Synthesis, characterization and X-ray structure of [Pd(SO 4)(dppp)]·H2O, a catalyst for the CO-ethene copolymerization [dppp = 1,3-bis(diphenylphosphino)propane]

Article abstract of DOI:10.1016/j.ica.2005.09.065

The title complex has been synthesized by first reacting dppp with Pd(AcO)₂ in acetone and then with NaHSO₄ in water. It has been characterized by IR, NMR and X-ray diffraction studies. The ³¹P NMR spectrum in DMSO shows a singlet at 16.62 ppm indicating that the two P atoms are equivalent and that the sulfate anion is weakly coordinating. The X-ray structure shows that the Pd atom is surrounded in an almost regular square planar environment by the two P atoms and by two O atoms of the sulfate anion and that the neutral complex is accompanied by a water molecule of crystallization. The Pd-P distances (2.217(1) and 2.233(1)) and the P-Pd-P angle (90.78(3)°) are close to those found in other complexes where the chelating diphosphine is the same. Also the Pd-O distances and the O-Pd-O bond angle are comparable to those of other relevant chelating ligands. In MeOH, the title complex, in combination with H₂SO₄, catalyses the CO-ethene copolymerization. The productivity reaches a maximum upon increasing the H₂SO₄/Pd ratio up to ca. 470 (7650 g of polyketone/g Pd h at 90°C and 45 atm, CO/ethene 1/1). The viscosity of the polyketone passes through a maximum of 0.95 dL/g in m-cresol when the above ratio is ca. 100. It has been proposed that acid promotes the copolymerization process by destabilizing the β- and γ-chelates intermediates involved in chain growing process, thus favoring the insertion of the monomers. At relatively high acid concentration the lowering of productivity and viscosity suggests that the sulfate anion competes with the monomers for the coordination to the metal center. In H₂O-CH₃COOH as a solvent the productivity strongly depends on the H₂O/CH₃COOH ratio, as it passes through a maximum of 12 000 g polymer/g Pd h in the presence of ca. 60% of H₂O. The productivity is significantly lower than that found when the acetate and chloride analogues are used (27 000 g polyketone/g Pd·h). Thus, it is likely that the sulfate anion assists significantly the copolymerization process even though the concentration of CH₃COOH/ CH₃COO^- is much preponderant.

Full text of DOI:10.1016/j.ica.2005.09.065

Inorganica Chimica Acta 358 (2005) 4555–4562
www.elsevier.com/locate/ica
Synthesis, characterization and X-ray structure
of [Pd(SO4)(dppp)] Æ H2O, a catalyst for the
CO-ethene copolymerization [dppp = 1,3-
bis(diphenylphosphino)propane]
a
b
b,
b
Gianni Cavinato , Andrea Vavasori , Luigi Toniolo *, Lucio Ronchin ,
b
c
Federico Dallꢀ Acqua , Alessandro Dolmella
a
Department of Chemical Sciences, University of Padua, via Marzolo 1, 35100 Padua, Italy
Department of Chemistry, University of Venice, Calle Larga S. Marta, Dorsoduro 2137, 30123 Venice, Italy
b
c
Department of Pharmaceutical Sciences, University of Padua, via Marzolo 5, 35131 Padua, Italy
Received 19 July 2005; received in revised form 26 September 2005; accepted 30 September 2005
Available online 11 November 2005
Abstract
The title complex has been synthesized by first reacting dppp with Pd(AcO)2 in acetone and then with NaHSO4 in water. It has been
characterized by IR, NMR and X-ray diffraction studies. The 31P NMR spectrum in DMSO shows a singlet at 16.62 ppm indicating that
the two P atoms are equivalent and that the sulfate anion is weakly coordinating. The X-ray structure shows that the Pd atom is sur-
rounded in an almost regular square planar environment by the two P atoms and by two O atoms of the sulfate anion and that the neu-
tral complex is accompanied by a water molecule of crystallization. The Pd–P distances (2.217(1) and 2.233(1)) and the P–Pd–P angle
(90.78(3)ꢀ) are close to those found in other complexes where the chelating diphosphine is the same. Also the Pd–O distances and the
O–Pd–O bond angle are comparable to those of other relevant chelating ligands.
In MeOH, the title complex, in combination with H2SO4, catalyses the CO-ethene copolymerization. The productivity reaches a max-
imum upon increasing the H2SO4/Pd ratio up to ca. 470 (7650 g of polyketone/g Pd h at 90 ꢀC and 45 atm, CO/ethene 1/1). The viscosity
of the polyketone passes through a maximum of 0.95 dL/g in m-cresol when the above ratio is ca. 100. It has been proposed that acid
promotes the copolymerization process by destabilizing the b- and c-chelates intermediates involved in chain growing process, thus
favoring the insertion of the monomers. At relatively high acid concentration the lowering of productivity and viscosity suggests that
the sulfate anion competes with the monomers for the coordination to the metal center.
In H2O–CH3COOH as a solvent the productivity strongly depends on the H2O/CH3COOH ratio, as it passes through a maximum of
12000 g polymer/g Pd h in the presence of ca. 60% of H2O. The productivity is significantly lower than that found when the acetate and
chloride analogues are used (27000 g polyketone/g Pd Æ h). Thus, it is likely that the sulfate anion assists significantly the copolymeriza-
tion process even though the concentration of CH3COOH/CH3COOꢀ is much preponderant.
ꢁ 2005 Elsevier B.V. All rights reserved.
Keywords: Palladium complex; Carbonylations; Ethene; X-ray crystal structures
1. Introduction
Cis-coordinated diphosphine cationic complexes of
Pd(II) can be highly active catalysts in the carbonylation
of ethene to a wide spectrum of important products
*
Corresponding author. Tel.: +39 041 2348553; fax: +39 041 2348517.
ranging from high molecular weight perfectly alternating
E-mail address: toniolo@unive.it (L. Toniolo).
0020-1693/$ - see front matter ꢁ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.09.065

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G. Cavinato et al. / Inorganica Chimica Acta 358 (2005) 4555–4562
polyketones (PK) or cooligomers, or lower carbonylated
derivatives down to monocarbonylated products such as
diethyl ketone (DEK) or propanoic acid and its esters
[1–3]. PK represents a new class of melt-processable ther-
moplastic with interesting physical–mechanical proper-
ties, biodegradability, impermeability towards gases and
organic solvents [4]. Mixtures of low molecular weight
products and DEK can find large use as low volatility
‘‘green’’ solvents [5]. Methyl propanoate (MP) is a poten-
tial intermediate to methyl methacrylate, an important
monomer produced annually as a multimillion tonne
scale worldwide [6].
rocene ligand, as when alkyl = methyl the catalyst is
very active in promoting high molecular weight PK,
whereas when alkyl = ethyl lower molecular weight PK
are obtained, and when alkyl = i-propyl the products
are MP and DEK [18].
Usually the catalyst is used in combination with an
excess of acid (acid/Pd = 10–50) and of an organic oxidant
such as benzoquinone (BQ/Pd = 50–100), to prevent
deprotonation of the active Pd–H species and to reoxidize
less active Pd(I) and Pd(0) species which unavoidably form
during the course of the catalysis. Possible deactivation
paths have been delineated in [2].
The reactions are usually performed using MeOH as
solvent, which acts also as a source of Pd–H or Pd–
OCH3 species, which start the catalysis [7]. Monocarb-
onylated products form through the so called ‘‘hydride’’
mechanism, which involves the insertion of the olefin
into a Pd–H bond with formation of a Pd-alkyl bond,
followed by the insertion of CO into the Pd-alkyl inter-
mediate with formation of a Pd-acyl species, which
undergoes alcoholysis or hydrolysis to the ester or the
acid with reformation of the Pd–H species starting the
catalytic cycle [8–11]. DEK forms when the insertion of
a second molecule of ethene into the Pd-acyl bond
occurs with formation of a Pd-alkylacyl intermediate, fol-
lowed by protonolysis [12–14]. If the Pd-alkylacyl inter-
mediate undergoes successive multiple sequential
insertions of CO and ethene the process of chain growing
proceeds through the intermediate formation of b- and
c-chelates till termination occurs with formation of coo-
ligomers or of PK [1–3,15]. Termination occurs via alco-
holysis or protonolysis. Protonolysis by MeOH yields the
polymer and a Pd–OCH3 species, which can start the so
called ‘‘carboalkoxy’’ mechanism, which involves the
insertion of CO with formation of a Pd–COOCH3 spe-
cies followed by successive multiple sequential insertion
of ethene and CO till termination [16].
Cis-coordination is ideal for promoting the insertion
reactions. In order to easy the coordination of the mono-
mers and the subsequent chain growing process, the cat-
ionic charge of the metal center has to be balanced by a
non- or weakly-coordinating anion, conjugated base of a
strong acid, for example TsOH (p-toluenesulfonic acid),
HBF4, HPF6, CF3COOH, CF3SO3H [7].
The selectivity of the carbonylation reaction depends
mainly on the nature of the ligand. C3-bridged diphenyl-
phosphine of the type reported on the title, or other
diaryl analogues, for example with ortho-methoxy sub-
stituents on the phenyl rings, are highly active in the
copolymerization process [17], whereas bulkier diphos-
phines such as t-BuP(CH2)3PBu-t or 1,2-bis[(di-tert-
butyl)phosphinomethyl]benzene are very active and
selective in promoting the methoxycarbonylation to MP
[8–11,17]. That steric bulk plays a role of paramount
importance in controlling the selectivity of the reaction
is also well illustrated by the results obtained using cat-
ionic Pd(II) complexes of 1,10-bis(dialkylphosphino) fer-
The influence of the anion has been less studied,
though the anion plays also a role in the catalysis. For
catalysts which promote the formation of low molecular
weight products the selectivity may depend dramatically
from the nature of the anion, as in the case of ethane-
bridged bis(phosphabicyclononane)-based catalyst that
changes from a low molecular weighꢀt polymer to MP
when the counter-anion is CH3SO3 or propanoate,
respectively [19].
The coordinating capacity depends not only on the nat-
ure of the anion, but also on that of the solvent. Polar sol-
vents help the dissociation of the anion from the cationic
active catalyst, thus favoring the polymerization process.
This is well illustrated by comparing the activity of the cat-
alysts [PdX2(dppp)] (X = TfO, TsO, TFA, AcO, Cl). In
MeOH the activity is comparable when X is weakly coordi-
nating like TfO, TsO, TFA (ca. 7000–6000 g polymer/g Pd h
at 90ꢀ, 45 bar, CO/ethene = 1/1). When X = AcO or Cl
the catalyst is ineffective [7], but becomes very active when
used in H2O–AcOH, ca. 40–50% molar ratio, 27000 g poly-
mer/g Pd Æ h, under the conditions just reported, in spite
of the lower solubility of the monomers in this medium
[20,21].
Also the activity of water-soluble catalysts based on the
hydrophilic bidentate phosphines is significantly influenced
by the nature of the anion [22–28].
The influence of the concentration of the acid has also
received less attention, though it has been reported that
for the Pd(AcO)2 precursor in combination with an excess
of PPh3 or with the monosulfonated derivative PPh2Ph
SO3H, in acetic acid and in the presence of TsOH (Pd/P/
TsOH = 1/8/80–800, 90 ꢀC, 1–50 bar), the reaction
switches from monocarbonylation with formation of pro-
pionic acid to oligocarbonylation products and to polyke-
tones as the pressure and concentration of TsOH increase.
The yield in polyketones also increases upon increasing the
acidity [29,30]. A similar acid effect has been found with the
dtbpe-based catalyst in MeOH for the production of DEK
in the presence of excess acid, as the selectivity lowers from
99% DEK to up to ca. 33%, ca. 60% being cooligomers
[14].
In this article, we report the synthesis and characteriza-
tion of the title complex and its catalytic activity in the CO-
ethene copolymerization in MeOH at different H2SO4/Pd
ratio or H2O–CH3COOH as solvents.

G. Cavinato et al. / Inorganica Chimica Acta 358 (2005) 4555–4562
4557
2. Experimental
autoclave, in order to prevent contamination by metallic
species because of corrosion of the internal surface of the
autoclave.
2.1. Reagents
In a typical experiment, 0.540 mg of (I) (8.57 ·
10ꢀ4 mmol) was added to 80 mL of MeOH (containing
H2SO4) or of AcOH–H2O (H2O = 55%, mol/mol) con-
tained in the bottle placed in the autoclave. The autoclave
was pressurized at room temperature with ca. 0.5 MPa of
the two gasses supplied as a 1/1 mixture from a tank reser-
voir. The autoclave was then heated to 90 ꢀC during ca.
10 min. The pressure was then adjusted to the desired value
(typically 4.5 MPa total pressure) and maintained constant
throughout the experiment (1 h, rate stirring 700 rpm) by
continuously supplying the gasses from the reservoir. At
the end of the experiment the autoclave was quickly cooled
and carefully depressurized. The slurry product was fil-
tered, washed with water and acetone and dried under vac-
uum at 80 ꢀC.
Methanol (purity >99.5%, 500 ppm water) and acetic
acid (100%, glacial) were purchased from Baker. Palla-
dium(II) acetate, and 1,3-bis(diphenylphosphine)propane,
1,1,1,3,3,3-hexafluoroisopropanol (99%) and CDCl3 were
Aldrich products. The complexes [PdX2(dppp)] (X = Cl,
AcO, TsO) were prepared as reported in literature
[31,32]. Carbon monoxide and ethylene were supplied by
SIAD Company (ꢁresearch gradeꢀ, purity >99.9%).
2.2. Equipment
Gas-chromatographic analysis of the gas phase was per-
formed on a Hewlett–Packard Model 5890, Series II chro-
matograph fitted with a 18 ft · 1/8 SS Silica Gel, 60/80
packed column.
The limiting viscosity number (LVN) measurements
were performed on a capillary viscosimeter at 25 ꢀC using
m-cresol as solvent.
2.5. X-ray diffraction analysis
A crystal suitable for the X-ray analysis was mounted on
the top of a glass fiber, coated with epoxy resin, and trans-
ferred to a STADI4 CCD automated diffractometer. Data
were collected at room temperature, using graphite mono-
IR spectra were recorded in nujol mull on a Nicolet
1
FTIR instrument mod. Nexus. H and 31P NMR spectra
of complex (I) were recorded on a Bruker AMX 300 spec-
trometer equipped with a BB multinuclear probe operating
˚
chromated Mo Ka radiation (k = 0.71073 A), and cor-
1
in the FT mode at 300 and 121.442 MHz for H and 31P,
rected for Lorentz, polarization and absorption effects.
The structure was solved by means of the heavy-atom
methods, using the SHELXTL-NT package [33,34], and was
refined by full-matrix least-squares methods on F2 with
SHELXL-97 [35]. All non-hydrogen atoms were refined aniso-
tropically; hydrogen atoms (apart from those of the water
molecule) were placed in calculated positions and refined as
riding model. A summary of crystal and refinement data is
reported in Table 1, while Table 2 lists some selected inter-
atomic distances and angles.
respectively.
1H and 13C NMR spectra of the copolymer were
recorded on a Bruker Avance 300 spectrometer in
1,1,1,3,3,3-hexafluoroisopropanol/CDCl3 (10/1) using the
1
Inverse H-Gated Decoupling Technique.
The X-ray diffraction analysis were recorded on a
STADI4 CCD automated diffractometer.
2.3. Synthesis of [Pd(SO4)(dppp)] Æ H2O (I)
Table 1
An acetone solution of dppp was added dropwise, under
stirring, at r.t., to an acetone solution of Pd(AcO)2 (3 mL,
112 mg, 0.50 mmol). The resulting yellow suspension was
allowed to stir for ca. 20. Upon addition of 160 mg of NaH-
SO4 Æ H2O (1.15 mol), dissolved in 1 mL of H2O, the yellow
solid first dissolved and then precipitation of a pale yellow
solid was observed. The suspension was stirred for ca.
30 min and then filtered. The yellow solid was washed with
acetone–water (5/2), ethyl ether and dried on vacuum. Yield
284 mg, 90%. Suitable crystal for X-ray structure determina-
tion have been obtained by recrystallization from MeOH–
CHCl3–petrol ether. Anal. Calc. for C27H28O5P2S: C,
51.21; H, 4.46; S, 5.07. Found: C, 51.13; H, 4.54; S, 4.59%.
Summary of crystal and structure refinement
Empirical formula
Formula weight
Crystal system; space group
C27H28O5P2PdS
632.89
monoclinic; P21/c (No. 14)
9.983(2)
15.634(3)
17.551(4)
95.92(3)
2724.6(9)
4; 1.543
0.71073; 9.11
3.3–29.5
6971
5037
6971/325
0.031
0.062
0.939
˚
a (A)
˚
b (A)
˚
c (A)
b (ꢀ)
3
˚
V (A )
Z; Dcalcd (Mꢀg1mꢀ3
)
˚
k (A), l (cm
h Range (ꢀ)
)
Reflections collected
Reflections observed (I > 2r(I))
Data/parameters ratio
a
R1 (observed reflections)
2.4. Carbon monoxide-ethene copolymerization
b
wR2 (observed reflections)
Goodness-of-fit on F2c
ꢀ3
˚
The copolymerization reactions were carried out by
using a Hastelloy C autoclave of ca. 250 ml provided with
a four-blade self-aspirating turbine. Solvent and catalyst
were added in a ca. 150 ml Pyrex bottle placed inside the
Largest peak (e A
)
0.89
P
P
a
R1 = iFoj ꢀ jFci/ jFoj.
P
P
2
2 1=2
b
c
wR2 ¼ ½ wðF 2o ꢀ F c2Þ = wðF 2oÞ ꢁ
.
P
2
1=2
GOF ¼ ½ wðjF 2oj ꢀ jF 2c jÞ ꢁ=ðNobsd ꢀ NparamsÞꢁ
.

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G. Cavinato et al. / Inorganica Chimica Acta 358 (2005) 4555–4562
Table 2
˚
Selected bond distances (A) and angles (ꢀ)
Pd–P(1)
Pd–P(2)
Pd–O(1)
Pd–O(2)
Pd–S
2.217(1)
2.233(1)
2.097(2)
2.093(2)
2.689(1)
S–O(1)
S–O(2)
S–O(3)
S–O(4)
1.519(2)
1.523(2)
1.438(2)
1.444(2)
P(1)–Pd–P(2)
O(1)–Pd–O(2)
P(1)–Pd–O(2)
P(2)–Pd–O(1)
90.78(3)
68.62(6)
166.90(5)
170.27(5)
Pd–P(1)–C(1)
Pd–P(2)–C(3)
Pd–O(1)–S
116.64(9)
114.81(9)
94.73(8)
94.76(8)
Pd–O(2)–S
3. Results and discussion
3.1. Characterization and X-ray structure of complex (I)
The IR spectrum shows bands at 3625 and 3389 cmꢀ1
for the stretching bands of H2O [32] and at 1235, 1099
and between 900 and 850 cmꢀ1 for the sulphate anion [36].
The 1H NMR spectrum in deuterated DMSO shows one
signal at 3.36 ppm (H2O), a broad signal at 2.88 ppm (4H,
PCH2), and a multiplet at 1.97 ppm (2H, CH2). The 31P
NMR in DMSO shows a singlet at 16.62 ppm and in
CDCl3–CD3OD a singlet at 18.45 ppm, thus indicating that
the two P atoms are magnetically equivalent. The 31P
NMR of the related complexes [Pd(TsO)2(dppp)] Æ 2H2O
[37] and [Pd(TsO)(H2O)(dppp)](TsO) [32] in CDCl3 show
a singlet in the same region, 16.0 and 17.5 ppm, respec-
tively, whereas the analogous complex [Pd(CF3COO)2
(dppp)] shows a singlet at 11.56 ppm [38]. It has been also
reported that addition of two equivalents of TsOH to an
equimolar mixture of Pd(AcO)2 and dppp in acetone solu-
tion causes a downfield shift of the phosphorous signal
from 11.1 to 17.5 ppm [7]. This deshielding of the P atoms
has been attributed to the shift of electron density from the
P atoms to a more electrophilic Pd centre which results
upon replacement of the acetate anions by TsOꢀ [7]. Thus
the position of the signal of complex (I) indicates that the P
atoms are deshielded and that the sulphate anion is labile.
This lability is likely to be related to the high catalytic
activity of complex (I), comparable to that of the tosylate
analogue (see below).
The X-ray structure shows that in the solid state the
neutral complex is accompanied by a crystallization water
molecule. An ORTEP [39] representation of the molecular
structure of the complex, together with the employed num-
bering scheme, is illustrated in Fig. 1.
The environment of the Pd atom is almost regular
square planar, and the chelate diphosphine ligand forms
upon coordination a six-membered ring, assuming a chair
(D3d) conformation. To the best of our knowledge, this is
the first example of a Pd(II) complex with a chelating sul-
fate dianion. A search in the Cambridge Crystallographic
Database (CCD) [40] for square planar Pd(II) complexes
showing the (P\P)O2 donor set and at least one oxygen
bound to a sulfur atom (P\P = bidentate neutral phospho-
rous ligand) returned five entries [11,41,42]. The main geo-
Fig. 1. The molecular structure of the complex; hydrogen atoms not
shown for clarity. Thermal ellipsoids at the 40% probability level.
metrical parameters for the reported structures are listed in
Table 3.
˚
The Pd–P distances (2.217(1) and 2.233(1) A) fit within
the reported experimental range and are similar to those
found in [Pd(OTf)(H2O)(dppp)] (OTf) [42]; the Pd–O dis-
˚
tances (2.097(2) and 2.093(2) A) are very short and close
to the values found in [Pd(OS(Me)O)(dppp)]2 (BAr4)2 [41]
˚
(2.078(5) and 2.088(5) A). However, in the present complex
the chelate ligand is dianionic and the formal oxidation
state of sulfur is +6, while compound [41] is a dinuclear
complex in which a monoanionic ligand bridges two Pd
units and the formal oxidation state of sulfur is +2. If com-
pound [41] is omitted, the Pd–O bond lengths found here
are the shortest reported so far. This is likely due to the
restraints imposed by chelation, or to the harder r donor
character of the dianionic ligand. The latter idea seems
somewhat reinforced by looking at data listed in Table 3,
where the S–O distances of the coordinating oxygens are
˚
ca. 0.06 A longer than those of the non-coordinating ones.
˚
In this work, the lengthening grows to 0.09 A, with the dis-
˚
tances (1.519(2) and 1.523(2) A) becoming the longest
reported so far.
With respect to bond angles, the P(1)–Pd–P(2) angle
(90.78(3)ꢀ) is similar to those reported in [Pd(OTf)-
(H2O)(dppp)](OTf) [42] and [Pd(OTs)(H2O)(dppp)](OTs)
[32], and is also close to the average of 92.5ꢀ found in about
thirty analogous Pd compounds [40]; on the other hand,
the O(1)–Pd–O(2) angle is similar to that of the chelating
methanesulfonate ligand in [Pd(g2–MeSO3)(dtbpx)]

G. Cavinato et al. / Inorganica Chimica Acta 358 (2005) 4555–4562
4559
Table 3
Geometrical parameters of reported square planar Pd(II) complexes
Pd–P
Pd–O
S–Oa
S–Ob
P(1)–Pd–P(2)
90.78 (3)
O(1)–Pd–(O2)
68.62 (6)
[Pd(g2-SO4)(dppp)] Æ H2O [this work]
[Pd(g2-MeSO3)(dtbpx)] (MeSO3)(MeSO3H) [11]c
[Pd(OS(Me)O)(dppp)]2 (BAr4)2 [41]e
[Pd(OTf)(H2O)(dppp)] (OTf) [42]
2.217 (1)
2.233 (1)
2.097 (2)
2.093 (2)
1.519 (2)
1.523 (2)
1.438 (2)
1.444 (2)
2.274 (2)
2.267 (2)
2.188 (4)
2.172 (4)
1.490d
1.492d
1.421d
100.59 (6)
90.12 (7)
90.98 (4)
88.61 (4)
65.7 (1)
90.9 (2)
86.1 (2)
85.9 (1)
2.235 (2)
2.239 (2)
2.078 (5)
2.088 (5)
1.494 (5)
1.507 (5)
2.237 (1)
2.228 (1)
2.106 (4)c
2.159 (3)
1.469 (4)
1.411 (5)
1.425 (5)
[Pd(OTs)(H2O)(dppp)] (OTs) [32]
2.236 (1)
2.236 (1)
2.138 (4)c
2.152 (3)
1.484 (3)
1.446 (4)
1.434 (5)
a
Refers to the coordinating oxygen(s).
Refers to non-coordinating oxygen(s).
Refers to the oxygen not bound to sulfur.
b
c
d
Data extracted from the Cambridge Crystallographic Database, standard deviations unavailable.
The reported metrical data refer to the one-half molecule defining the asymmetric unit.
e
Scheme 1. Catalytic cycles for the copolymerization process, with evidenced the termination step by protonolysis (a) and by methanolysis (b), and side
reactions to less active Pd(0) and Pd(I) species.
(MeSO3) (MeSO3H) [11]. As for the nonbonding interac-
tions, the crystallization water molecule is very loosely
bound to the sulfate O(4) oxygen, with a O–O separation
H2O–CH3COOH. In the first solvent the activity has been
tested using the precursor in combination with H2SO4. In
order to observe significant catalytic activity the acid must
be used in excess. Under the condition reported in Fig. 3
the productivity reaches a maximum of ca. 7650 g of poly-
mer/g of Pd h when the ratio H2SO4/Pd is ca. 470/1. This
behavior is similar to that observed when the analogous
acetate precursor is used in combination with formic acid,
though in this case the productivity reaches a plateau of
˚
of 2.835 A.
3.2. CO-C2H4 copolymerization
Complex (I) was used as catalyst precursor for the
CO-C2H4 copolymerization in MeOH or in a mixture of

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G. Cavinato et al. / Inorganica Chimica Acta 358 (2005) 4555–4562
ca. 7500 g polymer/g Pd h at a higher concentration of acid
(HCOOH/Pd ca. 3000) [43]. Differently, the tosylate pre-
cursor [Pd(TsO)(H2O)(dppp)](H2O) gives best results when
the ratio TsOH/Pd is significantly lower (8/1) and in the
presence of 800–1000 ppm of H2O [44].
As already mentioned, a Pd–H or a Pd–OCH3 species
initiate the catalytic cycle [7]. In principle, these species
should reform at the termination steps, which occur via
methanolysis, or via protonolysis by MeOH or H2O
(always present in the solvent), as shown in the Scheme 1
[16].
Methanolysis can occur via inter- or intra-molecular
nucleophilic attack by MeOH at the carbon atom of a
Pd-acyl moiety and produces an ester end-group and a
Pd–H+ species, which may deprotonate with formation of
less active Pd(0) and dimeric species [45,46].
Protonolysis involves the reaction of a b-chelate moiety
with methanol and prþoduces a polymer with keto end-
group and a Pd–OCH3 species. The mechanism proposed
to explain deuterium enrichment in the 2-position with
respect to 1-position involves a pre-equilibrium of the
b-chelate with its enolate isomer by a b-H elimination/
hydride migration and protonation to the more nucleo-
philic oxygen atom to give an enol, which rearranges to
the ketone [16,47]. When protonolysis occurs by H2O, the
binuclear complex ½Pdðl-OHÞðP–PÞꢁ22þ, which may form
if excess acid is not used, reacts with CO with regeneration
of a Pd–H+ species after CO2 evolution [48]. In turn the
hydride may undergo deprotonation with formation of a
less active dimer as shown in Scheme 1 [45,49,50].
The beneficial effect of H2SO4 on the productivity shown
in Fig. 2 is probably due to the possibility that the acid
inhibits the deprotonation of the active Pd–H+ species
which initiates the catalysis. The lowering of productivity
at relatively high H2SO4/Pd ratio suggests that the anion
may compete with the monomers for the coordination to
the metal center.
Since protonolysis is one of the chain termination reac-
tion, a lowering of the molecular weight of the copolymer
should be expected upon increasing the acid concentration.
For the catalytic system Pd(AcO)2/dppp/CF3COOH it was
also found that the molecular weight is not effected upon
increasing the acid/Pd ratio up to 100 [51]. It was con-
cluded that enolate formation is the rate determining step.
However, the results obtained with the system complex
(I)/H2SO4 show that the viscosity significantly increases
upon increasing the H2SO4/Pd ratio up to 100 and that
at higher ratio it decreases (see Fig. 2). These results sug-
gest that at relatively high concentration of the acid the
chain growing process may slow down because of the
above mentioned competition between the sulfate anion
and the monomers, whereas at relatively low acid concen-
tration the rate of the copolymerization process increases
upon increasing the acid concentration. We suggest that
protonation of the oxygen atom of the coordinating keto
groups might destabilize the b- or of the c-chelates, thus
favoring the insertion of the monomers. At this regard, it
is worth mentioning that the insertion of ethene is believed
to be the slow step of the copolymerization process [1].
As already mentioned the copolymerization is carried out
also in the presence of an organic oxidant, such as BQ,
which has the function to reoxidize the Pd(0) and Pd(I) spe-
cies. When the copolymerization is carried out in the pres-
ence of relatively high concentration of H2SO4, the use of
BQ is influent. In order to evidentiate a possible beneficial
effect of BQ it is necessary to carry out the copolymerization
with a relatively low acid/palladium ratio. Thus, for exam-
ple when H2SO4/Pd = 10, the productivity increases from
4000 g polymer/g Pd h in the absence of BQ to 8000 g.
In H2O–CH3COOH as a solvent the productivity
strongly depends on the H2O/CH3COOH ratio, as it passes
through a maximum of 12000 g polymer/g Pd h in the
presence of ca. 60% of H2O (Fig. 3). The productivity is
significantly lower than that found when the acetate and
8000
7000
6000
5000
4000
3000
2000
1000
0
1.0
productivity
LVN
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0
100 200 300 400 2000 3000 4000 5000 6000 7000 8000 9000
H2SO4/Pd (mol/mol)
Fig. 2. Effect of H2SO4 addition on the productivity and on the LVN in MeOH as solvent Run conditions: [Pd(SO4)(dppp)] Æ H2O = 2.8 · 10ꢀ3 mmol;
P = 4.5 MPa (CO/C2H4 = 1/1, at the working temperature); T = 90 ꢀC; MeOH = 80 mL; reaction time = 1 h; stirrer = 700 rpm.

G. Cavinato et al. / Inorganica Chimica Acta 358 (2005) 4555–4562
4561
30000
[P
d(AcO)2(dppp)]
[PdCl2(dppp)]
25000
20000
15000
10000
5000
0
[Pd(SO4)(dppp)].H2O
0
10
20
30
40
50
60
70
80
90
100
H2O, % (mol/mol)
Fig. 3. Effect of the concentration of water on the productivity in acetic acid as solvent. Run conditions: [Pd(SO4)(dppp)] Æ H2O = 8.5 · 10ꢀ4 mmol;
P = 4.5 MPa (CO/C2H4 = 1/1, at the working temperature); T = 90 ꢀC; H2O–AcOH: 80 mL; reaction time = 1 h; stirrer = 700 rpm. For comparison, the
productivities obtained using the precursors [Pd(AcO)2(dppp)], [PdCl2(dppp)] (see [20,21], respectively) are also shown.
[4] D.H. Weinkauf, P.A. Kinneberg, C.A. Ash, Proceedings of the 1995
ANTEC Conference, Boston, Massachusetts.
[5] R.A.M. Robertson, D.J. Cole-Hamilton, Coord. Chem. Rev. 225
chloride analogues are used [20,21]. Thus, in this solvent, it
is likely that the sulfate anion assists significantly the copo-
lymerization process even though the concentration of
(2002) 67.
CH3COOH/CH3COOꢀ is much preponderant.
[6] New MMA Technology, European Chemical News, Oct. 30–Nov. 5,
The relatively high molecular weight of the polymer pre-
vented to establish the nature of the end groups. The fact
that the copolymerization process occurs with concomitant
CO2 evolution suggests that the catalytic cycle starts from a
hydride and that termination occurs via protonolysis by
2000, p. 20.
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H2O or H3O+ with formation of Pd–OH+ or Pd–OH2
,
2þ
which regenerate the starting Pd–H+ species upon interac-
tion with CO followed by evolution of CO2 according to a
reaction strictly related to the water gas-shift reaction [52–
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The high activity may be due to the possibility that the
acid and H2O prevent deprotonation/dimerization of the
active hydride with formation of dimeric species of the type
shown in Scheme 1, thus ensuring a relatively high concen-
tration of the most active species. Moreover, also in this
case the acid through protonation of the oxygen atom of
the coordinating keto group of the b- or c-chelates may
favors the insertions of the monomers in the process of
chain growing.
The viscosity of the copolymer obtained under condi-
tions of highest productivity (H2O in CH3COOH = 60%)
is 0.65 dL/g, in line with the values found when MeOH is
used as solvent.
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