(II)-acyl and Pd(II)-carbomethoxy [19–27],
which are related to the so called “hydride” mechanism, outlined
above, and to the so called “carbomethoxy” mechanism. This ini-
tiates through the insertion of CO into a Pd-methoxy bond, which
gives a carbomethoxy intermediate, followed by the insertion of
the olefin with formation of an ethylcarbomethoxy-␤-chelate,
which, through the intermediacy of its enolate isomer, undergoes
protonolysis by MeOH yielding the ester and the Pd-methoxy
species back to the catalytic cycle.
Compared to above reported diphosphine-based catalysts, the
cationic catalytic system [Pd(TsO)2(PPh3)2]/PPh3 is moderately
active in the presence of a hydride source such as hydrogen,
water or p-toluensulphonic acid (TOF 1800 h−1 at 100 ◦C, 40 bar,
CO/E = 1/1, Pd/TsOH/P = 1/8/10, H2O 800 ppm in MeOH [24]).
Here, we report an investigation on the hydromethoxycarbony-
lation of E using the neutral complex cis-[Pd(SO4)(PPh3)2], which
turns into an active catalyst even under relatively low pressure
when used in combination with H2SO4.
for the hydromethoxycarbonylation of ethene (E) to methyl
propanoate (MP) (TOF 50,000 h−1, selectivity 99.98%, 80 ◦C, 10 bar,
CO/E = 1/1) [1,2] has opened the way to a new convenient environ-
mentally preferred route for the commercial production of methyl
methacrylate [3–5].
Other cationic Pd(II)–diphosphine systems have been reported
to be highly active and selective, for example the one based
on 1,3-bis(di-t-butylphosphino)propane (TOF = 25,000 h−1, 97.4%
selectivity at 120 ◦C and 40 bar, CO/E = 2/1) [6,7].
For the catalyst [PdH(MeOH)(dtbpx)](TfO), it has been demon-
strated that catalysis initiates through the insertion of the olefin
into the Pd–H bond with formation of a Pd-ethyl intermediate, fol-
lowed by the insertion of CO leading to a Pd-acyl species, which
undergoes methanolysis with formation of the product and of the
Pd–H species that initiates another catalytic cycle [8–10]. It has
been proposed that the steric bulkness of the diphosphino ligand
∗
Corresponding author. Tel.: +39 041 2348553; fax: +39 041 2348517.
E-mail address: toniolo@unive.it (L. Toniolo).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.10.016

G. Cavinato et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 180–185
181
2. Experimental
2.1. Materials
Carbon monoxide and ethene (purity higher then 99%)
were supplied by SIAD Spa (Italy). MeOH, NEt3, PPh3, H2SO4
(96%) and CD2Cl2 were purchased from Aldrich Chemicals.
Pd(OAc)2 was purchased from Chimet (Italy). NEt3 and the
solvents were of commercial grade and used without further
purification. Cis-[Pd(SO4)(PPh3)2] [28], trans-[PdCl2(PPh3)2] [29],
trans-[Pd(COEt)Cl(PPh3)2] [22], cis-[Pd(H2O)2(PPh3)2](TsO)2·2H2O
[22], cis-[Pd(TsO)2(PPh3)2] [22] and trans-[Pd(COEt)(TsO)(PPh3)2]
[26] were prepared according to methods reported in the literature.
2.2. General procedure
Fig. 1. Influence of the H2SO4/Pd ratio on the TOF. Run conditions: [Pd(SO4)(PPh3)2],
1.38 × 10−2 mmol; PPh3, 0.22 mmol; solvent MeOH, 80 mL; pCO = pE = 30 bar; tem-
perature, 100 ◦C; reaction time, 1 h.
The IR spectra were recorded in nujol mull on a Nicolet FTIR
1
instruments mod. Nexus. 1H and 31P { H} NMR spectra of the com-
plexes dissolved in CD2Cl2 (typically 10−2 mol L−1) were recorded
on a Bruker AMX 300 spectrometer equipped with a BB multin-
uclear probe operating in the FT mode at 300 and 121.5 MHz for
1
1
1H and 31P{ H}, respectively. 1H and 31P{ H} chemical shifts are
reported in ppm downfield of the deuterated solvent used as inter-
nal standard or of externally referenced to 85% H3PO4, respectively.
GC analysis was performed using a GC Agilent 7890A instru-
ment, equipped with a column HP-5 30 m × 0.320 mm, 0.25 ␮m,
conditions: oven 40 ◦C (10 min) to 180 ◦C (30 min) at 25 ◦C/min,
T(inj) = 250 ◦C, T(det) = 250 ◦C, flow = 2.2 mL/min, N2. Conductivity
measurements were carried out using an instrument Radiometer
Copenhagen CDM 83.
2.3. Hydromethoxycarbonylation of E
The catalytic reactions were carried out using a stainless steel
autoclave of ca. 250 mL provided with a four-blade self-aspirating
turbine. In order to avoid contamination by metallic species due to
the corrosion of the internal surface of the autoclave, the solvent
and the catalytic system were contained in a ca. 150 mL Pyrex bottle
placed inside the autoclave.
Fig. 2. Influence of added PPh3/Pd ratio on the TOF. Run conditions: [Pd(SO4)(PPh3)2],
1.38 × 10−2 mmol; H2SO4, 96% 151 mg, 1.48 mmol H2SO4; solvent MeOH, 80 mL;
pCO = pE = 30 bar; temperature, 100 ◦C; reaction time, 1 h.
In
a
typical experiment 10 mg of cis-[Pd(SO4)(PPh3)2]
added, up to a maximum of 1700 h−1 when the H2SO4/Pd ratio is
107/1.
(1.38 × 10−2 mmol), 58 mg of PPh3 (0.22 mmol) were added
to 80 mL MeOH containing 151 mg H2SO4 (96%) in the Pyrex bottle
placed into the autoclave. The autoclave was purged by pressuriz-
ing it with a mixture of CO and E (1/1, ca. 3 bar) and depressurizing
it to atmospheric pressure (this cycle was repeated 5 times at room
temperature). Then the autoclave was heated rapidly up to 100 ◦C
and then charged first with E and then with CO to the desired pres-
sure. The pressure was maintained constant throughout the batch
time (1 h) by continuously feeding CO and E in the ratio 1/1 from a
reservoir connected to the autoclave through a constant pressure
regulator. Then the autoclave was rapidly cooled to 5–10 ◦C and
the gas was carefully released. The liquid was analyzed by GC. The
H2O content in the reaction medium was measured before and
after the reaction by the Karl–Fischer method.
(1)
Fig. 2 shows the influence of the P/Pd ratio on the catalytic activ-
ity. Some decomposition to Pd metal occurs when the ratio is <10/1.
The catalytic activity is not inhibited even when using a relatively
large excess of PPh3. As a matter of fact, it increases of ca. 15% when
the P/Pd ratio is increased from 12 to 22. Under comparable reac-
tion conditions (Pd/P/acid = 1/10/10–30) the sulphate system is ca.
30% more active that the tosylate system [24].
The promoting effects of the acid and of PPh3 will be discussed
later (see the proposed catalytic cycle).
3. Results and discussion
3.1. Preliminary experiments
3.2. Influence of the pressure
Preliminary experiments were carried out under 60 bar of
total pressure with CO/E = 1/1, at 100 ◦C. Above this tempera-
ture partial decomposition of the catalyst occurs as evidenced
by the formation of Pd metal. The catalytic activity of the
[Pd(SO4)(PPh3)2]/H2SO4/PPh3 system in the hydrocarbomethoxy-
lation of E (reaction (1)) is strongly influenced by the H2SO4/Pd ratio
as shown in Fig. 1. It rises quickly from 103 h−1, when no H2SO4 is
The results shown in Figs. 1 and 2 induced us to study the influ-
ence of the pressure of CO and of E using the catalytic system in the
ratios Pd/P/H2SO4 = 1/18/100.
Fig. 3 shows the influence of total pressure of CO and E
(CO/E = 1/1). Upon increasing the pressure up to 6 bar the TOF
increases rapidly up to a maximum of 2168 h−1 and then it
decreases gently down to 1540 h−1 under 100 bar.

182
G. Cavinato et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 180–185
Fig. 6. Influence of concentration of H2O on the TOF. Run conditions:
[Pd(SO4)(PPh3)2], 1.38 × 10−2 mmol; PPh3, 0.22 mmol; H2SO4 96% 151 mg,
1.48 mmol H2SO4; solvent (MeOH + H2O) 80 mL; pCO = pE 10 bar; temperature,
100 ◦C; reaction time 1 h.
Fig. 3. Influence of the total pressure (CO/E = 1/1) on the TOF. Run condi-
tions: [Pd(SO4)(PPh3)2], 1.38 × 10−2 mmol; PPh3, 0.22 mmol; H2SO4, 96% 151 mg,
1.48 mmol H2SO4; solvent MeOH, 80 mL; temperature, 100 ◦C; reaction time, 1 h.
sure the TOF decreases gently (1656 h−1 or 1380 h−1 under 50 bar
of CO or E, respectively). These results suggest that CO and E are
only slightly competitive for coordination to the metal centre.
3.3. Influence of the water concentration
The initial concentration of H2O in the MeOH we used was
400 ppm. After catalysis under the conditions of Figs. 1–5 the con-
centration of H2O is higher (2500 ppm when the precursor and
the acid are used in the ratio Pd/H2SO4 = 1/150). Analogously, the
H2O concentration increases up to 2000 ppm under the condi-
tions of Fig. 2 when Pd/H2SO4/P = 1/110/18. The increasing of H2O
concentration is likely to be due to esterification of MeOH with
H2SO4. These observations prompted us to investigate the effect of
H2O added to the solvent. Under the conditions of Fig. 6, the TOF
decreases to 1700 h−1 when MeOH with 10% of H2O is used. This
fact suggests that H2O competes with the reacting molecules for
coordination to the metal centre.
Fig. 4. Influence of CO partial pressure at constant
E pressure. Run condi-
tions: [Pd(SO4)(PPh3)2], 1.38 × 10−2 mmol; PPh3, 0.22 mmol; H2SO4, 96% 151 mg,
1.48 mmol H2SO4; solvent MeOH, 80 mL; pE, 10 bar; temperature, 100 ◦C; reaction
time, 1 h.
3.4. Influence of the temperature
Figs. 4 and 5 show the influence of the pressure of CO or of E
keeping the pressure of the other gas at a constant value of 10 bar.
Again, the catalytic activity increases rapidly to reach a maximum
TOF = 2040 h−1 or 1960 h−1 upon increasing the pressure of CO or E
up to 5.5 or up to 10 bar, respectively. And again, under higher pres-
The catalytic activity falls rapidly below 70 ◦C (Table 1). The data
fit satisfactorily the Arrhenius plot only in the range 70–100 ◦C
(Fig. 7). In this range the activation energy is 22.6 kcal mol−1. At
60 ◦C the catalytic activity is significantly lower than that expected
from the Arrhenius plot. This fact suggests that at this temperature
the interaction of free PPh3 and the metal centre is such as to inhibit
the coordination of the molecules that lead to the formation of MP.
3.5. On [Pd(SO4)(PPh3)2]
The IR spectrum of cis-[Pd(SO4)(PPh3)2] has been already
reported. The sulphato ligand acts as bidentate in the expected four
Table 1
Influence of temperature on the TOF.
Experiment
Temperature (K)
TOF (h−1
)
1
2
3
4
5
373
363
353
343
333
2168
1200
513
149
8.4
Fig. 5. Influence of
E partial pressure at constant CO pressure. Run condi-
Run conditions: [Pd(SO4)(PPh3)2], 1.38 × 10−2 mmol; PPh3, 0.22 mmol; H2SO4, 96%,
151 mg, 1.48 mmol H2SO4; solvent MeOH, 80 mL; pCO = pE = 3 bar at the working
temperature; reaction time, 1 h.
tions: [Pd(SO4)(PPh3)2], 1.38 × 10−2 mmol; PPh3, 0.22 mmol; H2SO4, 96%, 151 mg,
1.48 mmol H2SO4; solvent MeOH 80 mL; pCO, 10 bar; temperature, 100 ◦C; reaction
time, 1 h.

G. Cavinato et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 180–185
183
phate ligand may be easily removed from the coordination sphere
of Pd(II). That the acid may interacts with the sulphate complex
has been proved by the fact that when the sulphate is treated with
1
H2SO4 (Pd/H2SO4 = 1/10) at room temperature the 31P{ H} signal
shifts immediately from 35.56 ppm to 40.38 ppm. This interaction
is likely to occur through hydrogen bridges between the sulphate
ligand and H2SO4. Thus, though the precursor is taken in a neu-
tral form, under the action of H2SO4 it transforms into a cationic
species and the catalytic cycle performs via cationic intermediates.
The acid has also another important function that will be discussed
later together with the proposed catalytic cycle.
3.6. On the catalytic cycle
The generally accepted catalytic cycles are shown in the
Scheme 1. They have been briefly outlined in Section 1. Hereafter,
we discuss the evidences that are in their favour.
Fig. 7. Arrhenius plot ln TOF versus 1/T relevant to the data on the Table 1.
When, after the formation of the carbomethoxy complex
through reaction (2), CO is replaced by E (4 bar) or by a mixture
of CO and E (1/1, 4 bar) no change is observed in the NMR spectra
even at 50 ◦C for ca. 30 min, which indicates that the complex is
rather stable and that the Pd–COOMe moiety is reluctant to insert
the olefin. It is worth noting that in the presence of E there are all
the ingredients necessary for the formation of MP, which however
does not occur.
co-ordination of Pd(II) [30], so that the complex has a cis-geometry.
1
The 31P{ H} NMR spectrum in CD2Cl2 shows a singlet at 35.56 ppm,
well above 30 ppm. It has been suggested that this is an indication
for a cis-geometry [31]. Therefore, the complex has a cis-geometry
also in solution.
In nitromethane solution the complex is not ionic
(ꢀM < 2 ohm−1 cm2 mol−1
, which is the same as the sol-
Instead, when the carbomethoxy complex is treated with
H2SO4 (Pd/H2SO4 = 1/5) at room temperature it slowly decom-
poses. The intensity of the 1H NMR of the Pd–COOMe moiety
lowers and at the same time a broad signal appears in the
vent), in contrast with the analogous tosylate complexes
cis-[Pd(TsO)2(PPh3)2] and cis-[Pd(H2O)2(PPh3)2](TsO)2·2(H2O)
(ꢀM 51.1 ohm−1 cm2 mol−1 and 41.8 ohm−1 cm2 mol−1, respec-
tively, at 25 ◦C, concentration 10−3 mol L−1). The fact that
cis-[Pd(TsO)2(PPh3)2] presents a conductivity close to that of
the cationic complex cis-[Pd(H2O)2(PPh3)2](TsO)2·2(H2O) sug-
gests that in the first complex the solvent displaces the TsO anion
which is weakly coordinating.
1
Pd–H region (ca. −6 ppm). The intensity of the 31P{ H} signal at
18.86 ppm of the carbomethoxy complex also lowers (in 15 min
is reduced of ca. 40%) and a new signal appears at 24.00 ppm,
which is close to that of the dinuclear complex [(PPh3)2Pd(␮-
H)(␮-CO)Pd(PPh3)2](CF3COO) [33] (see later). The instability of
the carbomethoxy complex and the formation of the hydride is
presumily due to adventitious H2O in the solvent and to that intro-
duced with the acid (reaction a on the Scheme 1). The fact that
in the absence of the acid the carbomethoxy is stable (see above)
suggests that the acid promotes reaction a. If some H2O is added
(Pd/H2SO4/H2O = 1/5/5) the Pd–COOMe moiety is destroyed in a
short time and does not reform even under 4 atm. of CO. Again, there
is formation of a Pd–H species which is revealed by the broad signal
at ca. −6 ppm, which turns into a quintet centred at −6.27 ppm (JH–P
40 Hz) when the temperature is lowered to −30 ◦C. Upon adding
3 equiv. of PPh3 at room temperature the quintet is replaced by a not
well resolved doublet of triplets (−6.78 ppm, JH–P 177 Hz) (see later
for the nature of these hydrides and Refs. [33,34]). Therefore, if dur-
ing catalysis there might be the formation of a Pd–COOMe species,
this would likely be transformed into a Pd–H species by the action
of H2O. All these evidences are not in favour of the “Pd–COOMe”
cycle.
It has been reported that cis-[Pd(TsO)2(PPh3)2] reacts with
CO (1 bar) in CD2Cl2/MeOH even at −78 ◦C to give trans-
[Pd(COOMe)(TsO)(PPh3)2] [31]. This fact also suggests that the TsO
anion is weakly coordinating. In contrast, the sulphate complex
1
is rather inert. After 1 h at room conditions the 31P{ H} and 1H
NMR spectra show, in addition to the signals of the starting sul-
phate, a new signal at 18.86 ppm and at 2.51 ppm, respectively.
1
The intensities of the 31P{ H} and 1H signals of the unreacted
sulphate and of the new complex are in the ratio 3/1 and 30/1,
respectively, which indicates that the sulphate complex is 25% con-
verted. Under 4 bar of CO the conversion is 100% in ca. 30 min.
The new signals at 18.86 ppm and at 2.51 ppm are close to those
of the carbomethoxy complexes trans-[Pd(COOMe)(X)(PPh3)2]
(X = Cl, TsO) and trans-[Pd(COOMe)(H2O)(PPh3)2](TsO) [23,27,31],
which suggests that the new complex is likely to be an analogous
carbomethoxy complex having a trans-geometry. Though it has not
been isolated, the new complex may be reasonably formulated as
trans-[Pd(COOMe)(SO4H)(PPh3)2]. Its formation may be depicted
by reaction (2), in which the proton released from MeOH is captured
by the sulphate ligand.
The other cycle starts from a Pd–H species. The NMR spectrum
of the sulphate complex in CD2Cl2/MeOH (6/1, v/v), either in the
presence of H2SO4 (Pd/H2SO4 = 1/5) or in its absence, does not
show any signal in the hydride region. Therefore, MeOH is not the
source of a hydride, although it can be even in the presence of an
acid [7]. Neither H2O is the source, because when it is added to
the NMR tube no hydride signal appears. In contrast, when CO is
admitted (4 bar, Pd/H2SO4/H2O = 1/5/5) the quintet at −6.27 ppm1
is detected. Therefore, H2O in combination with CO is the source
of the hydride, which may form through a reaction closely related
to the well known water gas shift reaction (WGSR) [34,35]. When
cis-[Pd(SO4)(PPh3)2]
+ CO + MeOH ꢀ trans-[Pd(COOMe)(HSO4)(PPh3)2]
(2)
The conductivity measurements and the reactivity with CO and
MeOH suggest that the sulphate ligand is not so labile as the tosy-
late one. It is well known that the tosylate anion possesses a weakly
coordinating ability, so that the metal centre presents easily avail-
able coordination sites, which is a factor of paramount importance
in promoting the catalytic activity [11–13,18,32]. And yet, the cat-
alytic activity of the neutral sulphate complex is higher than that of
the cationic tosylate one. The fact that cis-[Pd(SO4)(PPh3)2] is highly
active when used in combination with H2SO4 suggests that the sul-
1
In the presence of 3 equiv. of PPh3, the doublet of triplets reported above is
detected.

184
G. Cavinato et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 180–185
Scheme 1. “Pd–H” and “Pd–COOMe” catalytic cycles for the hydromethoxycarbonylation of ethene.
E is admitted (E/CO = 1/1, total pressure 4 bar) formation of MP is
observed (300% in the presence of 3 equiv. of PPh3 after 1 h at 40 ◦C).
Attempts to isolate cationic acyl or carbomethoxy complexes,
after catalysis had occurred in the autoclave, did not give satis-
factory results. Instead, upon adding a LiCl solution in MeOH a
white precipitate was formed. Its IR spectrum shows a band at
1684 cm−1. The 31P{1H} and the 1H NMR spectra in CD2Cl2 show
a signal at 20.55 ppm and at −0.11 (t, CH3, 3H, CH2CH3, J 6.82 Hz)
and at 1.89 ppm (q, CH2, 2H, CH2CH3, J 6.82 Hz). These data are
identical to those of trans-[Pd(COEt)Cl(PPh3)2] synthesized from
trans-[Pd(COEt)(TsO)(PPh3)2] and LiCl in MeOH [26] or by pres-
surizing trans-[PdCl2(PPh3)2] with CO in MeOH [22]. Therefore the
complex isolated after addition of LiCl is trans-[Pd(COEt)Cl(PPh3)2],
which is related to the “Pd–H” cycle. All these observations are in
favour of this mechanism. The results of a detailed NMR investiga-
tion on the mechanism of the catalytic hydrocarbomethoxylation
of E will be the object of a forthcoming article.
As far as the nature of the hydrides above mentioned is
concerned, we report here some results on the reactivity of
[Pd(CF3COO)2(PPh3)2] with CO in aqueous trifluoroacetic acid
solution. This system catalyzes the WGSR through the inter-
mediacy of [PdH(PPh3)3](CF3COO), which shows a doublet of
triplets at −7.0 ppm (JH–Ptrans 177 Hz, JH–Pcis 13.5 Hz) [33,34] in
the 1H NMR spectrum. This hydride reacts with CO, giving the
binuclear palladium carbonyl hydride complex [(PPh3)2Pd(␮-
H)(␮-CO)Pd(PPh3)2](CF3COO) (1H hydride signal at −6.3(q), JH–P
40 Hz [33]), which gives [PdH(PPh3)3]+ when is treated with an
excess of PPh3. Therefore, these are the hydrides generated in the
present study.
In addition to easy the displacement of the sulphate anion
mentioned above, the acid stabilizes [PdH(PPh3)3]+ against depro-
tonation thus providing a higher concentration of the hydride,
which leads to higher TOFs. Moreover, the fact that higher TOFs
are achieved in the presence of a relatively high excess of PPh3
suggests that this ligand does not impede access of the reacting
molecules to the coordination sites of Pd(II). In addition, PPh3 may
promote rearrangements in the catalytic intermediates in such a
way as to bring together the moieties involved in the insertion and
in the product-forming steps.
Thus the promoting effect of the acid is also more in favour of
the Pd–H cycle. In addition, it should be pointed out that in no
case there was any formation of dimethyl succinate, either under
relatively high pressure with the monomers in the ratio 1/1 (Fig. 3)
or with a significant excess of CO (CO/E = 5/1) (Fig. 4). Formation of
this diester would occur through insertion of E into a Pd–COOMe.
Therefore, this is another reason why catalysis to MP through the
“carbomethoxy” cycle is unlikely.
In spite of these experimental evidences the “Pd–COOMe”
mechanism cannot be excluded to occur. Indeed, it has
been found that the hydromethoxycarbonylation of
E using
the diphosphine precursor [Pd(H2O)2(dppf)](TsO)2 (dppf = 1,1ꢀ-
bis(diphenylphosphino)ferrocene) yields MP and oligoketoesters
as well as diesters such as dimethyl succinate and higher ketodi-
esters even when the precursor is used in combination with a
relatively large amount of TsOH (Pd/TsOH = 1/160) and with CO
and E in the ratio 1/1 [37]. Therefore, MP may form through both
mechanisms. The simultaneous occurrence of the two cycles has
been demonstrated in the hydromethoxycarbonylation of E using
a Pd(II)-[1,3-(iBu2P)2C3H6] cationic precursor [38]. However, it
should be pointed out that these complexes possess a cis-geometry,
which favours the insertion reactions.
It has been proposed that the formation of the dinuclear species
occurs through partial deprotonation of the mononuclear hydride
with formation of a Pd(0) complex, which reacts with the remaining
undeprotonate Pd(II)–hydride complex to give the formally Pd(I)
dinuclear complex (reaction (3)) [33].
Acknowledgement
+
+
[P3PdH]+−ꢀH P3Pd0COꢀ,−PP2Pd0(CO)[P PdH] ,−P
ꢀ
3
The financial support of MIUR (Rome) is gratefully acknowl-
edged.
[P2Pd(␮-H)(␮-CO)PdP2]+
(3)
Equilibria (3) help us to give a sound explanation on the promot-
ing effect of H2SO4 and on the role of PPh3 (cf. Figs. 1 and 2).
Though a fraction of added PPh3 may be subtracted from the reac-
tion medium because of the reaction with H2SO4 (equilibrium (4))
[36], it remains enough free PPh3 to stabilize the catalytic system,
because the phosphonium salt is a PPh3 buffer, which releases it
when necessary [36]. Thus, the combined action of PPh3 and H2SO4
may prevent the formation of dinuclear complexes as well as fur-
ther degradation to inactive Pd(0).
References
[1] G.R. Eastham, R.P. Tooze, X.L. Wang, K. Whiston, World Pat. 96/19434 (1996)
(to ICI).
[2] W. Clegg, G.R. Eastham, M.R.J. Elsegood, R.P. Tooze, X.L. Wang, K. Whiston W.,
J. Chem. Soc. Chem. Commun. (1999) 1877.
[3] B. Wassener, Special Report on the Industry’s campaign for a better image, FT
Chemicals, Wednesday, September 8, 2004, p. 3.
[4] Hydrocarbon Process. 82 (2003) 25.
[5] Plast. Rubber Wkly. (July) (2005).
[6] E. Drent, E. Kragtwijk, Eur. Pat. Appl. EP 495548 (1992) (to Shell).
[7] R.I. Pugh, E. Drent, Adv. Synth. Catal. 344 (2002) 837.
−
PPh3 + H2SO4 ꢀ [HPPh3]+HSO4
(4)

G. Cavinato et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 180–185
185
[8] G.R. Eastham, B.T. Heaton, J.A. Iggo, R.P. Tooze, R. Whyman, S. Zacchini, J. Chem.
[22] G. Cavinato, L. Toniolo, J. Organomet. Chem. 398 (1990) 187.
Soc. Chem. Commun. (2000) 609.
[23] R. Bertani, G. Cavinato, L. Toniolo, G. Vasapollo, J. Mol. Catal. 84 (1993) 165.
[24] A. Vavasori, G. Cavinato, L. Toniolo, J. Mol. Catal. 176 (2001) 11.
[25] G. Cavinato, A. Vavasori, L. Toniolo, F. Benetollo, Inorg. Chim. Acta 343 (2003)
183.
[26] G. Cavinato, L. Toniolo, A. Vavasori, J. Mol. Catal. A: Chem. 219 (2004) 233.
[27] E. Amadio, G. Cavinato, A. Dolmella, L. Ronchin, L. Toniolo, A. Vavasori, J. Mol.
Catal. A: Chem. 298 (2009) 103.
[28] A.I. Min’kov, O.A. Efimov, N.K. Eremenko, Akademii Nauk SSSR, Seriya Khimich-
eskaya 5 (1989) 998.
[29] J.M. Jenkins, J.C. Verkade, Inorg. Chem. 11 (1968) 108.
[30] J.J. Levison, S.D. Robinson, J. Chem. Soc. A (1971) 762.
[31] E. Amadio, G. Cavinato, A. Dolmella, L. Toniolo, Inorg. Chem. 49 (2010) 3721.
[32] A. Sen, Acc. Chem. Res. 26 (1933) 303.
[33] V.N. Zudin, V.D. Chinakov, V.M. Nekipelov, V.A. Likholobov, I. Yu, I. Yermakov,
J. Organomet. Chem. 289 (1985) 425.
[34] V.N. Zudin, V.D. Chinakov, V.M. Nekipelov, V.A. Rogov, V.A. Likholobov, Yu.I.
Yermakov, J. Mol. Catal. 52 (1989) 27.
[35] V.N. Zudin, V.A. Likholobov, Yu.I. Yermakow, Kinet. Katal. 20 (1979) 805.
[36] E.G. Chepaikin, A.P. Bezruchenko, A.A. Leshcheva, Kinet. Katal. 40 (1999)
313.
[37] C. Bianchini, A. Meli, W. Oberhauser, P.W.N.M. van Leeuwen, M.A. Zuideveld, Z.
Freixa, P.J. Kamer, A.L. Spek, O.V. Gusev, M. Kal’sin, Organometallics 22 (2003)
2409.
[9] W. Clegg, G.R. Eastham, M.R.J. Elsegood, B.T. Heaton, J.A. Iggo, R.P. Tooze, R.
Whyman, S. Zacchini, Organometallics 21 (2002) 1832.
[10] W. Clegg, G.R. Eastham, M.R.J. Elsegood, B.T. Heaton, J.A. Iggo, R.P. Tooze, R.
Whyman, S. Zacchini, J. Chem. Soc. Dalton Trans. (2002) 3300.
[11] P.W.N.M. van Leeuwen, M.A. Zuideveld, B.H.G. Swennenhuis, Z. Freixa, P.C.J.
Kamer, K. Goubitz, J. Fraanje, M. Lutz, A.L. Spek, J. Am. Chem. Soc. 125 (2003)
5523.
[12] G. Cavinato, L. Toniolo, A. Vavasori, in: M. Beller (Ed.), Catalytic Carbonylation
Reactions, Top. Organomet. Chem. 18 (2006) 125.
[13] E. Drent, P.H.M. Budzelaar, Chem. Rev. 96 (1996) 663.
[14] G.P. Belov, E.V. Novikova, Russ. Chem. Rev. 73 (2004) 267.
[15] A. Sen (Ed.), Catalytic Synthesis of Alkene–Carbon Monoxide Copolymers and
Cooligomers, Kluwer Academic Publishers, Dordrecht, 2003.
[16] V. von Kutepov, K. Bittler, D. Neubauer US Patent 3,437,676 (1969) (to Badische
Anilin and Soda-Fabrik).
[17] E.G. Chepaikin, A.P. Bezruchenko, A. Ben’ei, F. Io, Izv. Akad. Nauk SSSR, Ser. Khim.
(1989) 743.
[18] P. Kalck, M. Urrutigoity, O. Dechy-Cabaret, in: M. Beller (Ed.), Catalytic Carbony-
lation Reactions, Top. Organomet. Chem. 18 (2006) 97.
[19] R. Bardi, A. Del Pra, A.M. Piazzesi, L. Toniolo, Inorg. Chim. Acta 35 (1979) L345.
[20] R. Bardi, A.M. Piazzesi, G. Cavinato, P. Cavoli, L. Toniolo, J. Organomet. Chem.
224 (1982) 407.
[21] R. Bardi, A.M. Piazzesi, A. Del Pra, G. Cavinato, L. Toniolo, Inorg. Chim. Acta 102
(1985) 99.
[38] J. Liu, B.T. Heaton, J.A. Iggo, R. Whyman, J.F. Bickley, A. Steiner, Chem. Eur. J. 12
(2006) 4417.