Characterization and catalytic activity of trans-[Pd(COCH2CH3)(TsO)(PPh3)2 ], isolated from the hydro-methoxycarbonylation of ethene catalyzed by [Pd(TsO)2(PPh3)2]

Article abstract of DOI:10.1016/j.molcata.2004.04.014

trans-[Pd(COCH₂CH₃)(TsO)(PPh ₃)₂] (I) was isolated after running the hydro-methoxycarbonylation (HMC) of ethylene. Complex (I) catalyzed the HMC of ethylene to methylpropanoate. Catalysis occurred via an initial

Full text of DOI:10.1016/j.molcata.2004.04.014

Journal of Molecular Catalysis A: Chemical 219 (2004) 233–240
Characterization and catalytic activity of trans-[Pd(COCH2CH3)
(TsO)(PPh3)2], isolated from the hydro-methoxycarbonylation
of ethene catalyzed by [Pd(TsO)2(PPh3)2]
Gianni Cavinatoa, Luigi Toniolob,∗, Andrea Vavasorib
a
Department of Chemistry Science, University of Padua, via Marzolo 1, 35100 Padua, Italy
Department of Chemistry, University of Venice, Dorsoduro 2137, 30123 Venice, Italy
b
Received 20 February 2004; accepted 6 April 2004
Available online 3 July 2004
Abstract
The title complex (I) has been isolated after running the hydro-methoxycarbonylation (HMC) of ethene (4.5 MPa of CO/C2H4 = 1/1, 343 K)
in MeOH, catalyzed by [Pd(TsO)2(PPh3)2]. It has been characterized by IR, 1H and 31P NMR spectroscopy.
Complex (I) reacts with MeOH, saturated with CO, even at r.t., yielding methylpropanoate (MP) in stoichiometric amount and Pd(0)
complexes and/or trans-[Pd(COOCH3)(TsO)(PPh3)2] (II), the latter forms in the presence of PPh3 and of p-toluenesulfonic acid (TsOH);
complex (I) catalyses the HMC of ethene to MP; it catalyses also the HMC of a different olefin yielding also a stoichiometric amount of MP.
After catalysis, complex (I) is recovered as such or as [Pd(TsO)2(PPh3)2] (III), the latter forming when an excess of TsOH is used (Pd/TsOH
= 1/8).
Complex (II), a potential catalytic intermediate, has been prepared under conditions similar to those employed to synthesize complex (I),
except for the presence of ethene. This complex, dissolved in MeOH saturated with C2H4, does not yield MP at r.t., whilst at 353 K, it becomes
a catalyst precursor for the HMC of ethene, however, it is recovered as complex (I). The conversion of (II) to (I) occurs with CO2 evolution.
For the conversion of (I) to (II), it is proposed that: (i) (I) reacts with MeOH yielding MP and a Pd(II)-hydride; (ii) this reacts with TsOH
with hydrogen evolution and yielding complex (III); (iii) this reacts with CO and MeOH yielding (II). For the conversion of (II) to (I) it is
proposed that: (i) (II) reacts with H2O yielding MeOH and a Pd–COOH species; (ii) this evolves CO2 with formation of a Pd(II)-hydride;
(iii) sequential addition of ethene and CO gives (I).
In addition, it has also been found that catalysis is accompanied by formation of CO2 also when using complex (I) as catalyst and that the
catalytic activity passes through a maximum with increasing the concentration of water (TOF = 420 h−1 at 353 K, 4.5 MPa CO/C2H4 = 1/1,
(I)/PPh3/TsOH = 1/6/8, H2O = 800 ppm). It is proposed that: (i) catalysis occurs through initial formation of a Pd(II)–H species (which form
after CO2 evolution from a Pd–(COOH) species formed via interaction of H2O with CO), followed by the insertion of the olefin into the
Pd(II)–H bond to form a Pd(II)–(alkyl) intermediate, which in turn inserts CO with formation of an acyl complex of type (I), which reacts
with MeOH yielding the ester and Pd(II)–H back to the catalytic cycle; (ii) a carbomethoxy complex of type (II) does not play a major direct
role in the catalytic cycle; (iii) during the catalysis Pd(II)–H consuming side reactions occur with formation of Pd(II) species of type (II)
and/or (III); these species are reintroduced, as hydrides, back to the catalytic cycle via interaction with H2O and CO.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Palladium catalyst; Carbon monoxide; Ethene; Methoxycarbonylation; Methylpropanoate
1. Introduction
particular those having weakly coordinating anions bal-
ancing the cationic charge, so that the metal has easily
available coordination sites capable of activating the re-
acting molecules [1–5]. As a simplified rule catalysts with
monophosphine or chelating ligands which open one arm
give the ester, chelating ligands that retain cis coordina-
tion give the polyketone [6–8]. Due to its importance, this
catalysis has been extended also to the use of water-soluble
Pd(II)–phosphine complexes are efficient catalyst for
the carbonylation of olefins to esters or to polyketones, in
∗
Corresponding author. Tel.: +39 041 2348553;
fax: +39 041 2348517.
E-mail address: toniolo@unive.it (L. Toniolo).
1381-1169/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2004.04.014

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G. Cavinato et al. / Journal of Molecular Catalysis A: Chemical 219 (2004) 233–240
Scheme 1. Proposed catalytic cycle.
Pd(II)-sulfonated phosphine catalysts [9–11]. Hereafter,
we will focus on the catalysis to the ester. Two mecha-
nisms for the catalytic cycles have gained more acceptance
(Scheme 1).
of 99.98% at 353 K and 1 MPa of CO/C2H4 = 1/1)
[23–26].
In favor of the Pd(II)–H mechanism, it has been
also reported that the catalytic activity of the precursor
[Pd(TsO)2(PPh3)2], is significantly enhanced when the
catalysis is carried out in the presence of a hydride source,
such as H2O, p-toluenesulfonic acid (TsOH), H2 and that the
same precursor catalyses the hydrogenation of olefin [27,28].
It has been also found that the cationic carbomethoxy com-
plex trans-[Pd(COOCH3)(H2O)(PPh3)2](TsO), plays only
a minor role, if any, in the catalysis [29].
In favor of the Pd(II)–H mechanism, there are also the
findings that during the HMC of ethene using the pre-
cursor [Pd(TsO)2(PPh3)2] in combination with relatively
large amounts of PPh3 and TsOH there was formation
of phosphonium salts of the type (EtPPh3)(TsO) and
(EtCOCH2CH2PPh3)(TsO) mediated by the metal [30].
This paper deals with the characterization, reactivity and
catalytic activity of the title Pd(II)–acyl complex, which
has been isolated in the HMC of ethene catalyzed by
[Pd(TsO)2(PPh3)2].
According to a simplified scheme, that does not take
into account the geometry of the key active intermediates
and of the transient species, one mechanism (A) involves
the initial formation of a Pd(II)–H species, followed by se-
quential coordination and migratory insertion of the olefin
and CO with formation of a Pd(II)–acyl species and fi-
nal attack of the alkanol to give the ester and the hydride
back to the catalytic cycle [12]. In favor of this mecha-
nism, there are the findings that active Pd–acyl complexes
of the type trans-[Pd(COR)Cl(PPh3)2] have been isolated
from HMC experiments using trans-[PdCl2(PPh3)2] as pre-
cursor and that these acyl complexes react with an alka-
nol to give the corresponding ester and that they are active
when reused as catalysts [13]. Together with the acyl com-
plex, it has been isolated also a carboalkoxy complex of
the type trans-[Pd(COOR)Cl(PPh3)2] [13], a potential inter-
mediate, which involves the insertion of the olefin into the
Pd–COOR bond to form a Pd–alkyl intermediate, which un-
dergoes alkanol protonolysis to give the final product and the
Pd–alkoxy species that inserts CO giving Pd–COOR back
to the catalytic cycle (mechanism B) [14–16]. However, it
was found that the isolated carbomethoxy complex does not
react with an olefin, but rather it brings to the deactivation
to Pd(0) complexes [13]. Acid treatment of a deactivated
catalyst can restore activity through formation of Pd(II)–H
species, which give further support that catalysis undergoes
through mechanism A [17,18].
2. Experimental
2.1. Reagents
Carbon monoxide and ethene (purity higher then 99%)
were supplied by SIAD Spa (Italy). Pd(OAc)2 98%,
AgTsO, triphenylphosphine and p-toluenesulfonic acid
(TsOH·H2O) were purchased from Aldrich Chemicals.
Methanol (purity > 99.5%, 0.01% of water) and the other
solvent were purchased from Baker and used without further
purification. The deuterated solvents were Aldrich prod-
ucts. The complexes trans-[Pd(COCH2CH3)Cl(PPh3)2],
[Pd(TsO)2(PPh3)2] and trans-[Pd(COOCH3)(H2O)(PPh3)2]
(TsO) were prepared according to the methods reported in
the literature [13,22,29].
Other more recent studies, based on multinuclear NMR
and isotopic labeling investigations, are also in favor of
the hydride route [19–22], including also the exception-
ally active and selective system generated in situ, via the
reaction of [Pd(P–P)(dba)] (P–P = 1,2(CH2PBut2)2C6H4;
=
dba = trans-(PhCH CH)2CO) with methanesulfonic acid
for the HMC of ethene (TOF=50,000 h−1, selectivity

G. Cavinato et al. / Journal of Molecular Catalysis A: Chemical 219 (2004) 233–240
235
2.2. Equipment
Complex (II) was prepared by the same procedure
used to prepare complex (I), except for the presence of
The IR spectra were recorded on a Nicolet FTIR instru-
ments mod. Nexus. 1H and 31P NMR spectra were recorded
on a Bruker AMX 300 spectrometer equipped with a BB
multinuclear probe operating in the FT mode at 300 and
C2H4 (60.3 mg, yield 70%). The IR spectrum shows ␯
CO
at 1668 cm−1, close to that of the aquo analogous com-
plex trans-[Pd(COOCH3)(H2O)(PPh3)2](TsO) (␯
at
CO
1684 cm−1 [29]).
1
121.442 MHz for H and 31P, respectively. Products of the
Complex (I) was also synthesized directly by reacting
the chloride analogous trans-[Pd(COCH2CH3)Cl(PPh3)2]
(0.1 mmol) with AgTsO (0.11 mmol) in 2 ml of cold
dichloroethane under vigorous stirring. AgCl precipitated
immediately. The suspension was quickly filtered using
a micro-filter system. Complex (I) was precipitated by
dropping the solution into 20 ml of n-pentane under vigor-
ous stirring. The solid was separated by filtration, washed
with water, n-pentane and ether and dried under vacuum
(68.7 mg, yield 80%).
HMC reaction were analyzed by GC on a HP 5890 series II
apparatus equipped with a 30 m × 0.53 mm × 0.1 ␮m HP 5
column. In order to reveal CO2 (TCD detector), it was used
a5.5 m × 3.18 mm SS silica gel, 60/80 packed column un-
der these conditions: carrier gas: helium, 30 ml min−1; oven:
313 K (2 min) to 373 K at 15 K min−1
.
The catalytic experiments were carried out in a stainless
steel autoclave of ca. 250 ml of capacity, provided with a
self-aspirating turbine. In order to prevent contamination by
metallic species, due to corrosion of the internal surface of
the autoclave, the reagents were contained in a ca. 150 ml
Teflon-beaker placed into the autoclave. CO–ethene mixture
(1/1) was supplied from a gas reservoir (260 ml) connected
to the autoclave through a constant pressure regulator. The
autoclave was provided with a temperature control ( 0.5 K).
2.4. Synthesis of [Pd(CO)(PPh3)3] (IV) and
[Pd(CO)(PPh3)]3 (V)
The synthesis of these complexes is reported in litera-
ture [31,32]. Here we propose a new procedure. A mix-
ture of Pd(OAc)2 (0.1 mmol) and PPh3 (0.33 mmol) in 5 ml
of MeOH was added to a glass bottle placed in a stain-
less steel autoclave of ca. 100 ml. The autoclave was purged
with CO and pressurized with 5.0 MPa of CO at room tem-
perature, then heated to the working temperature (343 K)
while stirring for 30 min. The autoclave was then cooled to
room temperature and depressurised. The white solid prod-
uct was separated by filtration, washed and identified as
[Pd(CO)(PPh3)3] by the elemental analysis and IR spec-
troscopy (71.8 mg, yield 78%).
2.3. Synthesis of trans-[Pd(COCH2CH3)(TsO)(PPh3)2]
(I) and of trans-[Pd(COOCH3)(TsO)(PPh3)2] (II)
The complex [Pd(TsO)2(PPh3)2] (III) (0.1 mmol), to-
gether with PPh3 and TsOH ((III)/PPh3/TsOH = 1/0.9/0.2),
in 3 ml of MeOH with 5 equivalents of H2O was added to
a glass bottle placed into a stainless steel autoclave of ca.
50 ml. The autoclave was purged with carbon monoxide
and pressurized with 4.0 MPa of CO–ethene (1/1) at room
temperature. The autoclave was heated to 343 K for 20 min
under vigorous stirring, then cooled to room temperature.
The gas phase was analyzed by GC. CO2 was present in
an amount practically equivalent to Pd. The autoclave was
then depressurized and opened. In the liquid phase MP
was formed (TOF1 = 50 mol of ester/mol Pd h, h−1). Upon
addition of 50 ml of cold water, complex (I) precipitated. It
was separated by filtration, washed and dried under vacuum
(68.7 mg, yield 80%).
The same results are obtained starting from Pd(OAc)2/
PPh3/TsOH in the ratio 1/2.9/2.2.
By adding to the methanol solution 1 equivalent of LiCl
instead of water the complex trans-[Pd(COCH2CH3)Cl
(PPh3)2] precipitated in 85% yield (61.5 mg).
Complex (I) was synthesized also from [Pd(CO)(PPh3)3]/
TsOH (1/2.2) or [Pd(CO)(PPh3)]3/PPh3/TsOH (1/1.9/2.2)
under the conditions just reported (60.1 mg, yield 70% and
55.8 mg, yield 65%, respectively).
Using the same procedure but using 1 equivalent of PPh3,
a reddish solid precipitated. It was separated by filtration,
washed and identified as [Pd(CO)(PPh3)]3 by the elemental
analysis and IR (89.3 mg, yield 75%).
2.5. Conversion of complex (I) to complex (II) and vice
versa
Complex (I) (0.1 mmol) was allowed to react with
methanol (3 ml) containing 1 equivalent of PPh3 and 1
equivalent of TsOH, previously saturated with CO, for ca.
20ꢀ at r.t.. MP formed in quantitative yield (ester/(I) = 1/1).
Upon addition of cold water, complex (II) precipitated. The
suspension was filtered, the precipitate was washed with
cold water and dried under vacuum (60.3 mg, 70% yield). If
the same experiment is carried out in the absence of PPh3
and TsOH, [Pd(CO)(PPh3)]3 forms (107.1 mg, yield 90%)
in place of complex (I).
A methanol solution of the carbomethoxy complex
(II), PPh3 and TsOH (0.1 mmol of (II), (II)/PPh3/TsOH
= 1/0.9/1.2, in 3 ml of MeOH, 800 ppm H2O, previously
washed with CO/C2H4) was heated at 353 K under 2.0 MPa
of CO/C2H4 = 1/1, in a autoclave of 50 ml capacity, for
20ꢀ. The autoclave was then cooled to r.t. and slowly de-
1
This value is only indicative that catalysis occurs. The catalyst pre-
cursor was used in abnormal high concentration, stirring was provided
by a little magnetic bar. The main aim of this experiment was to be able
to isolate a complex after the reaction.

236
G. Cavinato et al. / Journal of Molecular Catalysis A: Chemical 219 (2004) 233–240
pressurised. A significant amount of MP formed (TOF1 =
50 h−1). In the gas phase CO2 was present, practically 1
equivalent with respect to Pd. After cooling and opening
the autoclave, complex (I) precipitated upon addition of
cold water. After usual work up, it was recovered in 65%
yield (55.8 mg).
cycle as schematized by reactions (1)–(3):
[Pd(TsO)2(PPh3)2]
C−−O−,H−2−O−a−s h−y−dr−id−e−so−u→rce [PdH(TsO)(PPh3)2]
(1)
(2)
−CO2,−TsOH
[PdH(TsO)(PPh3)2]
(i) C2H4,(ii) CO
−−−−−−−−→ [Pd(COCH2CH3)(TsO)(PPh3)2]
2.6. Catalytic hydro-methoxycarbonylation of ethene
(I)
In a typical experiment complex (I) (0.1 mmol), together
with PPh3 and TsOH (I/PPh3/TsOH = 1/6/8) was dissolved
in 50 ml of MeOH (800 ppm of H2O) in a Teflon beaker
placed in the autoclave. The solvent was degassed with a
mixture of CO/C2H4 before adding the catalytic precursor.
The autoclave was washed by pressurizing with a 1/1 mix-
ture of CO/C2H4 (ca. 0.5 MPa) and then depressurising to
atmospheric pressure (this cycle was repeated five times at
room temperature with stirring). After washing a gas sample
was analyzed by GC: no CO2 was detected. The autoclave
was then pressurized with 0.5 MPa of the gas mixture and
then heated to 353 K during ca. 10 min without stirring. The
pressure was then adjusted to the desired value (typically
4.5 MPa total pressure) and, while stirring, maintained con-
stant throughout the experiment (2 h, rate stirring 800 rpm)
by continuously supplying the gasses from the reservoir. Gas
samples were withdrawn at regular intervals of time (15ꢀ).
CO2 was unambiguously detected and its concentration reg-
ularly increased with time. At the end of the experiment, the
autoclave was quickly cooled and carefully depressurised.
The content of the beaker was analyzed by GC.
A Pd(0) complex, in combination with TsOH, was also
used as catalyst precursor. Moreover, complex (I) was used
also for the catalytic HMC of cyclohexene. A typical exper-
iment was carried out by following the same procedure just
described except for the presence of C2H4. Complex (I) was
dissolved in a mixture of 10 ml of cyclohexene and 40 ml
of MeOH (800 ppm of H2O) and the autoclave was washed
by pressurizing with CO; the experiments were carried out
at 2.0 MPa of CO. The results are reported in Table 3.
[Pd(COCH2CH3)(TsO)(PPh3)2] + MeOH
(I)
→ [PdH(TsO)(PPh3)2] + EtCOOMe
(3)
It is well known that H2O in combination with CO can
interact with Pd(II) to give a hydride, which is unstable to-
ward deprotonation with formation of Pd(0) species [33]. It
is also known that an acid can stabilize the hydride by shift-
ing equilibrium (a) toward left, however the acid can also
react with the hydride with H2 evolution and reformation of
the Pd(II) species [33,34]:
+
Pd(II) + CO + H2O−ꢀH Pd(II) − (COOH)
−−−C→O2 Pd(II) − Hꢀ Pd(0) + H+
(4)
(5)
(a)
Pd(II) − H + H+ ꢀ Pd(II) + H2
Reactions (4) and (5) are at the basis of the water
gas shift reaction (WGSR) catalyzed by a homogeneous
Pd(AcO)2/PPh3/CF3COOH/H2O system [33]. It has been
also reported that in the presence of C2H4 the evolution of
H2 is inhibited and catalysis takes a different way, as the
Pd(II)–H is trapped by the olefin to form a Pd(II)–Et inter-
mediate, which inserts CO with formation of a Pd(II)–acyl
intermediate, which inserts another molecule of C2H4 with
formation of a Pd(II)–C2H4COEt species, which is splitted
by the acid to yield diethylketone (DEK) and the initial
Pd(II) precursor [33].
In the present case, complex (III) is dissolved in MeOH
and the catalyst promotes the MP in place of DEK. Thus,
MeOH attack to a Pd(II)–acyl intermediate of type (I) (re-
action 3) is faster than the insertion of a second molecule of
C2H4.
3. Results and discussion
3.1. Synthesis and characterization of
trans-[Pd(COCH2CH3)(TsO)(PPh3)2] (I)
For the system [Pd(P–P)(dba)]–CH3SO3H it has been
proved that the alkanol is the source of hydride, as the hy-
dride forms only with primary or secondary alkanol and not
with tertiary ones, like t-BuOH [26]. We have found that
the acyl complex (I) forms also using t-BuOH in place of
MeOH, thus in our case the source of hydride is H2O in
combination with CO interacting with Pd(II) or the acid by
addition to Pd(0) (cf. reaction 4, equilibrium (a)).
As already mentioned, Pd(II)–H complexes are unstable
toward deprotonation with formation of Pd(0). As a mat-
ter of fact, complex (I) can be prepared starting also from
[Pd(CO)(PPh3)3] (or [Pd(CO)(PPh3)]3 in combination with
Complex (I) was isolated after running the HMC of ethene
in MeOH catalyzed by the system [Pd(TsO)2(PPh3)2]/PPh3/
TsOH or by Pd(AcO)2/PPh3/TsOH. (cf. 2.3). The forma-
tion of complex (I) (which occur with CO2 evolution,
see Section 2.3) and catalysis are likely to occur via a
Pd–hydride species which forms via interaction of CO with
H2O. The hydride adds C2H4 and CO to give a Pd(II)–acyl
intermediate of type (I), which undergoes attack of MeOH
yielding the ester and the Pd(II)–H back to the catalytic

G. Cavinato et al. / Journal of Molecular Catalysis A: Chemical 219 (2004) 233–240
237
Table 1
Elemental analysis, selected IR, 1H and 31P NMR data for complex (I)
Analysis (%)
IR (cm−1
)
1H NMR δ (ppm)
31P NMR δ (ppm)
C
H
S
Found
Calcd.
63.25
4.77
3.82
1688 (s, CO) 1230,
1035, 1013 (s, SO3)
0.13 t (CH3, 3H, CH2CH3)
1.92 s (CH3, 3H,TsO)
2.36 q (CH2, 2H, CH2CH3)
7.20–7.71 m (C6H5, 34H, PPh3 and TsO)
17.37 s (PPh3)
64.30
4.93
3.73
Abbreviations: s, singlet; t, triplet; q, quadruplet; m, multiplet. NMR spectra taken in deuterated benzene; δ (31P) values in ppm from external 85%
H3PO4, downfield shifts being taken as positive.
the relatively low value of the 31P chemical shift sug-
gests that complex (I) in benzene is a neutral species with
the TsO anion coordinating the metal. However, the 31P
NMR spectrum of complex (II) and of its aquo analogous
trans-[Pd(COOCH3)(H2O)(PPh3)2](TsO) in CD3OD shows
a singlet at δ = 16.51 [29].
PPh3) and TsOH, under HMC conditions. These results give
further support to the above suggestions for the formation
of complex (I).
The synthesis of complex (I) was carried out also by react-
ing the chloride analogue trans-[Pd(COCH2CH3)Cl(PPh3)2]
with AgTsO in dichloroethane. It has been characterized by
elemental analysis, IR and NMR spectroscopy (Table 1).
The IR spectrums shows a strong absorption at 1688 cm−1
3.2. Reactivity of trans-[Pd(COCH2CH3)(TsO)(PPh3)2]
assigned to the νC
of the acyl ligand [35–37]. The ab-
=
(I) and of trans-[Pd(COOCH3)(TsO)(PPh3)2] (II)
O
sorptions −in the range 1230–1013 cm−1 are characteristic
1
the –SO3 group of the TsO ligand [29,38–40]. The H
The reactivity of these two complexes has been tested in
order to give more insight into their role in the catalysis,
as both of them can be considered as model catalytic inter-
mediates. Some reactions have been carried out also in the
presence of PPh3 and TsOH, since they have been found to
have a beneficial effect on the catalytic activity [27,28]. The
results reported in Table 2 can be summarized as follows: (i)
at r.t. the acyl complex (I) reacts with MeOH to give the ex-
pected MP in quantitative yield and a Pd(0) complex (entry
1), eventually together with the carbomethoxy complex (II),
if in the presence of PPh3 (entries 2 and 3, see Section 3.3).
At higher temperature, in the presence of CO–ethene mix-
ture, complex (I), in the presence of an excess of TsOH
and PPh3, catalyses the HMC (TOF = 380 h−1), but after
the reaction, it is recovered as complex (III) (yield 98%,
entry 4, see also Section 3.4), while when (I)/TsOH = 1/1,
NMR spectrum in deuterated benzene shows a quartet at
2.36 ppm and a triplet at 0.13 ppm due to the CH2 and
to the CH3 protons of the acyl group and a singlet at
1.92 ppm due to the CH3 protons of the TsO ligand. The
31P NMR spectrum in deuterated benzene shows a singlet
at 17.37 ppm indicating that the two triphenylphosphine
ligands are in trans position. It has been reported that
the complex trans-[Pd(CF3COO)2(PPh3)2] in a low-polar
dioxane solvent exists as a covalent PdX2P2 species
(31P NMR at 16.5 ppm), while in a relatively high-polar
80% aqueous CF3COOH medium as a cationic species
[PdP2(S)X]+ or even [PdP2(S)2]2+ (δ = 36.5 ppm) [33].
Moreover, the 31P NMR spectrum of the chloride ana-
logue to complex (I), trans-[Pd(COCH2CH3)Cl(PPh3)2],
in deuterated benzene, shows a smglet at 19.25 ppm. Thus,
Table 2
Reactivity of Pd(II) and Pd(0) complexes
Entry No. Complex Additives TsOH/PPh3/ P (MPa)
T (K) Time
Isolated species, yield (%)
Pd (mol/mol)
(min)
CO + C2H4 CO/C2H4
(I) (II) (III) (IV) (V)
CH3CH2COOCH3
mmol TOF (h−1
)
1
2
3
4
5
6
7
8
9
(I)
(I)
(I)
(I)
0/0/1
0/1/1
1/1/1
8/6/1
1/1/1
0/0/1
0/1/1
2/1/1
1.2/0.9/1
0/1/1
1/0/1
0/0/1
0.1
0.1
0.1
4.0
4.0
0.1
0.1
0.1
2.0
0.1
0.1
0.1
1/0
1/0
1/0
1/1
1/1
0/1
0/1
0/1
1/1
1/0
1/0
1/0
293
293
293
353
353
293
293
293
353
293
293
293
10
10
10
30
30
10
10
10
20
10
10
10
–
–
–
–
80
–
–
–
65
–
–
–
45
70
–
–
–
–
98
–
–
–
–
–
–
–
30
20
–
10
–
–
10
–
90
–
5
2
10
–
–
10
0.1
0.1
0.1
–
–
–
–
–
–
–
–
–
–
380
80
–
–
–
(I)
–
(II)
(II)
(II)
(II)
(III)
(IV)
(IV)
90
65
60
–
90
75
–
Traces
–
–
50
–
–
10
11
12
–
–
80
–
–
–
–
–
20
–
Run conditions: catalyst precursor: 0.1 mmol; (I) = [Pd(COCH2CH3)(TsO)(PPh3)2]; (II) = [Pd(COOCH3)(TsO)(PPh3)2]; (III) = [Pd(PPh3)2(TsO)2];
(IV) = [Pd(CO)(PPh3)3]; (V) = [Pd(CO)(PPh3)]3; MeOH: 3 ml; H2O: 800 ppm.

238
G. Cavinato et al. / Journal of Molecular Catalysis A: Chemical 219 (2004) 233–240
it is recovered as such it is (yield 80%, entry 5; complex
(I) is recovered also running the experiment at r.t., in which
case the TOF is significantly lower, ca. 8 h−1); (ii) at r.t.
the carbomethoxy complex (II) does not react with C2H4
in MeOH, it does not yield any MP and it is quantitatively
recovered unreacted (entry 6) (or together with a Pd(0)
complex, if the test in carried out also in the presence of
PPh3 and TsOH (entries 7 and 8)); (iii) at higher temper-
ature complex (II) catalyses the HMC (TOF = 50 h−1),
but after the reaction, it is recovered as complex (I) (yield
65%, entry 9, see also Section 3.3); (iv) catalysis occurs
with evolution of CO2 (see also Sections 3.3 and 3.4).
These results prove that the acyl complex (I), though re-
active, has been isolated from catalytic experiments because
the subsequent reaction with MeOH is relatively slow and
support to the suggestion that catalysis occurs via a hydride
species rather then a carbomethoxy one (see Sections 3.3
and 3.4).
why the carbomethoxy complex (II) is obtained in less than
50% yield when the conversion of (I) is carried out without
adding any TsOH (entry 2).
3.4. Conversion of trans-[Pd(COOCH3)(TsO)(PPh3)2]
(II) to trans-[Pd(COCH2CH3)(TsO)(PPh3)2] (I)
In principle complex (II), by reacting with C2H4, could
give the ester according to reactions (7) and (8). However, in
MeOH, saturated with C2H4, at r.t., it does not give the ester
in detectable amounts and it is recovered almost quantita-
tively (entry 6). It does not yield any ester even the presence
of 2 equivalents of TsOH (entry 8).
trans-[Pd(COOMe)(TsO)(PPh3)2] + C2H4
(II)
ꢀ trans-[Pd(CH2CH2COOMe)(TsO)(PPh3)2]
(7)
trans-[Pd(CH2CH2COOMe)(TsO)(PPh3)2]
3.3. Conversion of trans-[Pd(COCH2CH3)(TsO)(PPh3)2]
(I) to trans-[Pd(COOCH3)(TsO)(PPh3)2] (II)
MeOH, CO
ꢀ
(II) + EtCOOMe
(8)
Starting from complex (II), the ester forms only upon
heating, under pressure of CO and C2H4, however the start-
ing complex is recovered as complex (I) in 65% yield (en-
try 9). The conversion occurs with evolution of almost one
equivalent of CO2 and without formation of hydrogen in a
detectable amount.
We propose that H2O displaces MeOH from the car-
bomethoxy ligand with formation of a carbohydroxy species,
which evolves CO2 [41], yielding a Pd(II)–hydride, which,
upon insertion of C2H4 and of CO, yields complex (I) which
catalyses the HMC.
In principle, the transformation of complex (II) into com-
plex (I) may occur also via formation of dimethyl carbonate.
The Pd(0) complex that forms, reaction 9, is reoxidized by
the acid to a Pd(II)–hydride and trapped by C2H4 and CO
to give complex (I). Subsequent hydrolysis of the carbonate
by H2O would give CO2. However, dimethyl carbonate was
not detected by GC, not even in trace amounts.
The results reported in Table 2 deserve further comments.
Under the conditions of entry 1, the formation of the Pd(0)
complex occurs within a few minutes, together with MP in
quantitative yield. If the same test is carried out in the pres-
ence of PPh3, the Pd(0) complex, that precipitates initially,
re-desolves in ca. 30ꢀ and, upon addition of cold water, the
carbomethoxy complex (II) precipitates. It is interesting to
note that in the absence of TsOH the carbomethoxy is re-
covered only in 45% yield (entry 2), whilst in the presence
of 1 equivalent of acid in 70% yield (entry 3). These results
can be rationalized as proposed below.
Since the acyl complex (I) catalyses the HMC of C2H4
(see Section 3.4), it is likely that it reacts with MeOH ac-
cording to reaction (3). However, as already mentioned, the
Pd(II)–H that forms is unstable and deprotonates at equi-
librium with Pd(0) and TsOH (cf. reaction (4), equilibrium
(a)). We suggest that the acid reacts with the hydride giving
complex (III) [33,34] (cf. reaction (5)), which reacts with
CO and MeOH to give the carbomethoxy complex (II):
trans-[Pd(COOMe)(TsO)(PPh3)2] + MeOH
→ Pd(0) + (MeO)2CO + TsOH
(9)
trans-[Pd(TsO)2(PPh3)2] + CO + MeOH
(III)
ꢀ trans-[Pd(COOMe)(PPh3)2(TsO)] + TsOH
(6)
3.5. Catalytic activity of
trans-[Pd(COCH2CH3)(TsO)(PPh3)2] (I)
(II)
Supporting these suggestions, there are the facts that com-
plex (II) forms in 90% from (III) in MeOH saturated with
CO, even in the absence of the acid (entry 10), and that it
forms also starting from [Pd(CO)(PPh3)3] (IV) in 75% yield
in the presence of TsOH, whilst in the absence of the acid,
unreacted Pd(0) complex is recovered (entries 11 and 12).
It should be noted that deprotonation (4, equilibrium (a))
yields only 1 equivalent of acid, while the combination of
reactions (3) with (5) and (6) shows that quantitative conver-
sion to complex (II) requires 2 equivalents: this may explain
Complex (I) has been tested as catalyst precursor in the
methoxycarbonylation of ethene:
=
CH2 CH2 + CO + CH3OH → CH3CH2COOCH3
(10)
Most of the experiments have been carried in MeOH in
the presence of 800 ppm of H2O, since it has been found
that this is the optimal concentration of H2O when using
the precursor (III) [27]. The results are reported in Table 3,
together with the results obtained using other precursors. In

G. Cavinato et al. / Journal of Molecular Catalysis A: Chemical 219 (2004) 233–240
239
Table 3
hydride is trapped by ethene, for example via reaction with
the acid [33,34]. As a matter of fact, when complex (I) is
used in high concentration and in the presence of an excess
of TsOH, at 80 ◦C under 4.0 MPa of CO/C2H4 = 1/1, a TOF
of 380 h−1 is obtained and it is recovered as complex (III)
(98%) (Table 2, entry 4).
Another intermediate consuming side reaction can be the
reaction of a Pd(II)–CH2CH3 intermediate with the acid that
gives ethane and a complex of type (III). As a matter of
fact, ethane has been detected by GC during the course of
catalysis.
As already mentioned, an excess of acid may consume
the active Pd(II)–H species with evolution of H2, but on
the other hand it may stabilize the hydrides preventing their
deprotonation to inactive Pd(0) species or reoxydise them
back to Pd(II)–H species. As matter of fact, also the inactive
Pd(0) complexes (IV), (V) and (VI) catalyze efficiently the
HMC of ethene if an excess of TsOH is used (Table 3, entries
11–13).
Catalytic activity of Pd(II) and Pd(0) complexes
Entry no.
Complex
TsOH/Pd
(mol/mol)
PPh3/Pd
(mol/mol)
TOF
(h−1
Pdmet
)
1
2
3
4
5
6
7
8
9∗
10∗
11
12
13
(I)
(I)
(I)
(I)
(II)
(II)
(III)
(III)
(I)
(III)
(IV)
(V)
(VI)
–
10
–
10
10
–
10
–
1
–
–
8
8
8
–
8
–
8
8
–
8
30
45
40
420
420
Traces
410
Traces
210
220
410
400
Yes
Yes
No
No
No
Yes
No
Yes
No
No
Yes
No
No
1
10
10
10
330
Run conditions: catalyst precursor: 0.1 mmol; MeOH: 50 ml; H2O:
800 ppm; T: 353 K; P: 4.5 MPa (CO/C2H4 = 1/1); reaction time: 2 h; stir-
rer speed: 700 rpm; (VI) = [Pd(PPh3)4]. *MeOH = 40 ml; cyclohexene
= 10 ml; P = 2.0 MPa (CO).
In conclusion, the results presented in this paper give fur-
ther support to the suggestion that catalysis occurs via an
initial Pd(II)–H species. In addition they suggest that H2O
promotes the catalysis, even when complex (I) is used as pre-
cursor, by converting species of type (II) and (III), that are
likely to form during the catalysis, to Pd–(COOH) species
and hence to active Pd(II)–H species.
absence of TsOH and PPh3, complex (I) leads, during 2 h,
to a TOF of 30 h−1 (entry 1). This value is only indicative
since under these conditions the complex decomposes to
palladium metal. By adding an excess of TsOH only a slight
increase of TOF is observed (45 h−1, entry 2). Also in this
case decomposition to palladium metal occurs. Practically,
the same result is obtained when PPh3 is added (without
TsOH), but the formation of Pd metal is avoided (entry 3).
The addition of PPh3 and of TsOH affords the best TOF
(420 h−1, entry 4), practically the same as that achieved
when using complex (II) or (III) under the same conditions.
However, these complexes in the absence of TsOH and PPh3
are inactive (entries 5–8) [27,29].
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