Regioselective hydromethoxycarbonylation of terminal alkynes catalyzed by palladium(II)-tetraphos complexes

Article abstract of DOI:10.1021/om3003812

An in situ generated dinuclear palladium hydride complex bearing cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphanyl)cyclobutane catalyzed the hydromethoxycarbonylation of terminal alkynes, giving the corresponding branched α,β-unsaturated ester (A) with high regioselectivity.

Full text of DOI:10.1021/om3003812

Article
pubs.acs.org/Organometallics
Regioselective Hydromethoxycarbonylation of Terminal Alkynes
Catalyzed by Palladium(II)−Tetraphos Complexes
Werner Oberhauser,*,† Andrea Ienco,† Francesco Vizza,† Barbara Trettenbrein,‡ Dennis Oberhuber,‡
Christof Strabler,‡ Teresa Ortner,‡ and Peter Bruggeller*,‡
̈
†Istituto di Chimica dei Composti Organometallici (ICCOM-CNR), Area di Ricerca CNR di Firenze, via Madonna del Piano 10,
50019 Sesto Fiorentino, Italy
‡Institut fur Allgemeine, Anorganische und Theoretische Chemie, Universitat Innsbruck, Innrain 80-82, 6020 Innsbruck, Austria
̈
̈
S
* Supporting Information
ABSTRACT: An in situ generated dinuclear palladium hydride
complex bearing cis,trans,cis-1,2,3,4-tetrakis(diphenyl-
phosphanyl)cyclobutane catalyzed the hydromethoxycarbonyla-
tion of terminal alkynes, giving the corresponding branched
α,β-unsaturated ester (A) with high regioselectivity.
he hydromethoxycarbonylation of terminal alkynes to give
Scheme 1. The tetracationic Pd(II)−aqua complex 1 was
obtained upon protonation of the corresponding in situ
synthesized Pd(II) acetate complex8 with p-toluenesulfonic
acid (p-TsOH) with the concomitant release of acetic acid. The
coordination of two water molecules to each palladium center
T
α,β-unsaturated carboxylic acids or esters is a highly valuable
synthetic process, which finds its major application in the synthesis
of methyl methacrylate (MMA)1 and its derivatives (i.e., applica-
tion in polymer synthesis)2 or in the synthesis of pharmaceutical
products such as the optically pure anti-inflammatory drugs
(S)-ibuprofen and (S)-naproxen.3 Hydromethoxycarbonylation re-
actions of terminal alkynes are generally mediated by Pd(II)-
phosphane-based catalyst precursors, which yield a plethora of
possible products that originate from mono- and dicarbonylation
reactions.4 The chemo- and regioselectivity of the catalytic car-
bonylation reactions are hence important issues to be addressed.
The most significant examples of regio- or chemoselective catalytic
hydromethoxycarbonylation reactions of terminal alkynes reported
so far are (i) the regioselective conversion of propyne into MMA
by the highly active Pd(II)/2-PyPPh2 catalyst system,1 (ii) the
synthesis of linear α,β-unsaturated carboxylic esters mediated by
Pd(II)/diphosphane catalyst precursors (i.e., acceptable substrate
conversion as well as a diphosphane bite angle dependent regio-
selctivity were obtained only with at least a 3-fold excess of diphos-
phane ligand with respect to Pd(II)),5 (iii) the chemoselective
synthesis of alkynylcarboxylic esters by PdCl2/Cu-based6a and
Pd(II)-TROPP (TROPP = 5-(diphenyl)phosphanyl-5H-dibenzo-
[a,d]cycloheptene)-based catalysts.6b The latter catalyst system
was shown to be stereospecific when p-benzoquinone was used
as oxidizing agent. Herein we report the synthesis and success-
ful application of dinuclear Pd(II)-dppcb (dppcb = cis,trans,cis-
1,2,3,4-tetrakis(diphenylphospanyl)cyclobutane)7 complexes for
the regioselective hydromethoxycarbonylation of terminal
alkynes.
1
in 1 was confirmed by a corresponding H NMR spectrum,
acquired in CD2Cl2, which showed a broad hump at 3.2 ppm
integrating for approximately eight hydrogen atoms. Moreover,
conductivity measurements of 1 in MeOH, which is the
reaction medium for the catalytic hydromethoxycarbonylation
reactions, corroborated its behavior as a 1:4 electrolyte.
The neutral bimetallic compound 2 was obtained upon
reaction of dppcb with [PClMe(η4-COD)] in CH2Cl2. The
31P{1H} NMR spectrum of 2 exhibited two singlets centered at
70.3 and 54.7 ppm and two doublets at 69.4 and 56.1 ppm with
a J(PP) value of 5.8 Hz. This spectroscopic finding is in
accordance with the occurrence of two geometrical isomers
(i.e., cis and trans isomers (centrosymmetric)) in an 1:2 ratio.
On the basis of the significantly different trans influences of
chloride and a methyl group in square-planar Pd(II) complexes,
the high-frequency 31P NMR signals at 70.3 and 69.4 ppm were
assigned to the phosphorus atoms located trans to chloride.
The reaction of 2 with Ag(OTs) in CH2Cl2 gave the
corresponding bis-cationic isomers of 3 in the same ratio as
1
those of 2. The H NMR spectrum of 3, acquired in CD2Cl2,
showed the absence of a signal which might be attributed to
palladium-coordinating water molecules. As a consequence,
tosylate coordinates to palladium in the solid state and CD2Cl2.
On the other hand, 3 was completely soluble in MeOH and
behaved as a 1:2 electrolyte in the latter solvent.9 The bis-
cationic complex 4, which was straightforwardly obtained,
RESULTS AND DISCUSSION
The dinuclear Pd(II) complexes 1−3 bearing dppcb were syn-
thesized by following the synthetic procedures shown in
■
Received: May 7, 2012
Published: June 25, 2012
© 2012 American Chemical Society
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Scheme 1
showed only in DMSO-d6 or CD3CN a slight solubility. The
dynamic allyl coordination to palladium in 4 was evidenced in
solution by broad H NMR singlets for the allyl hydrogen
cycles defined by Pd(1), P(1), P(2), C(1), and C(2) and their
symmetry-equivalent counterparts.
1
A comparison of the envelope angles (i.e., defined as the
interplanar angle between the least-squares planes through
P(1), P(2), C(1), C(2) and Pd(1), P(1), P(2), respectively)
found in trans-2·4DMF and 4·CH2Cl2 showed only a slight
dependence on the nature of the ancillary ligand present (i.e.,
26.25(0.13)° in trans-2·4DMF and 32.61(0.10)° in 4·CH2Cl2).
A direct consequence of the envelope conformation of the
palladium−diphosphane cycles is a fixed spatial orientation of
the phenyl rings with respect to the palladium coordination
plane defined by Pd(1), P(1), and P(2) with the axially
oriented phenyl rings (i.e., C(11) and C(31)) pointing in the
same direction.
atoms at 3.62 and 5.96 ppm.
The single-crystal X-ray structure analyses of trans-2·4DMF10
(Figure 1) and of 4·CH2Cl2 (Supporting Information)
Catalyst precursors 1 and 3 have been screened in the
hydromethoxycarbonylation reaction of selected terminal
alkynes carried out in neat MeOH or a 9:1 solvent mixture
of CH3CN and MeOH. The optimal CO pressure was revealed
to be 150 psi, and catalytic reactions carried out under syngas
conditions (i.e., CO/H2 (150/150 psi); a hydrogen pressure
higher than 150 psi gave no beneficial effect in terms of sub-
strate conversion) showed significantly higher substrate conver-
sion in comparison to identical reactions conducted in the
absence of hydrogen.5b The catalytic performance of dppcb-
based precursors (i.e., 1 and 3) (Table 1) has been compared
with that of [Pd(H2O)2(PP)](OTs)2 (PP = meso-2,3-bis-
(diphenylphosphanyl)butane (5) and 1,2-bis(diphenylphosphanyl)-
ethane (6)), which are characterized by a comparable ligand bite
angle of ca. 85° but a typical twist conformation of the five-
membered palladium−diphosphane cycle, as shown by the
corresponding single-crystal X-ray structure analysis (Supporting
Information). In addition, the catalytic performance of 1 has
been compared with that of [Pd(H2O)2(dppf)](OTs)2 (7; dppf =
1,1′-bis(diphenylphosphanyl)ferrocene),11 since dppf-based palladium
precursors are known to be among the most active catalysts for the
hydromethoxycarbonylation of terminal alkynes.5
Figure 1. ORTEP plot of trans-2·4DMF. Thermal ellipsoids are shown
at the 30% probability level. Solvent molecules and hydrogen atoms
except for the cyclobutane carbon ring have been omitted, and only
the ipso carbon atoms of the phenyl rings are shown for clarity.
Selected bond lengths (Å) and angles (deg): Pd(1)−P(1) =
2.2969(10), Pd(1)−P(2) = 2.2246(11), Pd(1)−Cl(1) = 2.3581(12),
Pd(1)−C(3) = 2.191(3), C(1)−C(2) = 1.564(5), C(1)−C(2)#
=
1.556(4); P(1)−Pd(1)−P(2) = 85.42(4), C(3)−Pd(1)−Cl(1) =
86.61(11). Symmetry-equivalent position: (#) −x, −y, −z.
confirmed the planarity of the cyclobutane carbon ring and the
envelope conformation of the five-membered palladium−diphosphane
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a
Table 1. Pd(II)-Phosphane Catalyzed Hydromethoxycarbonylation of Terminal Alkynes
b
c
conversn (%) [TOF]
selectivity br. ester (%)
1 h/2 h
entry
cat. precursor
substrate (R)
phenyl
p(H2) (psi)
1 h
2 h
1
2
1
1
1
1
1
1
1
1
1
1
3
3
5
5
5
6
6
6
6
6
7
0
150
0
31 [31]
n.d.
54 [27]
95 [48]
n.d.
99/99
phenyl
n.d./99
99/n.d.
99/99
d
3
d
phenyl
49 [196]
65 [260]
n.d.
4
phenyl
150
150
150
150
150
150
150
0
100 [200]
80 [40]
97 [49]
98 [49]
51 [26]
20 [40]
40 [20]
24 [12]
94 [47]
37 [19]
54 [27]
n.d.
5
6
7
8
4-methoxyphenyl
4-chlorophenyl
2-methoxyphenyl
hexyl
n.d./99
n.d./99
n.d./99
n.d./93
87/87
n.d.
n.d.
n.d.
d
9
hexyl
12 [48]
n.d.
10
11
12
13
14
decyl
n.d./93
99/99
phenyl
15 [15]
n.d.
phenyl
150
0
n.d./99
85/85
phenyl
20
phenyl
150
150
0
n.d.
n.d./85
86/n.d.
85/85
d
15
phenyl
6 [24]
14
16
17
18
19
20
phenyl
16 [8]
phenyl
150
150
150
150
150
n.d.
36 [18]
21 [11]
46 [23]
37 [19]
16 [32]
n.d./84
n.d./84
n.d./45
n.d./52
2-methoxyphenyl
hexyl
n.d.
n.d.
decyl
n.d.
d
21
phenyl
12 [48]
30/30
a
b
Conditions: catalyst precursor (0.01 mmol Pd), substrate (1.00 mmol), MeOH (10.0 mL), CO (150 psi), T (70 °C). Substrate conversion
c
d
determined by GC using n-decane as internal standard. TOF defined as mmol of substrate converted ((mmol of precursor) h)−1. Conditions:
precursor (0.005 mmol Pd), substrate (2.00 mmol), 9:1 solvent mixture of CH3CN and MeOH (10.0 mL).
Under the applied experimental conditions the correspond-
ing α,β-unsaturated branched and linear esters (Table 1) were
the only carbonylation products obtained. Two experimental
facts were found: (i) the conversion was increased on con-
ducting catalytic reactions in the presence of hydrogen gas
(i.e., hydrogen gas stabilizes Pd−H species and unlike protic
acids avoids the hydration of alkynes giving ketones) and
(ii) analogous reactions carried out in the presence of
2,6-dimethoxybenzoquinone gave not even traces of the α,β-
unsaturated esters, the corresponding alkynyl-methoxycarboxy-
late6 being obtained instead. This experimental fact proves that
Pd−OCH3 is not the catalytically active species.12 As a con-
sequence, the unsaturated esters were obtained by a Pd−H-based
catalytic cycle,13 as shown in Scheme 2.
Scheme 2
The palladium hydride (Scheme 2, A), which is the key
intermediate in the catalytic cycle, was generated from either
Pd−aqua complexes (i.e., 1, 5−7) by a water-gas shift (WGS)
reaction14 or from a palladium−alkyl species12 (i.e., 3) upon
CO insertion and a subsequent methanolysis reaction releasing
methyl acetate, the presence of which in the catalyst solu-
tion was proved by GC-MS analysis. The stabilizing effect
of p-TsOH for the catalyst, formed upon the WGS reaction of
Pd−aqua complexes, is nicely demonstrated by carrying out
catalytic reactions of 1 and 3 in MeOH in the absence of
hydrogen pressure (Table 1, entries 1 and 11).
Regardless of the substrate and reaction medium employed,
dppcb-based precursors gave the highest regioselectivity for the
branched esters (i.e., 99% and 93% for aromatic and aliphatic
terminal alkynes, respectively) (Table 1). In order to rationalize
this experimental result, the corresponding regioselectivity-
determining Pd−hydride alkyne intermediate15 (i.e., the
terminal alkyne hydrogen atom is pointing toward the Pd−H
moiety) (Scheme 2, B) in the case of dppcb-based precursors
was mimicked by a geometry-optimized model compound that
bears cis-1,2-bis(diphenylphosphanyl)cyclobutane (L) as a
model ligand.
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Importantly, the model ligand exhibits all the characteristics
of monocoordinated dppcb (i.e., rigid and planar cyclobutane
carbon ring, which was theoretically realized by forcing the
dihedral angle of the four carbon atoms to be 0°). Both possible
model compounds of [Pd(H)(L)(C6H5C2H)]+ were calculated
using the B97-D method, which takes into account the
dispersion forces.16 The most interesting structural features arising
from the model compound calculated for the pro-branched
isomer, as shown in Figure 2, are (i) the carbon−carbon triple
the catalytic activities of 1, 5, and 6 under identical experi-
mental conditions unambiguously showed that an increasing
rigidity of the diphosphane ligand (i.e., dppe < meso-2,3-dppb <
dppcb) led to an increased substrate conversion. This experi-
mental finding is in accordance with slower overall deactivation
processes triggered by phosphane on−off coordination to
Pd(II) in the case of dppcb. Operando high-pressure 31P{1H}
NMR experiments carried out with 1 and 6 in MeOH-d4 in the
presence of phenylacetylene under syngas conditions showed
17
only in the case of 6 the formation of [Pd(dppe)2](OTs)2
(i.e., completely inactive under the chosen experimental
conditions) at 70 °C as a result of the formation of Pd(0)
and the concomitant release of diphosphane, which coordinates
a cationic [Pd(dppe)]2+ unit. In the absence of experimental
evidence for the formation of a related Pd(II) complex in the
case of dppcb, we calculated computational models for both
possible conformers 8a,b (Figure 3), using the model ligand
introduced above (L).
Figure 2. Optimized theoretical model of the pro-branched [Pd(H)-
(L)(C6H5C2H)]+. Short intramolecular contacts between H11 and
some carbon atoms of the equatorial ligand phenyl ring are highlighted
in red. Selected bond and intramolecular distances (Å) as well as
angles (deg): Pd−C1 = 2.21, Pd−C2 = 2.47, Pd−P1 = 2.27, Pd−P2 =
2.39, Pd−H = 1.58, C1−C2 = 1.25, H11···C20 = 2.75, H11···C21 =
2.69, H11···C22 = 2.84, H11···C25 = 2.93; P1−Pd−P2 = 84.3,
C1−Pd−C2 = 30.2.
bond of phenylacetylene is tilted with respect to the Pd
coordination plane of 46.4° and (ii) the short intra-
molecular interactions between H11 of the substrate phenyl
ring and four carbon atoms of one equatorially oriented ligand
phenyl ring are shorter than 3 Å. As a consequence, the model
compound with the terminal alkyne hydrogen atom pointing
toward Pd−H is 2.00 kcal mol−1 (i.e., energy difference in
methanol) lower in energy in comparison to its counterpart
(Supporting Information). The importance of the dispersion
force was evidenced by comparing the same optimized models
using the classical B3LYP method. In the latter case the short
interactions between the phenyl rings of phenylacetylene and of
the ligand are lost and the energy difference between the two
isomers is significantly 2.25 kcal mol−1 in favor of the model
compound, with the phenyl ring pointing toward Pd−H
(Supporting Information). Analogous theoretical calculations
with 1-hexyne instead of phenylacetylene showed that the
energy difference is slightly in favor of the model compound,
showing the butyl carbon chain which points toward Pd−H
(0.28 kcal mol−1). The latter theoretical result is in line with the
lower regioselectivity for the branched esters found for aliphatic
in comparison to aromatic terminal alkynes (Table 1). In
addition, it is rational to assume that an increasing diphosphane
flexibility significantly decreases weak interactions between the
substrate and the ligand phenyl rings. Accordingly, dppe- and
meso-2,3-dppb-based catalyst precursors showed a lower
regioselectivity versus the branched esters in comparison to
the dppcb counterpart (Table 1).
Figure 3. Optimized theoretical models of the possible conformers of
[Pd(L)2]2+. The axial and equatorial phenyl rings are red and green,
respectively. Selected bond distances (Å) and angles (deg) for 8a
(left): Pd−P(1) = 2.38, Pd−P(2) = 2.36, Pd−P(3) = 2.35, Pd−P(4) =
2.36; P(1)−Pd−P(2) = 84.6, P(3)−Pd−P(4) = 85.1. Selected bond
distances (Å) and angles (deg) for 8b (right): Pd−P(1) = 2.36, Pd−
P(2) = 2.37, Pd−P(3) = 2.43, Pd−P(4) = 2.39; P(1)−Pd−P(2) =
83.8, P(3)−Pd−P(4) = 82.3.
The rigidity of the model ligand induces strain, due to
equatorial phenyl−phenyl repulsions. The strain present in
both model compounds is minimized by either a stark distor-
tion of the square-planar Pd coordination geometry, as in the
case of 8a (i.e., interplanar angle between P(1)−Pd−P(2) and
P(3)−Pd−P(4) of 23.8°, while the analogous angle in 8b is
8.4°), or by a significant increase of the Pd−P bond lengths of
one ligand, as in the case of 8b (i.e., Pd−P(1) = 2.36 Å,
Pd−P(2) = 2.37 Å vs Pd−P(3) = 2.43 Å and Pd−P(4) =
2.39 Å). Nevertheless, 8a is 2.71 kcal mol−1 more stable than
8b. At this point it is important to consider that the formation
of 8a,b is kinetically much more disfavored in comparison
to [Pd(dppe)]2+, since in the case of the rigid ligand two
coordination sites have to be provided simultaneously.
CONCLUSION
■
The fixed orientation of the phenyl rings in the Pd−dppcb
hydride alkyne complex favors the Markovnikov insertion of
the terminal alkynes into the Pd−H bond, yielding branched
α,β-unsaturated esters with high regioselectivity (up to 99%),
which is unique for an isolated Pd complex. The overall rigidity
of coordinated dppcb notably stabilizes the Pd(II) center in the
The highest substrate conversion was obtained with 1,
significantly exceeding that of 7 (Table 1, entry 4 vs 21), by
carrying out catalytic reactions in a 9:1 solvent mixture of
CH3CN and MeOH under syngas conditions. A comparison of
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(found) for C40H42O8P2S2Pd: C, 54.42; H, 4.76. Found: C, 54.38;
H, 4.60.
course of the catalytic carbonylation reaction, precluding the
formation of catalytically inactive Pd−bis(chelate) complexes.
Catalytic Reactions. A homemade stainless steel autoclave (80.0 mL)
was charged with the catalyst precursor (0.01 mmol Pd), before it was
sealed and evacuated by a vacuum pump. Afterward a deaerated
solution of the terminal alkyne (1.00 mmol) in MeOH (10.0 mL) and
n-decane (100.0 μL) was introduced into the autoclave by suction,
followed by pressurizing the autoclave with CO (45 psi) and heating it
to 70 °C by means of an oil bath. Once the reaction temperature was
reached, the CO pressure was adjusted to 150 psi, followed by
charging the autoclave with H2 (150 psi) when needed. After the
desired reaction time, the autoclave was cooled to room tempera-
ture by means of an ice/water bath, the gas pressure released, and the
catalytic solution analyzed by GC and GC-MS.
Theoretical Calculations. Structural optimizations were carried
out at the hybrid density functional theory (DFT) level using the
Gaussian03 suite of programs.21 Both B97-D16 and the Becke three-
parameter hybrid exchange-correlation function,22 containing the
nonlocal gradient correction of Lee, Yang, and Parr (B3LYP)23 were
used for the pro-branched and pro-linear models of the phenyl and
hexyl precursors. In the case of conformers 8a,b only the calculations
at the B97-D level were reported. Single-point energies for the opti-
mized models were recalculated also in methanol by using the
conductor-like polarizable continuum model (CPCM).24 Calculations
of the frequencies were performed to validate the nature of the opti-
mized stationary points. The negative frequencies found in the
calculations tried to break the imposed constraint on the cyclobutane
rings. The Stuttgart/Dresden effective core potential was used for
metals.25 The basis set used for the remaining atomic species was
6-31G(d,p).26
EXPERIMENTAL SECTION
■
General Procedures. All reactions were carried out under a
nitrogen atmosphere using standard Schlenk techniques. [PdClMe-
(η4-COD)],18 [PdCl(η3-C3H5)]2,19 PdCl2(dppe),20 PdCl2(meso-2,3-
dppb)],20 [Pd(H2O)2(dppf)](OTs)2,11 and dppcb7 were prepared
according to literature methods. 1H, 13C{1H}, and 31P{1H} NMR
spectra were obtained on a Bruker Avance DRX-400 spectrometer at
400.13, 100.62, and 161.98 MHz. Chemical shifts (δ) are reported in
ppm relative to TMS (1H and 13C NMR) or 85% H3PO4. Infrared
spectra were recorded on a FT-IR Perkin-Elmer BX spectrometer.
Carbonylation reactions were performed in homemade 80 mL stain-
less steel autoclaves, equipped with a magnetic stirrer and temperature
and pressure controller. GC analyses were performed on a Shimadzu
GC-14A gas chromatograph equipped with a flame ionization detector
and a 30 m (0.25 mm i.d., 0.25 μm film thickness) SPB-1 Supelco
fused silica capillary column, using n-decane as internal standard. GC-
MS analyses were performed on a GC-MS QP2010S instrument which
was equipped with the same capillary column. Elemental analyses were
carried out with a NA 1500 Carlo Erba elemental analyzer.
[Pd2(H2O)4(dppcb)](OTs)4 (1). Pd(OAc)2 (92.2 mg, 0.411 mmol)
was added to a deaerated solution of dppcb (162.4 mg, 0.205 mmol)
in CH2Cl2 (10 mL) at room temperature. The reaction solution was
stirred for 4 h at room temperature. Afterward water (5 mL) and
p-TsOH·H2O (234.5 mg, 1.233 mmol) were added, giving a biphasic
reaction mixture which was vigorously stirred at room temperature for 2 h.
Afterward the organic phase was separated from the water phase,
washed with water, dried over MgSO4, and then concentrated to
dryness, giving the product as a beige solid. Yield: 288.9 mg (80%).
Anal. Calcd (found) for C80H80O16P4S4Pd2: C, 54.54; H, 4.54. Found:
C, 54.32; H, 4.63. ΛM = 0.051 Ω−1 cm2 mol−1.
ASSOCIATED CONTENT
■
S
* Supporting Information
Synthesis of [Pd2Cl2Me2(dppcb)] (2). PdClMe(η4-COD)
(108.6 mg, 0.410 mmol) was added to a deaerated solution of dppcb
(162.4 mg, 0.205 mmol) in CH2Cl2 (6 mL) at room temperature.
During the reaction (1 h) the product precipitated from solution as an
off-white product. To complete precipitation, n-pentane (5 mL) was
added, and the product was separated by filtration and dried by a flow
of nitrogen. Yield: 182 mg (85%). Anal. Calcd (found) for C54H50Cl2-
P4Pd2: C, 58.63; H, 4.52. Found: C, 58.43; H, 4.60.
Synthesis of [Pd2(OTs)2Me2(dppcb)] (3). Compound 2 (120.0 mg,
0.108 mmol) was suspended in deaerated CH2Cl2, followed by the
addition of Ag(OTs) (63.3 mg, 0.227 mmol). The suspension was
allowed to react for 2 h. Then AgCl was removed by filtration of the
suspension through Celite and the resulting solution was concentrated
to dryness, giving an off-white solid. Yield: 104.1 mg (70%). Anal.
Calcd (found) for C68H64O6P4S2Pd2: C, 59.29; H, 4.64. Found: C,
59.15; H, 4.56. ΛM = 0.029 Ω−1 cm2 mol−1.
Figures, tables, text, and CIF files giving NMR characterization
of 1−6, operando high-pressure 31P{1H} NMR spectroscopic
studies with 1 and 6, and single crystal X-ray structure analyses
of 2·4DMF, 4·CH2Cl2, 5, 6, and theoretical model compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: werner.oberhauser@iccom.cnr.it (W.O.); Peter.
Brueggeller@uibk.ac.at (P.B.).
Notes
The authors declare no competing financial interest.
Synthesis of [Pd2(η3-C3H5)2(dppcb)] (4). To a deaerated solution
of dppcb (80.0 mg, 0.101 mmol) in CH2Cl2 (20 mL) was added
[PdCl(η3-C3H5)]2 (37.0 mg, 0.101 mmol) at room temperature. The
reaction solution was stirred for 1 h, followed by the addition of
Tl(PF6) (74.1 mg, 0.212 mmol). The reaction was continued for 1/2 h.
Then the suspension was filtered through Celite and the solution con-
centrated to half of its original volume, causing the partial precipitation
of the product. On addition of petroleum ether the precipitation was
complete and the off-white product separated by filtration and dried in
vacuo. Yield: 117.0 mg (84%). Anal. Calcd (found) for C58H54F12P6Pd2: C,
50.58; H, 3.92. Found: C, 50.62; H, 3.99.
Synthesis of [Pd(H2O)2(meso-2,3-dppb)](OTs)2 (5) and
[Pd(H2O)2(dppe)](OTs)2 (6). PdCl2(meso-2,3-dppb) and PdCl2(dppe)
(0.132 mmol) were suspended in deaerated CH2Cl2 (10 mL) at room
temperature. To the latter suspension was added Ag(OTs) (77.3 mg,
0.277 mmol), and the suspension was stirred for 2 h, followed by its
filtration through Celite and the concentration of the resulting solution
to dryness, giving brownish and yellow powders, respectively. 5: yield
93.8 mg (78%). Anal. Calcd (found) for C42H46O8P2S2Pd: C, 55.38;
H, 5.05. Found: C, 55.28; H, 4.98. 6: yield 99.0 mg (85%). Anal. Calcd
ACKNOWLEDGMENTS
■
A.I. acknowledges the ISCRA-CINECA HP grant “H2act-
HP10BEG2NO” for computational resources. This research
was also financially supported by the Fonds zur Foerderung der
wissenschaftlichen Forschung (FWF) and the Forschungsfoer-
derungsgesellschaft (FFG), Vienna, Austria, the Tiroler
Wissenschaftsfonds (TWF), Innsbruck, Austria, and the
companies Verbund AG and D. Swarowski KG.
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