Palladium(II) Complexes of the Hemilabile Pincer Ligand PPh(o-C6H4SMe)2as Highly Active and Recyclable -Mizoroki–Heck Catalysts

Article abstract of DOI:10.1002/ejoc.201501354

Pd^IIcomplexes with PPh(o-C₆H₄SMe)₂acting as a pincer ligand in an S,S′,P- or S,P-coordination mode have been synthesized. The complexes include [PdCl₂{PPh(o-C₆H₄SMe)₂}] (1), [Pd(Me)Cl{PPh(o-C₆H₄SMe)₂}] (2), and [Pd(X){PPh(o-C₆H₄SMe)₂}]A [A = CF₃SO₃, X = Cl (3); A = CF₃SO₃, Me (4); A = PF₆, X = Me (5)]. Insertion of CO into the Pd–Me bond of these complexes leads to [Pd(COMe)Cl{PPh(o-C₆H₄SMe)₂}] (6) and [Pd(COMe){PPh(o-C₆H₄SMe)₂}]A [A = CF₃SO₃(7); A = PF₆(8)]. Some of these complexes are good Mizoroki–Heck catalysts (for the reactions between iodobenzene and methyl acrylate or styrene). The most active catalyst was found to be [Pd(Me)Cl{PPh(o-C₆H₄SMe)₂}] (2). This catalyst could be retained on a silica gel column, recovered, and reused several times without losing activity.

Full text of DOI:10.1002/ejoc.201501354

DOI: 10.1002/ejoc.201501354
Full Paper
Recyclable Heck Catalyst
Palladium(II) Complexes of the Hemilabile Pincer Ligand
PPh(o-C6H4SMe)2 as Highly Active and Recyclable Mizoroki–Heck
Catalysts
Asunción Luquin,[a,b] Noelia Castillo,[b] Elena Cerrada,[b] Francisco L. Merchan,[b]
Julian Garrido,[a] and Mariano Laguna*[b]
Dedicated to the memory of Professor Roberto Sanchez-Delgado
Abstract: PdII complexes with PPh(o-C6H4SMe)2 acting as a [Pd(COMe){PPh(o-C6H4SMe)2}]A [A = CF3SO3 (7); A = PF6 (8)].
pincer ligand in an S,S′,P- or S,P-coordination mode have been Some of these complexes are good Mizoroki–Heck catalysts (for
synthesized. The complexes include [PdCl2{PPh(o-C6H4SMe)2}] the reactions between iodobenzene and methyl acrylate or
(1), [Pd(Me)Cl{PPh(o-C6H4SMe)2}] (2), and [Pd(X){PPh(o- styrene). The most active catalyst was found to be
C6H4SMe)2}]A [A = CF3SO3, X = Cl (3); A = CF3SO3, Me (4); A = [Pd(Me)Cl{PPh(o-C6H4SMe)2}] (2). This catalyst could be retained
PF6, X = Me (5)]. Insertion of CO into the Pd–Me bond of these on a silica gel column, recovered, and reused several times
complexes leads to [Pd(COMe)Cl{PPh(o-C6H4SMe)2}] (6) and without losing activity.
Introduction
ing complexes have interesting structures, although they tend
not to be very reactive, probably due to the fact that the strong
M–S bonds at the thiolate ends prevent stoichiometric or cata-
lytic reactions. In a search for more reactive complexes with
pincer ligands based on the same P,S,S′ framework, we selected
PPh(o-C6H4SMe)2 as a possible ligand. Although coordination of
this ligand through the P atom is easy and gives strong P–M
bonds, the thioether SMe moieties are likely to bond to the
same metal much more weakly. This behaviour is known as
hemilability, and it allows the ligand, which can bond to a metal
centre in a tricoordinate pincer mode, to easily change to a
bicoordinate mode in the presence of another ligand
(Scheme 1).
The chemistry of polydentate ligands with sulfur and phos-
phorus donor atoms in their frameworks has attracted increas-
ing attention, as, apart from the inherent interest in complexes
with M–S bonds,[1] the presence of two different coordination
centres enhances their reactivity, and gives them interesting
properties.[2–4] Sulfur-ligated transition-metal complexes form
the inorganic biologically active centres of some metalloprote-
ins and enzymes, and these metalloproteins and enzymes have
found applications in pharmacology.[5–7] Sulfur-ligated transi-
tion-metal complexes have also been used as lubricants[2] and
as catalysts in different processes,[3] and many have interesting
structures and reactivities.[4]
The original tricoordinate mode should be quickly recovered
when the new ligand is removed. This change from tri- to di-
Some of us have described nickel and palladium complexes
of the P,S,S′ pincer donor ligand[8] PPh(o-C6H4S)22–. The result-
Scheme 1. Hemilability of the ligand.
[a] Departamento de Química Inorgánica, Instituto de Síntesis Química y
Catálisis Homogénea, Universidad de Zaragoza – CSIC,
Plaza S. Franciso s/n, 50009 Zaragoza, Spain
[b] Departamento de Química Aplicada, Universidad Pública de Navarra,
Campus de Arrosadía s/n, 31006 Pamplona, Spain
E-mail: mlaguna@unizar.es
coordination, or from di- to monocoordination, could be taken
advantage of in insertion reactions, or to test the efficiency of
such complexes as catalysts in different C–C coupling reactions.
Jeffrey and Rauchfuss[9] introduced the term “hemilabile li-
gand” in 1979 to refer this type of polydentate ligand, and since
then, numerous reviews of different scaffolds and reactivity
pathways have been published.[10] Although the term can be
a.luquin@unavarra.es
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejoc.201501354.
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used for both di- and tricoordinate ligands, the latter tend to 331 and 296 cm–1). One signal in the 31P{1H}NMR spectrum at
be more likely to show this property. Pincer ligands, in which δ = 50.0 ppm splits into two, with similar intensities, at δ = 49.9
the three coordinating moieties bond to a metal centre in a T- and 49.5 ppm, when the temperature is reduced to –55 °C. In
shaped form, frequently show this type of behaviour, and have the 1H NMR spectrum, the S-methyl groups appear at δ =
been studied in many catalytic procedures.[11]
2.80 ppm, although decreasing the temperature to –55 °C pro-
duces four signals: two, with an intensity of around 35 % each,
at δ = 3.24 and 2.62 ppm, and the other two, with an intensity
of around 15 % each, at δ = 2.81 and 2.54 ppm. Similar behav-
iour has been described by Abel and coworkers,[13] who pro-
posed two different explanations: pyramidal inversion of the
coordinated S atom, and internal S-methyl replacement chang-
ing between coordinated and uncoordinated S-methyl groups.
In this paper, we describe some PdII complexes with PPh(o-
C6H4SMe)2. The ligand acts as a pincer ligand in an S,S′,P- or
S,P-coordination mode, as confirmed by the X-ray structures of
[PdCl2{PPh(o-C6H4SMe)2}] (1) and [Pd(Me)Cl{PPh(o-C6H4SMe)2}]
(2). Complexes with Pd–methyl bonds are able to insert CO to
give acyl derivatives, as confirmed by the X-ray structure of
[Pd(COMe)Cl{PPh(o-C6H4SMe)2}] (6). Some of these complexes
are good Mizoroki–Heck-type catalysts, with [Pd(Me)Cl{PPh(o-
C6H4SMe)2}] (2), which could be recovered and reused several
times, being the most active.
Complex 2 shows a similar behaviour. The IR spectrum shows
a single absorption at around ν = 302 cm–1 for the Pd–Cl bond.
˜
A signal in the 31P{1H} NMR spectrum at δ = 42.8 ppm splits
into two [a main signal, with an intensity of around 90 %, at
δ = 42.3 ppm and another, less intense (10 %) signal at δ =
45.0 ppm] in the low-temperature spectrum. The 1H NMR spec-
trum shows a signal for the S-methyl groups at δ = 2.56 ppm.
When the temperature is lowered, this signal splits into four,
with two signals at δ = 2.81 and 2.40 ppm, which have an
intensity of around 42 % each, and two signals at δ = 3.25 and
2.52 ppm, with an intensity of around 8 % each. As the spectro-
scopic behaviour of complex 2 is similar to that of complex 1,
the behaviour of compound 2 is likely to have the same expla-
nation as that given by Abel et al. for complex 1. X-ray crystal
structural analysis of complexes 1 and 2 (see next section and
Results and Discussion
Synthesis and Characterization
All complexes synthesised in this work were derived from the
tridentate ligand bis(o-methylthiophenyl)phenylphosphane,
which was first prepared by Workman et al. and Livingstone et
al.[12] Treatment of this phosphane with the metal salts
Na2[PdCl4] and [Pd(Me)Cl(COD)] (COD = cyclooctadienyl) gave
the complexes [PdCl2{PPh(o-C6H4SMe)2}] (1) (which was previ-
ously prepared by Livingstone et al.[12b] and by Abel et al.[13]
)
and [Pd(Me)Cl{PPh(o-C6H4SMe)2}] (2), respectively, in which the Figures 1 and 2) confirmed the structure of 1 proposed by Abel
phosphane acts as a bidentate ligand. The IR spectrum of com- and coworkers, and the structure of 2 proposed by us
plex 1 shows absorptions corresponding to Pd–Cl[13] bonds (ν = (Scheme 2).
˜
Scheme 2. i) Na2[PdCl4]; ii) PdClMe(COD); iii) AgCF3SO3; iv) AgCF3SO3 or TlPF6; v) and vi) bubbling CO; vii) AgOCOCH3.
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Complexes 1 and 2 have at least one bond to a chloride retains a cis configuration, and this was confirmed by X-ray
atom, which should be easy to remove to obtain tricoordination structural analysis (see below).
of the phosphane. Thus, treatment of compound 1 with
AgCF3SO3 led to [PdCl{PPh(o-C6H4SMe)2}]CF3SO3 (3) as a yellow
solid. The H NMR spectrum of complex 3 includes a signal in
the region of the S-methyl groups that does not change as the
temperature is decreased, and the 31P{1H}NMR spectrum has a
signal at δ = 80.0 ppm that also remains unchanged as the
temperature is decreased. The IR spectrum shows signals for
the triflate ion[15] and the Pd–Cl moiety. These findings suggest
that the structure of complex 3 is as shown in Scheme 2.
To test whether it was possible to insert CO2, reactions were
carried out by bubbling CO2 gas or by adding solid CO2 to a
solution of 4 in dichloromethane. In both cases, it was possible
to observe a new product that could be the insertion com-
1
1
pound by H NMR spectroscopy, although the yield never ex-
ceeded 5 %. In an attempt to improve the yield of the insertion
reaction, an experiment was carried out in supercritical CO2 for
24 h. The 31P{1H} NMR spectrum of the crude product of the
reaction with compound 4 showed a new signal at δ =
53.8 ppm with an intensity of 12 % relative to the starting mate-
The reaction to remove the chlorine atom from complex 2
to force the phosphane to act as a pincer ligand to the metal
centre was carried out with two different salts, namely
AgCF3SO3 and TlPF6. This gave the yellow solids [Pd(Me){PPh(o-
C6H4SMe)2}]CF3SO3 (4) and [Pd(Me){PPh(o-C6H4SMe)2}]PF6 (5),
respectively. The signals for the S-methyl groups in the 1H NMR
spectra appear as singlets [δ = 2.56 (4) and 2.53 (5) ppm], which
do not change when the temperature is decreased. The
31P{1H}NMR signals for complexes 4 and 5 appear at δ = 43.2
and 43.1 ppm, respectively. The IR spectra show signals for the
triflate[15] ion in compound 4, and the hexafluorophosphate[16]
ion in 5. These data, as well as those obtained for complex 3,
suggest that complexes 4 and 5 are salts in which the phos-
phane acts as a pincer ligand, with all three coordination posi-
tions joined to the metal.
1
rial. The H NMR spectrum shows a doublet at δ = 2.92 ppm
that could be due to the methyl group bonded to the oxygen
of the COO,[18c] and another signal at δ = 2.48 ppm probably
due to the S-methyl group. Increasing the reaction time had
no effect on the proportions of product and starting material.
Attempts to separate the new complex from the reaction mix-
ture by recrystallization from different solvents, or to separate
on an alumina column were unsuccessful, and only starting
material 4 was obtained in both cases.
In order to determine which of the two possible products
was obtained in this reaction of 4 with supercritical CO2, the
reaction of complex 3 with silver acetate was carried out. The
new complex [Pd(OOCMe){PPh(o-C6H4SMe)2}]CF3SO3 (9) shows
a signal in the 31P{1H}NMR spectra at δ = 77.1 ppm, thus show-
ing that this compound is not the reaction product formed in
supercritical CO2. Complexes 2 and 5 did not react with CO2 in
our hands, even under supercritical conditions.
It is well known that small molecules such as CO and CO2,[17]
amongst others, can insert into metal–methyl bonds. We report
here the insertion of CO, and, partially, of CO2, into the Pd–Me
bond in complexes 2, 4, and 5. When the insertion reaction was
carried out by bubbling CO through a dichloromethane solu-
tion of those complexes for 2 h, products [Pd(COMe)Cl{PPh(o-
C6H4SMe)2}] (6), [Pd(COMe){PPh(o-C6H4SMe)2}]CF3SO3 (7), and
[Pd(COMe){PPh(o-C6H4SMe)2}]PF6 (8) were obtained as the pure
insertion products. The IR spectra for these complexes in di-
Crystal Structures of [PdCl2{PPh(o-C6H4SMe)2}] (1)
[Pd(Me)Cl{PPh(o-C6H4SMe)2}] (2), and [Pd(COMe)Cl-
{PPh(o-C6H4SMe)2}] (6)
The structures of complexes 1, 2, and 6 were determined by
single-crystal X-ray diffraction analysis, and their crystal struc-
tures are shown in Figures 1, 2, and 3, respectively. All three
complexes show similar structures, with the bis(o-methylthio-
phenyl)phenylphosphane acting as a bidentate P,S-ligand, with
a slightly distorted square-planar arrangement around the pal-
ladium centre, in all cases.
The metal atoms lie outside the plane defined by the four
atoms that conform the square-planar arrangement in each
case by 0.0232(6), 0.0133(7) and 0.00133(13) Å, respectively. The
bond angles P(1)–Pd(1)–S(1) [87.21(4)° in 1, 86.96(3)° in 2, and
87.01(5)° in 6], S(1)–Pd(1)–C(l) [91.87(4)° in 1, 92.24(3)° in 2, and
92.54(5)° in 6], and P(1)–Pd(1)–C(l) [179.05(4)° in 1, 178.23(3)° in
2, and 178.77(4)° in 6] deviate slightly from 90 and 180°. In
contrast, the bond angles S(1)–Pd–X [X = Cl(2) in 1, C(1) in 2,
and C(3) in 6] in 1 and 2 [174.10(4) and 175.54(7)°, respectively]
deviate much more markedly. The P–Pd–S angles [87.21(4)° in 1,
86.96(3)° in 2, and 87.01(5)° in 6] are typical for neutral (P,S)PdII
complexes,[19] and smaller than those found in cationic (P,S)PdII
derivatives (94.6–96.9°).[20]
chloromethane solution show an absorption at ν = 1721 cm–1
˜
for compound 7, and at ν = 1693 cm–1 for complexes 6 and 8.
˜
The 1H NMR spectra show a downfield shift of the S-methyl and
Pd-CO-methyl signals. However, while the shift of the S-methyl
signals (around δ = 2.60 ppm) is the same in all three com-
plexes, the Pd-CO-methyl signals show different behaviour, with
the shift in complex 6 (δ = 2.22 ppm) being lower than those
in complexes 7 (δ = 2.91 ppm) and 8 (δ = 2.84 ppm). This
different behaviour could be due to the different position of
the acyl group, which is trans to the P atom in complexes 7
and 8, but cis in complex 6. Similar examples of this dissimilar
behaviour can be found in the literature.[18] The 31P{1H} NMR
spectrum of complex 6 shows a singlet at higher field (δ =
22.4 ppm) than that of the starting material (2, δ = 42.8 ppm),
whereas for complexes 7 (δ = 54.4 ppm) and 8 (δ = 54.3 ppm),
the singlets appear at lower field (4, δ = 43.2 ppm, and 5, δ =
43.1 ppm). Although some examples of this behaviour can also
be found in the literature, a plausible explanation is still lacking.
Although complex 6 could have the CO-methyl group in a
position trans or cis to the P atom, the reaction gives only one
The Pd–S distances [Pd–S(6) 3.193 Å in 1, Pd–S(2) 3.370 Å in
isomer. Bearing in mind the fact that the starting material has 2, and Pd–S(2) 3.460 Å in 6] to the free thioether group are
a cis configuration, it is reasonable to assume that complex 6 too long to be considered bonds. Consistent with the weaker
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Figure 3. Crystal structure of [Pd(PhP(C6H4SMe)2)Cl(COMe)] (6). Displacement
parameter ellipsoids represent 50 % probability. Selected bond lengths [Å]
and angles [°]: Pd(1)–C(3) 2.024(4), Pd(1)–P(1) 2.2299(13), Pd(1)–Cl(1)
2.3616(14), Pd(1)–S(1) 2.4276(12), S(1)–C(5) 1.778(4), S(1)–C(1) 1.804(5), S(2)–
C(2) 1.761(6), C(3)–Pd(1)–P(1) 91.62(13), C(3)–Pd(1)–Cl(1) 88.83(13), P(1)–
Pd(1)–Cl(1) 178.77(4), C(3)–Pd(1)–S(1) 178.62(12), P(1)–Pd(1)–S(1) 87.01(5),
Cl(1)–Pd(1)–S(1) 92.54(5). H atoms are omitted for clarity.
Figure 1. Crystal structure of [Pd(PhP(C6H4SMe)2)Cl2] (1). Displacement param-
eter ellipsoids represent 50 % probability. Selected bond lengths [Å] and an-
gles [°]: Pd(1)–P(1) 2.2031(11), Pd(1)–S(1) 2.2722(10), Pd(1)–Cl(2) 2.3053(10),
Pd(1)–Cl(1) 2.3520(11), S(1)–C(19) 1.799(4), S(6)–C(20) 1.810(4), P(1)–Pd(1)–S(1)
87.21(4), P(1)–Pd(1)–Cl(2) 87.28(4), S(1)–Pd(1)–Cl(2) 174.10(4), P(1)–Pd(1)–Cl(1)
179.05(4), S(1)–Pd(1)–Cl(1) 91.87(4), Cl(2)–Pd(1)–Cl(1) 93.64(4). H atoms are
omitted for clarity.
lie within the range of Pd–P distances that have been reported
for related complexes.[14,21] The Pd–S distances differ markedly
between the complexes, being shorter in complex 1 [Pd–S(1)
2.2722(10) Å in 1, 2.3684(8) Å in 2, and 2.4276(12) Å in 6] due
to the weak trans influence of the chlorine atom, and the
stronger trans influence of the CO-methyl group. The distances
for complexes 2 and 6 are even longer than the distances re-
ported for similar complexes.[14,22–27] The Pd–C distance of
2.024(4) Å in 6 is shorter than that observed in 2 [2.155(2) Å],
although both are longer than the distances found in related
acylpalladium derivatives with P-ligands or thioether-contain-
ing ligands.[28]
Despite the potential tridenticity of the ligand, it behaves as
a monochelating P,S ligand in all three complexes, as observed
in other square-planar complexes of the type [MX2{PPh(o-
C6H4SMe)2}] (M = Pd, X = Cl, Br, I; M = Pt, X = Cl).[14] Pyramidal
inversion of the coordinated sulfur atoms in the latter com-
plexes is accompanied by an exchange of the pendant and
bound methylthio groups (as seen by NMR spectroscopy).
Figure 2. Crystal structure of [Pd(PhP(C6H4SMe)2)Cl(Me)] (2). Displacement pa-
rameter ellipsoids represent 50 % probability. Selected bond lengths [Å] and
angles [°]: Pd(1)–C(1) 2.155(2), Pd(1)–P(1) 2.1927(8), Pd(1)–S(1) 2.3684(8),
Pd(1)–Cl(1) 2.3695(8), S(1)–C(2) 1.806(3), C(1)–Pd(1)–P(1) 90.04(7), C(1)–Pd(1)–
S(1) 175.54(7), P(1)–Pd(1)–S(1) 86.96(3), C(1)–Pd(1)–Cl(1) 90.84(7), P(1)–Pd(1)–
Cl(1) 178.23(3), S(1)–Pd(1)–Cl(1) 92.24(3). H atoms are omitted for clarity.
Catalytic Activity
PdII pincer complexes are usually good catalysts for C–C cou-
pling reactions.[29] Indeed, complexes with P and S donor atoms
are especially good catalysts, because it is possible to decoordi-
structural trans influence of S-donor ligands, the Pd–Cl bond nate the S atom to form unsaturated species that may be active
lengths trans to the phosphorus atom [2.3520(11) Å in 1, in homogeneous catalysis. We chose Mizoroko–Heck-type reac-
2.3695(8) Å in 2, and 2.3616(14) Å in 6] are longer than that tions[30] to test some of these complexes as catalysts, due to
observed in complex 1 trans to the thioether group [Pd–Cl(2) the mild reaction conditions and the low toxicity and cost of
2.3053(10) Å]. Similar differences have been observed in the reagents used. Moreover, this reaction is particularly flexi-
[PdI2{Ph2P(C6H4SMe)}].[21] The palladium–phosphorus distances ble, as it allows a wide variety of solvents (reactions can be even
of 2.2031(11) Å (in 1), 2.1927(8) Å (in 2), and 2.2299(13) Å (in 6) done without solvent), bases, and conditions, and the Mizoroki–
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Heck coupling products can be used as intermediates in the
synthesis of materials, natural products, and biologically active
compounds.[31] We selected two reactions for this study, using
iodobenzene together with methyl acrylate or styrene to give
methyl cinnamate and stilbene, respectively, as the only reac-
tion products (Figures 4 and 5).
Table 1. Initial screening of the catalytic reaction between iodobenzene and
styrene.
Entry Catalyst
[mol-%]
Solvent/base
Time Conv. Products
[h]
[%]
1
2
3
4
5
6
7
8
1 (1)
2 (1)
3 (1)
4 (1)
DMA/NaOAc
DMA/NaOAc
DMA/NaOAc
DMA/NaOAc
DMA/NaOAc
DMF/NaOAc
DMF/K2CO3
DMF/CsF
DMF/NBu3
DMF/NBu3
DMF/NBu3
DMF/NBu3
DMF/NBu3
18
18
18
18
18
1
1
1
1
1
50
90
59
49
51
5
36
61
100
29
33
19
31
35 % A; 15 % B
51 % A; 21 % B; 18 %C
40 % A; 19 % B
A
5 (1)
37 % A; 14 % B
2 (10–2
2 (10–2
2 (10–2
2 (10–2
1 (10–2
3 (10–2
4 (10–2
5 (10–2
)
)
)
)
)
)
)
)
A
A
A
A
A
A
A
A
9
Figure 4. Catalytic reaction between iodobenzene and methyl acrylate.
10
11
12
13
1
1
1
and a TOF of 4.2 × 103 runs h–1. Although these values are not
much higher that those reported by Yamada et al.,[32a] they are
within the range of other results reported with ppm-level cata-
lyst loadings; the loading of Pd catalyst 2 is 10 ppm[32,30g] in
this reaction.
Figure 5. Catalytic reaction between iodobenzene and styrene.
The complexes chosen for the initial screening with methyl
acrylate were complexes 1 and 2, in which the phosphane acts
as a bidentate ligand, and complexes 3, 4, and 5, in which the
We tested the reaction between iodobenzene and styrene
phosphane acts as a pincer ligand. The catalytic conditions (Figure 5) in order to attempt a less activated reaction. Initially
were catalyst (1 mol-%), DMA (N,N-dimethylacetamide) as sol- we tested the same catalyst [Pd(Me)Cl{PPh(o-C6H4SMe)2}] (2)
vent, and NaOAc as base, at 118 °C for 18 h (Table 1, entries 1– under the best conditions found for the first reaction, after
5). Although mixtures of products were obtained in all cases showing that this combination does not react in the absence
(Figure 6), [Pd(Me)Cl{PPh(o-C6H4SMe)2}] (2) (Table 1, entry 2) of catalyst (see Supporting Information, Table S2, entries 1 and
was found to be the most active (90 % conversion), and there- 2). Two solvents, DMA (118 °C) and DMF (153 °C), were tested,
fore this complex was selected for subsequent studies. The re- and it is noteworthy that the Z isomer was the only isomer
action was run with 10–2 mol-% of 2 as catalyst in DMF at 153 °C present in the reaction mixture. Both NaOAc and K2CO3 were
and a reaction time of 1 h, with different bases, including tested as bases, and the best results were obtained with the
NaOAc, K2CO3, CsF, and NBu3. Only the trans isomer was formed latter, in DMA as solvent, with 10–2 mol-% of the catalyst, and
in all cases, with conversions of 5, 36, 61, and 100 %, respec- with a reaction time of 24 h (81 % conversion). This represents
tively (Table 1, entries 6–9). Complexes 1, 3, 4, and 5 were a TON of 8100, and a TOF of 338 runs h–1. In this case, the yields
tested under the same conditions, and gave conversions of 29, did not vary with the water content of the solvent, and similar
33, 19, and 31 %, respectively (Table 1, entries 10–13). It is results were obtained whether working under air or under an
worth noting that only the trans isomer was obtained in all argon atmosphere.
cases, and [Pd(Me)Cl{PPh(o-C6H4SMe)2}] (2) gave a turnover
number (TON) of 104, and a turnover frequency (TOF) of 104
runs h–1 (Table 1, entry 9).
Once the reaction conditions had been optimized, and we
had found that the reaction works equally well in air, and better
in the presence of a small amount of water, we decided to
Although the reactions shown in Table 1 were run under an examine catalyst recovery. Catalyst recyclability is one of the
argon atmosphere, similar results were obtained in the pres- most interesting challenges in Mizoroki–Heck reactions.[29e,30]
ence of air. As inconsistent results were obtained depending on To this end, once the reaction was finished, the reaction mixture
the water content of the DMF used, we decided to carry out was passed through a chromatography column containing silica
experiments in which the water concentration was changed gel. We found that when diethyl ether was used as eluent,
(see Supporting Information, Table S1, entries 1–4). The best [Pd(Me)Cl{PPh(o-C6H4SMe)2}] (2) was retained as a yellow band
results were obtained with a water concentration of 0.015 %. A at the very beginning of the column. Switching to acetone then
catalyst loading of 10–3 mol-% and DMF with a water content allowed the catalyst to be eluted and recovered unchanged (1H
of 0.015 % gave conversions of 16, 65, and 100 % after 6, 18, and 31P NMR spectroscopy). Thus, after one reaction run, the
and 24 h, respectively. This represents a TON higher than 105, solution was passed through a silica gel column, eluting with
Figure 6. Catalytic reaction between iodobenzene and styrene. Mixture of products obtained in the initial screening.
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diethyl ether, to trap the catalyst on the column. The solvent
containing the organic products was collected, and the conver-
sion was determined by 1H NMR spectroscopy. The catalyst was
then removed from the column with acetone, the solution was
evaporated to dryness, and, after the addition of DMA, iodo-
benzene, and styrene, the reaction was run again. The results
collected in Table 2 show that the recovered catalyst remained
active, and, indeed, increased in activity for each run, thus
showing that complex [Pd(Me)Cl{PPh(o-C6H4SMe)2}] (2) is in fact
a precatalyst. This behaviour allowed us to rule out the possibil-
ity that nanoparticles were the actual catalyst, as they should
be retained on the column. The repeatability of these cycles
results in an accumulative catalytic activity as high as TON =
66400, and, as more cycles could be run, this is not the limit.
Nevertheless this recovery strategy is not general, and should
be tested for each reaction, because, for example, it does not
work for the reaction between iodobenzene and methyl acryl-
ate.
Figure 7. Reaction of styrene and iodobenzene to obtain stilbene as a func-
tion of time. Catalyst loading of 10–2 mol-% in red, and 3 × 10–3 mol-% in
black.
Table 2. Experiments with catalyst recovery for the reaction of iodobenzene
with styrene.[a]
Entry
Catalyst
Time [h]
Conv. [%]
TON
Accumulated TON
Table 3. Experiments with catalyst recovery for the reaction of 4-bromoaceto-
phenone with styrene.[a]
1
2
3
4
5
6
7
24
24
24
24
24
8
83
85
100
100
100
96
8300
8500
10000
10000
10000
9600
8300
16800
26800
36800
46800
56400
66400
R1
R2
R3
R4
R5
R6
Entry
Catalyst
Time [h]
Conv. [%]
TON
Accumulated TON
1
2
3
4
24
24
24
24
49
27
15
13
4900
2700
1500
1300
4900
7600
9100
10400
R1
R2
R3
5
100
10000
[a] Conditions: catalyst 2 (10–2 mol-%), K2CO3, DMA. RN = recovered from
experiment number N.
[a] Conditions: catalyst 2 (10–2 mol-%), K2CO3, DMA, reflux. RN = recovered
from experiment number N.
To find out why our catalyst became better after each recov-
ery, we prepared two reactions using the conditions of Table 2,
entry 1, with 10–2 mol-% of the catalyst in both experiments.
To one of these reaction mixtures we added 25 mg of the same
silica gel used in the column. After 24 h, the conversions were
83 and 84 %, nearly the same as that obtained in Table 2, en-
try 1. These results ruled out the possibility that the presence
of or interaction with SiO2 could change the characteristics of
our catalyst. The increased speed should then be due to the
transformation of the precatalyst (i.e., 2) in the real catalyst. This
point was evidenced by checking the transformation of styrene
and iodobenzene in stilbene as a function of time at a catalyst
loading of 10–2 mol-% of 2 (Figure 7, in red; see also Supporting
Information, Table S3). It can be seen that the catalyst becomes
more effective with time, and that conversion reaches 93 % at
24 h without reaching a plateau. When a catalyst loading of
3 × 10–3 mol-% was used, the reaction profile was similar, and
100 % conversion was reached after 42 h (Supporting Informa-
tion, Table S4, and Figure 7 in black).
This catalyst 2 was active even when an aryl bromide was
used instead of an aryl iodide, although in this case we used
the activated substrate 4-bromoacetophenone. After 24 h un-
der similar conditions (Table 3, entry 1), the conversion was
49 %, which means a TON of 4900 and a TOF of 204 runs h–1.
When the catalyst was recycled in the way described above
(Table 3, entries 2–4), the catalyst lost activity, probably because
this time it was retained on the column.
To find out whether the retention or loss of activity was due
to the presence of bromide instead iodide, we repeated the
reaction with 4-iodoacetophenone (Table 4, entry 1). In just 2 h,
this reaction gave a conversion of 79 %, corresponding to a
TON of 7900, and a TOF of 3950 runs h–1. Here, successive recy-
cling of the catalyst gave even better results, as was seen with
the other catalytic reactions involving iodobenzene. After
five recycling cycles, the accumulative TON reached 52500,
which is not the limit, and the TOF in Table 4, entry 6 was
18600 runs h–1. This behaviour of the iodide derivatives should
show that the precatalyst in these reactions is different from
that in the reactions of the bromide derivatives.
Table 4. Experiments with catalyst recovery for the reaction of 4-iodoaceto-
phenone with styrene.[a]
Entry
Catalyst
Time [h] Conv. [%]
TON
Accumulated TON
1
2
3
4
5
6
2
2
1
1
0.5
0.5
79
95
81
100
77
93
7900
9500
8100
10000
7700
9300
7900
17400
25500
35500
43200
52500
R1
R2
R3
R4
R5
[a] Conditions: catalyst 2 (10–2 mol-%), K2CO3, DMA, reflux. RN = recovered
from experiment number N.
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(Nujol): ν = 331 (Pd–Cl), 296 (Pd–Cl) cm–1. 1H NMR (CDCl3): δ = 8.33–
Conclusions
˜
6.70 [m, 13 H, (C6H5)P(C6H4SMe)2], 2.80 (s, 6 H, SMe) ppm. 31P NMR:
δ = 50.0 ppm. MS (LSIMS+): m/z (%) = 495 (37) [M – Cl]+, 459 (40)
Organometallic palladium(II) complexes with a pincer ligand
have been synthesised and characterized. They react with CO
to give COMe insertion complexes. Three complexes were char-
acterized by X-ray crystallographic analysis. Some of the com-
plexes were tested as catalysts for Mizoroki–Heck C–C coupling
reactions. The best precatalyst was found to be
[Pd(Me)Cl{PPh(o-C6H4SMe)2}] (2), which catalyses the addition
reactions of iodobenzene to methyl acrylate or styrene in the
presence of water and air. In the former reaction we observed
a TON as high as 105. In the case of the iodobenzene addition
to styrene, we recovered catalyst 2 by passing the reaction mix-
ture through a silica gel column to separate the reaction pro-
ducts from the catalyst. Catalyst 2 was recovered and reused
without losing activity. In our hands, a catalytic activity up to
TON = 66400 was found for the formation of stilbene by the
reaction of iodobenzene and styrene at a Pd catalyst loading of
100 ppm. Although complex 2 is a good precatalyst for the
reaction of 4-bromoacetophenone and styrene, in this case, the
recovery of the catalyst led to a lower conversion. The use of a
short chromatography column (SiO2, Al2O3, or others) to re-
cover the catalyst could have more applications, but it is not a
general procedure, and should be tested in each case. It could
be of interest for many homogeneous catalytic reactions.
Even when the catalyst retained on the column could not be
reused again, this method still represents a good way to sepa-
rate a metallic homogeneous catalyst from the final products,
recovering the precious metal involved in the catalysis, and
avoiding metal contamination of organic components. Work is
in progress to extend this approach to other processes.
[M – 2Cl]+, 445 (62) [M – Me – 2Cl]+. Λm = 1.9 S cm2 mol–1
.
[Pd(Me)Cl{PPh(o-C6H4SMe)2}] (2): PPh(o-SMeC6H4)2 (0.035 g,
0.1 mmol) was dissolved in dichloromethane (10 mL), and
[PdClMe(COD)] (0.026 g, 0.1 mmol) was added. The yellow solution
was stirred overnight, and then it was concentrated under reduced
pressure. n-Hexane (10 mL) was added to give 2 (0.037 g, 73 %) as
a pale yellow solid. C21H22ClPPdS2 (511.35): calcd. C 49.4, H 4.3, S
12.5; found C 49.0, H 3.9, S 12.4. IR (Nujol): ν = 302 (Pd–Cl) cm–1. 1H
˜
NMR (CDCl3, –55 °C): δ = 7.76–6.90 [m, 13 H, (C6H5)P(C6H4SMe)2],
2.81 (s, 3 H, SMe), 2.40 (s, 3 H, SMe), 0.92 (d, 3 H, PdMe) ppm. 31P
NMR: δ = 42.8 ppm. 13C NMR (CDCl3): δ = 141.8, 134.7, 134.5, 134.1,
134.1, 133.8, 133.5, 133.4, 132.2, 131.5, 131.5, 131.2, 129.5, 129.3,
129.2, 129.1, 128.7, 127.2, 21.6, 4.9 ppm. MS (LSIMS+): m/z (%) = 987
(7) [2M – 2Cl + 2Me + S]+, 497 (100) [M – Cl – Me + S]+, 475 (83)
[M – Cl]+, 445 (80) [M – 2Me – Cl]+. Λm = 14.6 S cm2 mol–1
.
[PdCl{PPh(o-C6H4SMe)2}]CF3SO3 (3): Compound (0.053 g,
1
0.1 mmol) was dissolved in dichloromethane (10 mL), and
AgCF3SO3 (0.025 g, 0.1 mmol) was added. The yellow solution was
stirred for 8 h, and then the AgCl was removed by filtration. The
solution was concentrated to ca. 5 mL. Diethyl ether (10 mL) was
added to give 3 (0.039 g, 60 %) as a yellow solid. C21H19ClF3O3PPdS3
(645.38): calcd. C 39.1, H 2.9, S 14.9; found C 38.6, H 3.2, S 14.4. IR
(Nujol): ν = 1282 (CF3SO3), 1224 (CF3SO3), 1148 (CF3SO3), 636
˜
(CF3SO3), 315 (Pd–Cl) cm–1. 1H NMR (CDCl3): δ = 7.98–6.70 [m, 13 H,
(C6H5)P(C6H4SMe)2], 2.20 (s, 6 H, SMe) ppm. 31P NMR: δ = 80.0 ppm.
MS (LSIMS+): m/z (%) = 497 (86) [M – CF3SO3]+, 475 (30) [M + Me –
CF3SO3]+, 461 (51) [M – Cl – CF3SO3]+, 445 (79) [M – Me – Cl –
CF3SO3]+, 505 (17) [M + Me3 – Cl – CF3SO3]+. Λm = 81.8 S cm2 mol–1.
[Pd(Me){PPh(o-C6H4SMe)2}]CF3SO3 (4): Compound 2 (0.051 g,
0.1 mmol) was dissolved in dichloromethane (10 mL), and
AgCF3SO3 (0.025 g, 0.1 mmol) was added. The yellow solution was
stirred for 8 h, and then the AgCl was removed by filtration. The
solution was concentrated to ca. 5 mL. Diethyl ether (10 mL) was
added to give 4 (0.040 g, 64 %) as a yellow solid. C22H22F3O3PPdS3
(624.96):calcd.C42.3,H3.5,S15.3;foundC42.0,H3.2,S14.9.IR(Nujol):
Experimental Section
General Methods: C, H, N, and S analyses were measured with a
Perkin–Elmer 2400 microanalyser. Infrared spectra were recorded
with a Perkin–Elmer 883 spectrophotometer over the range ν =
ν = 1279 (CF3SO3), 1224 (CF3SO3), 1114 (CF3SO3), 637 (CF3SO3) cm–1.
˜
˜
4000–200 cm–1, using Nujol mulls between polyethylene sheets.
NMR spectra were recorded with Varian Unity 300 and Bruker ARX
300 spectrometers, in CDCl3 or CD2Cl2 at room temperature unless
otherwise stated (300 MHz for 1H, 121.4 MHz for 31P, and 75 MHz
for 13C). Chemical shifts are cited relative to SiMe4 (1H, 13C) or H3PO4
(85 % aq.; external, 31P). Mass spectra were recorded with a VG Au-
tospec instrument, by liquid secondary-ion mass spectrometry
(LSIMS+), using nitrobenzyl alcohol as matrix, and a cesium gun.
Supercritical CO2 experiments were run as described elsewhere.[33]
1H NMR (CDCl3): δ = 7.70–6.89 [m, 13 H, (C6H5)P(C6H4SMe)2], 2.56
(s, 6 H, SMe), 0.92 (s, 3 H, PdMe) ppm. 31P NMR: δ = 43.2 ppm.
13C NMR (CDCl3): δ = 141.1, 137.2–127.3, 121.6, 30.0, 27.8 ppm. MS
(LSIMS+): m/z (%) = 491 (29) [M + Me – CF3SO3]+, 461 (43) [M – Me
– CF3SO3]+, 445 (85) [M – 2Me – CF3SO3]+, 355 (100) [M – Pd – Me
– CF3SO3]+. Λm = 89.6 S cm2 mol–1
.
[Pd(Me){PPh(o-C6H4SMe)2}]PF6 (5): Compound
2
(0.051 g,
0.1 mmol) was dissolved in dichloromethane (10 mL), and TlPF6
(0.035 g, 0.1 mmol) was added. The yellow solution was stirred for
8 h, and then the TlCl was removed by filtration. The solution was
concentrated to ca. 5 mL. Diethyl ether (10 mL) was added to give
5 (0.042 g, 68 %) as a yellow solid. C21H22F6P2PdS2 (620.86): calcd.
Molar conductivities were measured in acetone in 5 × 10–4
tions.
M solu-
Materials:
The
starting
materials
PhP(o-C6H4SMe)2,[12]
were prepared by
[Pd(Me)Cl(COD)],[34] Na2[PdCl4], and AgCF3SO3
[35]
C 40.6, H 3.5, S 10.3; found C 40.7, H 3.4, S 9.9. IR (Nujol): ν = 837
˜
published procedures. All other reagents were commercially avail-
able, and were used as supplied.
(PF6), 759 (PF6), 558 (PF6), 490 (PF6) cm–1. 1H NMR (CDCl3): δ = 7.70–
6.90 [m, 13 H, (C6H5)P(C6H4SMe)2], 2.53 (s, 6 H, SMe), 0.93 (d, 3 H,
PdMe) ppm. 31P NMR: δ = 43.1 ppm. MS (LSIMS+): m/z (%) = 475 (51)
[M – PF6]+, 461 (41) [M – Me – PF6]+, 445 (100) [M – 2Me – PF6]+,
505 (12) [M + 2Me – PF6]+, 583 (9) [M + Pd – PF6]+. Λm = 59.5
Preparation of Compounds
[PdCl2{PPh(o-C6H4SMe)2}] (1): PPh(o-C6H4SMe)2 (0.035 g,
0.1 mmol) was dissolved in dichloromethane (10 mL), and
Na2[PdCl4] (0.029 g, 0.1 mmol) was added. The yellow solution was
heated at reflux overnight. The resulting yellow solid precipitate
was collected by filtration to give 1 (0.049 g, 92 %). C20H19Cl2PPdS2
(531.77): calcd. C 45.2, H 3.6, S 12.0; found C 45.7, H 3.7, S 11.9. IR
S cm2 mol–1
.
General Procedure for the Synthesis of 6–8: Carbon monoxide
was bubbled through a dichloromethane solution (10 mL) of 2
(0.051 g, 0.1 mmol), 4 (0.062 g, 0.1 mmol), or 5 (0.062 g, 0.1 mmol)
Eur. J. Org. Chem. 2016, 789–798
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for 1 h, and the colour of the solution changed from yellow to
green. The green solution was concentrated to ca. 5 mL. Diethyl
ether (10 mL) was added to give gave a green solid, which was
isolated under an argon atmosphere.
X-Ray Structure Determination of [PdCl2{PPh(o-C6H4SMe)2}] (1),
[Pd(Me)Cl{PPh(o-C6H4SMe)2}] (2), and [Pd(COMe)Cl{PPh(o-
C6H4SMe)2}] (6): A summary of the fundamental crystal and refine-
ment data of the compounds 1, 2, and 6 is given in Table 5. Crystals
of 1 and 2 were mounted on a glass fiber in a Bruker–Siemens
Smart CCD diffractometer, and on an Enraf Nonius Kappa CCD area
detector in the case of 6. Data were collected using graphite mono-
chromated Mo-Kα radiation (λ = 0.71073) with a nominal crystal-to-
detector distance of 4.0 cm. Structures were solved by direct meth-
ods with the aid of successive difference Fourier maps, and were
refined using the SHELX97[36] and SHELXTL 5.1 software packages.
Hydrogen atoms were assigned to ideal positions and refined using
a riding model. The diffraction frames were integrated using the
SAINT[37] package, and corrected for absorption with SADABS.[38]
Complex 2 contained a disordered acetone molecule for which the
carbon atom of the carbonyl group was disordered over four posi-
tions. The C–C and C–O distances were fixed at 1.54 and 1.2 Å
respectively. The carbon atom of the free thioether group shows
disorder, and was refined with a 60:40 site occupancy.
[Pd(COMe)Cl{PPh(o-C6H4SMe)2}] (6): Yield 0.046
g
(85 %).
C22H22ClOPPdS2 (539.36): calcd. C 49.0, H 4.1, S 11.9; found C 48.7,
1
H 3.7, S 11.5. IR (CH2Cl2): ν = 1693 (CO) cm–1. H NMR (CDCl3): δ =
˜
7.76–6.90 [m, 13 H, (C6H5)P(C6H4SMe)2], 2.62 (s, 6 H, SMe), 2.22 (d,
J = 0.9 Hz, 3 H, Pd(CO)Me) ppm. 31P NMR: δ = 22.4 ppm.
[Pd(COMe){PPh(o-C6H4SMe)2}]CF3SO3 (7): Yield 0.059 g (90 %). IR
1
(CH Cl ): ν = 1721 (CO) cm–1. H NMR (CDCl3): δ = 7.76–6.90 [m, 13
˜
2
2
H, (C6H5)P(C6H4SMe)2], 2.62 (s, 6 H, SMe), 2.91 [s, 3 H, Pd(CO)Me]
ppm. 31P NMR: δ = 54.4 ppm. MS (LSIMS+): m/z (%) = 475 (53) [M –
CO – CF3SO3]+, 460 (70) [M – Me – CO – CF3SO3]+.
[Pd(COMe){PPh(o-C6H4SMe)2}]PF6 (8): Yield 0.053
C22H22F6OP2PdS2 (648.87): calcd. C 40.7, H 3.4, S 9.9; found C 40.2,
H 3.5, S 9.4. IR (CH2Cl2): ν = 1693 (CO) cm–1 1H NMR (CDCl3): δ =
g (82 %).
.
˜
7.76–6.90 [m, 13 H, (C6H5)P(C6H4SMe)2], 2.59 (s, 6 H, SMe), 2.84 [d,
J = 2.8 Hz, 3 H, Pd(CO)Me] ppm. 31P NMR: δ = 54.3 ppm. MS
(LSIMS+): m/z (%) = 475 (40) [M – CO – PF6]+, 461 (56) [M – Me –
CO – PF6]+, 445 (92) [M – CO – 2Me – PF6]+.
CCDC 1401688 (for 1), 1401689 (for 2), and 1401690 (for 6) contain
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre.
[Pd(OOCMe){PPh(o-C6H4SMe)2}]CF3SO3 (9): Compound
3
(0.064 g, 0.1 mmol) was dissolved in dichloromethane (10 mL), and
AgOOCMe (0.016 g, 0.1 mmol) was added. The yellow solution was
stirred for 8 h, and then the AgCl was removed by filtration. The
solution was concentrated to ca. 5 mL. Diethyl ether (10 mL) was
added to give 9 (0.049 g, 73 %) as an orange solid. C23H22F3O5PPdS3
(668.97): calcd. C 41.3, H 3.3, S 14.4; found C 41.1, H 3.2, S 14.5. IR
General Procedure for Catalytic Reactions: A DMA (or DMF) solu-
tion (3 mL) of iodobenzene (2 mmol), methyl acrylate (3 mmol) or
styrene (3 mmol), and the prescribed amount of the catalyst, was
introduced into a Schlenk tube under argon or in open air, depend-
ing on the experiment. Base (NaOAc, K2CO3, NBu3, or CsF; 2.2 mmol)
was added to the stirred reaction mixture. The tube was closed,
(Nujol): ν = 1690 (CO), 1282 (CF3SO3), 1224 (CF3SO3), 1148 (CF3SO3), and the temperature was raised to the reflux point of the solvent
˜
636 (CF3SO3) cm–1 1H NMR (CDCl3): δ = 8.47–7.27 [m, 13 H, (118 °C for DMA, 153 °C for DMF). After the prescribed reaction
.
(C6H5)P(C6H4SMe)2], 3.11 (s, 6 H, SMe), 2.08 (s, 3 H, OCOMe) ppm.
31P NMR: δ = 77.1 ppm. MS (LSIMS+): m/z (%) = 520 (78) [M –
CF3SO3]+, 505 (26) [M – Me – CF3SO3]+.
time, the mixture was cooled to room temperature, and brine
(15 mL) was added. The mixture was extracted with diethyl ether
(2 × 20 mL), and the combined organic layer was washed again
Table 5. Crystal data and data collection and refinement for complexes [PdCl2(PhP(C6H4SMe)2)] (1), [PdCl(Me)(PhP(C6H4SMe)2)] (2), and
[PdCl(COMe)PhP(C6H4SMe)2] (6).
1
2
6
Empirical formula
Molecular weight
C20H19Cl2PPdS2
531.74
C22.5H23.5ClO0.5PPdS2
538.85
C23H23Cl4OPPdS2
658.70
Colour, appearance
yellow, needle
monoclinic, P21/n
12.826(2)
11.1395(19)
14.459(3)
90
orange, block
monoclinic, C2/c
29.5299(18)
8.8389(5)
20.2073(12)
90
orange, tablet
triclinic, P
9.4247(19)
11.663(2)
13.287(3)
99.80(3)
Space group
a [Å]
b [Å]
c [Å]
α [°]
β [°]
97.886(3)
90
2046.2(6)
4
1.726
0.19 × 0.12 × 0.062
1.453
117.8570(10)
90
4663.1(5)
102.94(3)
100.74(3)
1363.8(5)
2
1.604
0.3 × 0.15 × 0.07
1.299
γ [°]
V [Å]3
Z
8
D (calcd.) (g cm–3
)
1.535
0.30 × 0.25 × 0.17
1.167
Crystal size [mm3]
μ [mm–1
]
Temperature [K]
100(2)
100(2)
150(2)
θ Range [°]
1.99 ≤ θ ≤ 23.29
9354
2948 [R(int) = 0.0298]
0.0272
0.0615
0.931
3.08 ≤ θ ≤ 23.27
10645
3347 [R(int) = 0.0140]
0.0266
0.0697
1.100
3.12 ≤ θ ≤ 27.51
15220
6091 [R(int) = 0.0718]
0.0532
0.1508
1.052
Number of data collected
Number of unique data
R1 [F2 > 2σ(F2)]
[a]
[b]
ω R2 (all data)
¯
S[c] (all data)
Residual ρ [e Å3]
0.598 and –0.392
0.855 and –0.452
2.420 and –1.59
[a] R1(F) = Σ ||Fo| – |Fc||/Σ |Fo|. [b] ωR2(F2) = Σ [ω(Fo – Fc2)2]/Σ [w(Fo2)2]1/2¸ w–1 = [σ2(Fo2) + (αP)2 + bP], where P = [max(Fo2,0) + 2Fc2]/3. [c] S = [Σ [ω(Fo – Fc2)2]/
2
2
¯
¯
¯
(n – p)]1/2, where n is the number of reflections, and p the number of refined parameters.
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erlands, 2007; c) M. Albrecht, G. van Koten, Angew. Chem. Int. Ed. 2001,
40, 3750; Angew. Chem. 2001, 113, 3866; d) M. E. van der Boom, D. Mil-
stein, Chem. Rev. 2003, 103, 798; e) B. Rybtchinski, D. Milstein, Angew.
Chem. Int. Ed. 1999, 38, 870; Angew. Chem. 1999, 111, 918; f) A. Vigalok,
D. Milstein, Acc. Chem. Res. 2001, 34, 798; g) D. Milstein, Pure Appl. Chem.
2003, 75, 2003; h) M. Gadliardo, D. J. M. Snelders, P. A. Chase, R. J. M.
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anhydrous MgSO4, filtered, and then concentrated in vacuo. The
residue was analysed by 1H NMR spectroscopy. The reaction conver-
sion was calculated by comparing the integration of the signal at
δ = 7.04 (t, 2 H) ppm from iodobenzene, with the signal at δ = 6.40
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catalytic reaction were iodobenzene and styrene, the signals used
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[12]
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The experiments to recover the catalysts took place as follows. Once
the reaction was finished, the mixture was cooled, and then concen-
trated to 1 mL. Then the mixture was placed onto a chromatogra-
phy column, and eluted with diethyl ether (30 mL) to recover the
organic products, and then with acetone (80 mL) to recover the
catalysts. The solution containing the catalyst was evaporated to
dryness and used in another run. The organic solution containing
the products was concentrated and analysed by 1H NMR spectro-
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Acknowledgments
Authors thank the Gobierno de Aragon (Spain) for financial sup-
port (Applied Organometallic Chemistry, E101 group).
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[19]
Keywords: Homogeneous catalysis · Ligand design · Pincer
ligands · S ligands · P ligands · Palladium
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Received: November 5, 2015
Published Online: December 15, 2015
Eur. J. Org. Chem. 2016, 789–798
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