Characterization of DMSO coordination to palladium(II) in solution and insights into the aerobic oxidation catalyst, Pd(DMSO)2(TFA) 2

Article abstract of DOI:10.1021/ic301799p

Recent studies have shown that Pd(DMSO)₂(TFA)₂ (TFA = trifluoroacetate) is an effective catalyst for a number of different aerobic oxidation reactions. Here, we provide insights into the coordination of DMSO to palladium(II) in both the solid state and in solution. A crystal structure of Pd(DMSO)₂(TFA)₂ confirms that the solid-state structure of this species has one O-bound and one S-bound DMSO ligand, and a crystallographically characterized mono-DMSO complex, trans-Pd(DMSO)(OH ₂)(TFA)₂, exhibits an S-bound DMSO ligand. ¹H and ¹⁹F NMR spectroscopic studies show that, in EtOAc and THF-d ₈, Pd(DMSO)₂(TFA)₂ consists of an equilibrium mixture of Pd(S-DMSO)(O-DMSO)(TFA)₂ and Pd(S-DMSO) ₂(TFA)₂. The O-bound DMSO is determined to be more labile than the S-bound DMSO ligand, and both DMSO ligands are more labile in THF relative to EtOAc as the solvent. DMSO coordination to Pd^II is substantially less favorable when the TFA ligands are replaced with acetate. An analogous carboxylate ligand effect is observed in the coordination of the bidentate sulfoxide ligand, 1,2-bis(phenylsulfinyl)ethane to Pd^II. DMSO coordination to Pd(TFA)₂ is shown to be incomplete in AcOH-d₄ and toluene-d₈, resulting in Pd^II/DMSO adducts with <2:1 DMSO/Pd^II stoichiometry. Collectively, these results provide useful insights into the coordination properties of DMSO to Pd^II under catalytically relevant conditions.

Full text of DOI:10.1021/ic301799p

Article
pubs.acs.org/IC
Characterization of DMSO Coordination to Palladium(II) in Solution
and Insights into the Aerobic Oxidation Catalyst, Pd(DMSO)2(TFA)2
Tianning Diao, Paul White, Ilia Guzei, and Shannon S. Stahl*
Department of Chemistry, University of Wisconsin - Madison, 1101 University Avenue, Madison, Wisconsin 53706, United States
S
* Supporting Information
ABSTRACT: Recent studies have shown that Pd-
(DMSO)2(TFA)2 (TFA = trifluoroacetate) is an effective
catalyst for a number of different aerobic oxidation reactions.
Here, we provide insights into the coordination of DMSO to
palladium(II) in both the solid state and in solution. A crystal
structure of Pd(DMSO)2(TFA)2 confirms that the solid-state
structure of this species has one O-bound and one S-bound
DMSO ligand, and a crystallographically characterized mono-
DMSO complex, trans-Pd(DMSO)(OH2)(TFA)2, exhibits an
S-bound DMSO ligand. 1H and 19F NMR spectroscopic studies show that, in EtOAc and THF-d8, Pd(DMSO)2(TFA)2 consists
of an equilibrium mixture of Pd(S-DMSO)(O-DMSO)(TFA)2 and Pd(S-DMSO)2(TFA)2. The O-bound DMSO is determined
to be more labile than the S-bound DMSO ligand, and both DMSO ligands are more labile in THF relative to EtOAc as the
solvent. DMSO coordination to PdII is substantially less favorable when the TFA ligands are replaced with acetate. An analogous
carboxylate ligand effect is observed in the coordination of the bidentate sulfoxide ligand, 1,2-bis(phenylsulfinyl)ethane to PdII.
DMSO coordination to Pd(TFA)2 is shown to be incomplete in AcOH-d4 and toluene-d8, resulting in PdII/DMSO adducts with
<2:1 DMSO/PdII stoichiometry. Collectively, these results provide useful insights into the coordination properties of DMSO to
PdII under catalytically relevant conditions.
INTRODUCTION
■
A number of homogeneous palladium catalysts have been
developed over the past decade for selective aerobic oxidation
of organic molecules.1 One of the earliest reported catalyst
systems consists of Pd(OAc)2 in DMSO as the solvent, which
was first reported by the groups of Larock and Hiemstra in the
mid-1990s.2 This simple catalyst system has been used in a
variety of oxidative transformations, including alcohol oxida-
tion, intramolecular hetero- and carbocyclization of alkenes,
and cycloalkenylation of silyl enol ethers.3 In these reactions,
DMSO has been proposed to serve as a ligand and is believed
to play a vital role in stabilizing Pd0 and promoting the
reoxidation of Pd0 by O2.4 Characterization of the coordination
properties of DMSO in these reactions is complicated by the
large excess of DMSO and insights thus far have been limited to
computational studies.5
Sulfoxides, including DMSO and bis-sulfoxides, have been
used as ligands or additives to promote a variety of Pd-catalyzed
oxidation reactions, often using benzoquinone or AgI as the
for dehydrogenation reactions and oxidative amination, while
stoichiometric oxidants.6 Recently, we have reported that
Pd(OAc)2 in combination with trifluoroacetic acid was used as
the catalyst in the biaryl-coupling reaction.
The coordination chemistry of DMSO to late transition
metals has been the subject of considerable investigation.10
Depending on the identity of the metal and other ancillary
ligands, DMSO can exhibit S-, O-, or bridging μ-S,O-bound
Pd(DMSO)2(TFA)2, in which DMSO is present in catalytic
quantities, is an effective catalyst system for a number of
oxidation reactions capable of using O2 as the oxidant,
including α,β-dehydrogenation of carbonyl compounds (eq
1)7 and oxidative amination of alkenes (eqs 2 and 3).8 A similar
catalyst system has been reported by Buchwald and co-workers
for chelate-directed C−H arylation of anilides (eq 4).9
Pd(TFA)2 (TFA = trifluoroacetate) was the palladium source
Received: August 16, 2012
Published: October 23, 2012
© 2012 American Chemical Society
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coordination modes. Soft metals, such as RuII, OsII, RhI, and
PtII, typically favor DMSO coordination via the sulfur atom, but
O-coordination has also been observed.10 Representative d8
metal−DMSO complexes include [Rh(S-DMSO)2(O-
DMSO)2][PF6],11 [Pt(bpy)(S-DMSO)Cl][BF4] (bpy = 2,2′-
bipyridine) and [Pd(phen)(O-DMSO)Cl][PF6] (phen =
phenanthroline).12 IR and H NMR spectroscopic methods
1
provide a valuable complement to X-ray crystallography in
establishing the coordination mode of DMSO to transition
metals.10,13 In the aforementioned complexes, S-coordinated
DMSO ligands exhibit S−O stretching frequencies that range
from 1080 to 1154 cm−1, while O-bound DMSO ligands exhibit
lower vibrational frequencies, ranging from 862 to 997 cm−1
1
Figure 1. Molecular structure of Pd(S-DMSO)(O-DMSO)(TFA)2,
with 50% probability ellipsoids. Selected bond lengths, Å: Pd1−O1:
2.0158(15), Pd1−O3: 2.0170(16), Pd1−O6: 2.0554(16), Pd1−S1:
2.1994(5), S1−O5: 1.4633(17), S2−O6: 1.5509(16).
(Table 1). In the H NMR spectra, S-bound DMSO ligands
Table 1. IR and 1H NMR Spectroscopic Data of DMSO and
Transition Metal-Coordinated DMSO Reported in the
Literature10
cm−1 and 923 cm−1. These bands, which are blue- and red-
shifted relative to free DMSO, νS−O = 1005 cm−1, correspond
to the S−O stretching frequencies of the S- and O-bound
DMSO ligands, respectively.19
IR (S−O) (cm−1
)
1H NMR (ppm)
free DMSO
1005
∼2.53
S-DMSO ligands
O-DMSO ligands
1080−1154
862−997
3.80−3.30
3.03−2.59
A mono-DMSO-ligated PdII complex was obtained from an
EtOAc solution of Pd(TFA)2 containing only 1 equiv of
DMSO. X-ray diffraction analysis established the structure of
trans-Pd(S-DMSO)(OH2)(TFA)2 (Figure 2). The H2O
exhibit 1H NMR chemical shifts approximately 1 ppm
downfield relative to free DMSO (∼2.53 ppm), whereas O-
bound DMSO exhibits a smaller downfield shift (Table 1).14
PdII complexes have been identified with both S- and O-
DMSO coordination. For example, in Pd(DMSO)2Cl2 both
DMSO ligands are S-bound,15 whereas [Pd(phen)(O-DMSO)-
Cl][PF6] features an O-bound DMSO.12 Complexes containing
both S- and O-bound DMSO ligands are also known, including
[Pd(DMSO)4](BF4)2, which has two S-bound and two O-
bound DMSO ligands.15
Pd(DMSO)2(TFA)2 was first characterized by IR spectros-
copy in the solid state in 1965 by Wilkinson and co-workers,16
and the data provided evidence for S-bound DMSO. Cotton
and co-workers then reported a crystal structure of this
complex, which revealed the presence of trans-DMSO ligands,
one S-bound and one O-bound.17 In light of the growing
significance of DMSO-ligated PdII complexes in aerobic
oxidation reactions and the difficulty in characterizing the
DMSO coordination chemistry in DMSO solvent, we wanted
to establish the coordination chemistry of Pd(DMSO)2(TFA)2
in solution. Here, we report the characterization of this complex
Figure 2. Molecular structure of Pd(S-DMSO)(OH2)(TFA)2, with
50% probability ellipsoids. The half molecule of cocrystallized solvent
ethyl acetate is not shown. Selected bond lengths, Å: Pd1−O1:
2.0205(13), Pd1−O3: 2.0171(13), Pd1−O5: 2.0564(13), Pd1−S1:
2.1979(8), S1−O6: 1.4677(13).
molecule, coordinated trans to the S-DMSO ligand, is hydrogen
bonded to the carboxylates of adjacent molecules (see
Supporting Information for details). An IR spectrum of the
solid Pd(S-DMSO)(OH2)(TFA)2 exhibits an absorption band
at 1155 cm−1, consistent with S-coordination of DMSO (cf.
Table 1). The lack of absorption bands between 860 and 1000
cm−1 is consistent with the absence of an O-bound DMSO
ligand.
1
by H and 19F NMR spectroscopy in catalytically relevant
solvents, EtOAc, THF, AcOH, and toluene. The results are
supplemented by X-ray crystallographic and IR spectroscopic
data, and they provide valuable insights into the structure and
dynamic properties of this complex in solution.
RESULTS AND DISCUSSION
■
The Solution-Phase Structure of Pd(TFA)2/DMSO in
EtOAc. (A). Spectroscopic Data. Discrepancies can exist
between solid-state and solution structures of transition-metal
complexes. For example, the DMSO ligand in [Pd(bpy)-
(DMSO)Cl][PF6] (bpy = 2,2′-bipyridine) was determined to
be O-bound by X-ray crystallography; however, a mixture of O-
X-Ray Crystal Structures of DMSO-Ligated Pd(TFA)2
Complexes. Deep-orange crystals were obtained from a
solution of Pd(TFA)2 and 2 equiv of DMSO in EtOAc, and
X-ray diffraction analysis established the structure of trans-
Pd(S-DMSO)(O-DMSO)(TFA)2 (Figure 1), which exhibits
only slight variations relative to the structure originally reported
by Cotton and workers.17 The S-bound DMSO ligand has a
shorter S−O bond [1.4633(17) Å] and the O-bound DMSO
ligand has a longer S−O bond [1.5509(16) Å] relative to that
of free DMSO (1.52 Å18). The solid-state IR absorption
spectrum of this complex reveals absorption bands at 1149
1
and S-DMSO ligated species was observed in CD3NO2 by H
NMR spectroscopy.12 In order to probe the similarities and
differences between solid-state and solution structures of
1
Pd(TFA)2/DMSO complexes, a combination of H and 19F
NMR spectroscopy was used to determine the coordination
properties of DMSO to PdII in catalytically relevant solvents.
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1
Figure 3. H NMR spectra of Pd(TFA)2 in EtOAc in the presence of various quantities of DMSO at −60 °C. Conditions: [Pd(TFA)2] = 30 mM
(6.6 mg, 0.02 mmol), EtOAc = 0.65 mL, −60 °C, [DMSO] = 0, 15, 30, 39, 48, 60, and 90 mM.
Figure 4. 19F NMR spectra of Pd(TFA)2 in EtOAc in the presence of various quantities of DMSO at −60 °C. Conditions: [Pd(TFA)2] = 30 mM
(6.6 mg, 0.02 mmol), EtOAc = 0.65 mL, −60 °C, [DMSO] = 0, 15, 30, 39, 48, 60, and 90 mM.
The Pd(DMSO)2(TFA)2-catalyzed dehydrogenation of
cyclohexanone was performed in EtOAc (eq 1), and our initial
dynamic processes. Pd(TFA)2 completely dissolves in EtOAc
1
1
and no H resonances are observed in the H NMR spectrum
between 2.3 and 3.7 ppm (Figure 3). Addition of 0.5 equiv of
DMSO relative to Pd results in the appearance of a singlet at
3.47 ppm. This peak grows upon addition of another 0.5 equiv
of DMSO (1 equiv total), with concomitant formation of three
spectroscopic studies were carried out in this solvent.7
A
titration experiment was carried out by adding DMSO to a
solution of Pd(TFA)2 in EtOAc-d0. The initial titration
experiments were performed at −60 °C in order to slow
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1
Figure 5. NMR spectra of Pd(TFA)2/DMSO (1:1.6) in EtOAc at various temperatures. (A) H NMR spectra; (B) 19F NMR spectra. Conditions:
[Pd(TFA)2] = 30 mM (6.6 mg, 0.02 mmol), [DMSO] = 48 mM (1.6 equiv), EtOAc = 0.65 mL, T = −60, −50, −40, −20, 0, and 24 °C.
new smaller peaks at 3.54, 3.42, and 2.95 ppm. Further addition
of DMSO results in diminishment of the peak at 3.47 ppm and
growth of the peaks at 3.54, 3.42, and 2.95 ppm. With 2 equiv
of DMSO, the peak at 3.47 ppm is not present, but a new peak
at 3.08 ppm is evident.20 A broad peak, corresponding to free
DMSO, appears at 2.64 ppm and the peak at 2.95 ppm is
broadened in the presence of 3 equiv DMSO.
The same DMSO titration solutions were analyzed by 19F
NMR spectroscopy (Figure 4). In EtOAc, Pd(TFA)2 exhibits a
singlet at −76.76 ppm. Multiple minor species are evident
between −73.50 and −75.30 ppm, the integrations of which
vary at different temperatures and Pd(TFA)2 concentrations. A
singlet at −74.74 ppm appears upon addition of 0.5 equiv of
DMSO. Further addition of DMSO leads to decreased
integration of the Pd(TFA)2 peak at −76.76 ppm, with
concomitant growth of the peak at −74.74 ppm and appearance
of two new singlets at −74.80 and −75.00 ppm. With 2 and 3
equiv of DMSO, the peaks at −74.80 and −75.00 ppm are the
sole peaks present in the 19F NMR spectrum.
previously characterized S-DMSO ligands (cf. Table 1), and
integration of this peak and the corresponding new TFA peak
in the 19F NMR spectrum at −74.74 ppm relative to an internal
standard (C6H5F) reflects formation of a 1:1 DMSO/
Pd(TFA)2 complex (2a, eq 6), analogous to Pd(S-DMSO)-
(OH2)(TFA)2 (cf. Figure 2). An S-DMSO-ligated Pd(TFA)2
dimer, such as 2b, is an alternative possible structure,
particularly in light of analogous crystallographically charac-
The temperature dependence of equilibra between different
Pd(TFA)2/DMSO complexes has been investigated with the
sample containing 1.6 equiv of DMSO, in which multiple
species are evident in the NMR spectra. Both 1H and 19F NMR
spectra exhibit peak broadening as the temperature is increased
1
(Figure 5). At temperatures above −20 °C, the H peaks at
3.47 and 3.42 ppm coalesce into one peak with concomitant
broadening of the peak at 2.95 ppm. Similar coalescence occurs
for the 19F peaks at −74.74 and −74.80 ppm. In contrast, the
1H peak at 3.54 ppm and 19F peak at −75.00 ppm remain sharp
throughout the temperature range.
22
terized structures, [Pd(S-DMSO)(OAc)2]2 and the phenyl-
sulfoxide-ligated Pd(TFA)2 dimer (Scheme 1).23,24
The peaks that appear in the 1H and 19F NMR spectra upon
addition of ≥1 equiv of DMSO are consistent with bis-DMSO-
(B). Structural Assignments. Assignments of the peaks in
the H NMR spectra presented above are facilitated by the
Scheme 1. Dimeric Carboxylate-Bridged PdII Complexes
Reported in the Literature22,23
1
literature precedents, and peaks in the 19F NMR spectra may be
correlated to those in the 1H NMR spectra and assigned
accordingly. The solid-state structure of Pd(TFA)2 is trimeric;21
however, Wilkinson has reported osmometry data suggesting
that Pd(TFA)2 dissociates into a monomer 1 in EtOAc (eq
5).16 The peak that appears at 3.47 ppm upon addition of
DMSO is assigned to an S-bound DMSO ligand by analogy to
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1
coordinated Pd species. The H peaks at 3.54 ppm and 3.42
ppm represent two different S-bound DMSO ligands, whereas
the peak at 2.95 ppm is consistent with an O-bound DMSO
ligand. The 1H resonances at 3.42 ppm and 2.95 ppm correlate
with the peak at −74.80 ppm in the 19F NMR spectra, and their
respective integrations correspond to a ligand ratio of 1 S-
DMSO/1 O-DMSO/2 TFA, consistent with the assignment of
this species as Pd(S-DMSO)(O-DMSO)(TFA)2 3a (eq 7). The
study reveals that 2a and 3a undergo fast exchange at elevated
temperature (eq 8). Ligand exchange of the labile O-DMSO
ligand in 3a with 2a, possibly catalyzed by the EtOAc solvent,
can account for these observations. The kinetics of this
exchange process will be discussed further below. The lack of
peak broadening of 3b suggests that analogous exchange of the
S-bound DMSO ligands in 3b does not take place on the NMR
time scale.
The temperature dependence plots shown in Supporting
Information (SI), Figure S8, also reveal that the concentration
of 3b decreased with concomitant increase of 3a at increased
temperature, indicating a temperature-dependent equilibrium
between 3a and 3b (eq 9). The equilibrium constants at various
remaining S-bound DMSO peak at 3.54 ppm correlates with an
19F peak at −75.00 ppm, and integrations of these peaks
correspond to a 1:1 DMSO:TFA ligand ratio. This complex is
assigned as Pd(S-DMSO)2(TFA)2 3b, in which both DMSO
ligands coordinate via the sulfur atom (eq 7). Both cis and trans
geometries of bis-(S-DMSO) PdII and PtII complexes are
known.25,26 Examples include trans-Pd(S-DMSO)2Cl2,25b trans-
Pd(S-DMSO)2(Ar)(TFA) (Ar = 2,4,5-(MeO)3C6H2)6d and cis-
Pd(S-DMSO)2(NO3)2.26b We speculate that 3b has a trans
geometry because it originates from displacement of the solvent
ligand in trans-Pd(S-DMSO)(sol)(TFA)2 2a. Moreover, DFT
calculations indicate that the trans geometry is more stable than
the cis geometry by 2.6 kcal/mol.27
temperatures are calculated based on the 19F NMR spectra.
van’t Hoff analysis reveals a linear relationship between the
ln(Keq) and 1/T (Figure S9, SI) and enables determination of
ΔH and ΔS, (−2.7 kcal/mol and −13 eu, respectively). The
relatively large negative ΔS suggests that 3b has a more ordered
structure than 3a.
On the basis of the above assignments, the 1H and 19F NMR
data can be used to plot the concentrations as a function of
1
added DMSO (Figure 6), and the plot derived from the H
NMR spectra is in good agreement with that derived from the
19F NMR spectra. To summarize the results, initial addition of
DMSO results in formation of a mono-DMSO-ligated Pd-
(TFA)2 complex with DMSO coordinated via the S atom.
Higher concentrations of DMSO leads to conversion of this
species into an equilibrium mixture of bis-DMSO-ligated
Pd(TFA)2 complexes 3a and 3b. The bis-DMSO coordination
to PdII is sufficiently favored that unbound DMSO is not
evident until >2 equiv of DMSO is added.
In the presence of 3 equiv of DMSO, the significant line
broadening is evident for the peaks corresponding to unbound
DMSO and the O-bound DMSO ligand in 3a. This observation
suggests that fast exchange takes place between them and
further implies that the O-bound DMSO ligand is more labile
than the S-bound DMSO ligand. Complementary observations
are obtained from the variable-temperature spectra of Pd-
(TFA)2 in the presence of 1.6 equiv of DMSO (Figure 5). This
The catalytic oxidation of cyclohexanone in EtOAc employs
Pd(TFA)2 with 2 equiv of DMSO at 60 °C. The data presented
above suggest that only bis-DMSO coordinated Pd compounds
3a and 3b are present under these conditions. At 60 °C,
significant peak broadening is observed among all species (data
not shown), suggesting that fast exchange can take place
between both S- and O-bound DMSO ligands at this
temperature. The equilibrium constant between 3a and 3b
was calculated to be 0.17 at 60 °C, based on the van’t Hoff
analysis described above (eq 10). Therefore, under the catalytic
conditions of dehydrogenation, 3a is favored over 3b by a 6:1
ratio.
Effect of the Anionic Ligand: Coordination Chemistry
of DMSO to Pd(OAc)2 in EtOAc. The use of trifluoroacetate
as an anionic ligand is important to the success of the Pd/
DMSO catalyst systems in eqs 1−3;7,8 replacement of
Pd(TFA)2 with Pd(OAc)2 has been shown to result in
Figure 6. Concentrations of Pd(TFA)2/DMSO complexes present in EtOAc as a function of added [DMSO] on the basis of 1H and 19F NMR data,
(A) and (B), respectively. Conditions: [Pd(TFA)2] = 30 mM (6.6 mg, 0.02 mmol), EtOAc = 0.65 mL, −60 °C, [DMSO] = 0, 15, 30, 39, 48, 60, and
90 mM.
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coordination might partially cleave the trimeric Pd(OAc)2
structure forming linear trimers, such as complexes 5a and 5b
(eq 11). A number of structurally similar acetate-bridged
decreased yields. These observations prompted us to analyze
the coordination chemistry of DMSO to Pd(OAc)2 in EtOAc.
Pd(OAc)2 fully dissolves in EtOAc to form an orange
solution. Titration of DMSO into the solution results in the
appearance of two broad peaks at 3.50 and 3.43 ppm in the 1H
NMR spectra, consistent with S-bound DMSO ligands,
together with a peak at 2.60 ppm corresponding to free
DMSO (Figure 7). Further addition of DMSO increases the
trimeric Pd complexes have been reported previously in the
literature.28 If this assignment is correct, the two distinct H
1
signals could arise from the isomeric structures, 5a and 5b.
Despite the tentative nature of the Pd(OAc)2/DMSO
structural assignments, a distinction between DMSO coordi-
nation to Pd(TFA)2 and Pd(OAc)2 is clearly evident. The
differences presumably reflect the different basicity of the
carboxylate ligands. The more basic acetate ligand should be a
more effective bridging ligand, and cleavage of the trimeric
structure by DMSO will be less favored. Such effects
undoubtedly contribute to the different activities of Pd(TFA)2
and Pd(OAc)2 in the catalytic reactions. More detailed
understanding of the catalytic implications of these observation
will require further investigation.
The Solution-Phase Structure of Pd(TFA)2/DMSO in
THF-d8. Pd(DMSO)2(TFA)2 in THF serves as an effective
catalyst for oxidative amination reactions (eq 2),8 and we
performed DMSO titration experiments similar to those
described above with a solution of Pd(TFA)2 in THF-d8. The
spectra reveal trends similar to those observed in EtOAc, as
well as differences associated with dynamics of the exchange
processes (Figures 9 and 10). Addition of 0.5 equiv of DMSO
Figure 7. 1H NMR spectra of Pd(OAc)2 in EtOAc with various
quantities of DMSO at 24 °C. Conditions: [Pd(OAc)2] = 30 mM (4.4
mg, 0.02 mmol), EtOAc = 0.65 mL, 24 °C, [DMSO] = 0, 7.5, 15, and
30 mM.
1
leads to the appearance of a resonance at 3.29 ppm in the H
spectrum and a peak at −74.73 ppm in the 19F NMR spectrum.
These peaks are assigned to the mono-DMSO complex 2a.
Addition of more DMSO results in the appearance of peaks at
2.76 ppm and 3.35 ppm in the 1H NMR spectra, and a peak at
−74.97 ppm in the 19F NMR spectra. Meanwhile, the peak at
concentration of bound DMSO ligands; however, the sum of
the bound DMSO ligands maximizes at a DMSO/PdII ratio of
approximately 2:3, when ≥10 equiv of DMSO have been added
(Figure 8).
The solid-state structures of Pd(TFA)2 and Pd(OAc)2 have
both been characterized previously to be trimeric by X-ray
crystallography,21 and Pd(OAc)2 has been proposed to remain
trimeric in EtOAc.16 The 2:3 DMSO:PdII stoichiometry evident
from the titration experiments in Figure 8 suggests that DMSO
1
3.29 ppm in the H NMR spectra and −74.73 ppm in the 19F
NMR spectra shift upfield. On the basis of the analysis of the
EtOAc solutions, the peak at 3.35 ppm in the 1H NMR spectra
and −74.97 ppm in the 19F NMR spectra are assigned to Pd(S-
1
DMSO)2(TFA)2, 3b. The peak at 2.76 ppm in the H NMR
spectra corresponds to an O-bound DMSO ligand, consistent
with formation of Pd(S-DMSO)(O-DMSO)(TFA)2, 3a.
The S-bound DMSO ligand of 3a was not observed as an
independent peak but was manifested as an upfield shift of the
S-bound DMSO resonance of 2a. This observation reflects fast
exchange between 2a and 3a (cf. eq 8), resulting in coalescence
of their S-bound DMSO resonances. Similarly, 2a and 3a
exhibit a single broadened peak at −74.73 ppm in the 19F NMR
spectra. Analogous coalescence of 2a and 3a has been observed
in EtOAc at temperatures higher than 20 °C (cf. Figure 5).
When >2 equiv of DMSO are present, however, this peak
appears at −74.81 ppm and is considerably sharpened,
consistent with complete conversion of 2a into 3a (and 3b).
Complexes 2a and 3a do not interconvert rapidly with 3b at
−60 °C; however, when the spectra were recorded at room
temperature, fast exchange was observed among all three
species (2a, 3a, and 3b; Figure S13 [SI]).
Figure 8. Titration curves of DMSO into the solution of Pd(OAc)2 in
EtOAc at 24 °C. Conditions: [Pd(OAc)2] = 30 mM (4.4 mg, 0.02
mmol), EtOAc = 0.65 mL, 24 °C, [DMSO] = 0, 15, 30, 60, 120, 300,
and 600 mM.
The broad peak corresponding to 2a and 3a exhibits a
chemical shift that depends on the relative concentrations of 2a
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Figure 9. 1H NMR spectra of Pd(TFA)2 in THF-d8 with various quantities of DMSO at −60 °C. Conditions: [Pd(TFA)2] = 15 mM (3.3 mg, 0.01
mmol), THF-d8 = 0.65 mL, −60 °C, [DMSO] = 0, 7.5, 15, 19.5, 24, 30, and 60 mM.
Figure 10. 19F NMR spectra of Pd(TFA)2 in THF-d8 with various quantities of DMSO at −60 °C. Conditions: [Pd(TFA)2] = 15 mM (3.3 mg, 0.01
mmol), THF-d8 = 0.65 mL, −60 °C, [DMSO] = 0, 7.5, 15, 19.5, 24, 30, and 60 mM.
and 3a. We reasoned that with 0.5 equiv of DMSO, when 3a
has not yet started to form, the peak at 3.29 ppm in the H
NMR spectrum and −74.73 ppm in the 19F NMR spectrum
arise solely from 2a. With 4 equiv of DMSO, when 2a is
1
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1
Figure 11. Titration curve of DMSO into the solution of Pd(TFA)2 in THF-d8. (A) Pd species derived from H NMR spectra; (B) Pd species
derived from 19F NMR spectra. Conditions: [Pd(TFA)2] = 15 mM (3.3 mg, 0.01 mmol), THF-d8 = 0.65 mL, −60 °C, [DMSO] = 0, 7.5, 15, 19.5,
24, 30, and 60 mM.
1
consumed, the peaks at 3.26 ppm in the H NMR spectrum
and −74.81 ppm in the 19F NMR spectrum solely represent 3a.
The concentrations of 2a and 3a can be calculated on the basis
of the chemical shift of the merged peaks, and the
concentrations of all of the individual species are plotted as a
function of added DMSO in Figure 11. In the presence of ≥2
equiv of DMSO at −60 °C, an equilibrium mixture of the two
bis-DMSO coordinated PdII complexes, 3a and 3b, is present in
very similar concentrations.
Comparison of the Kinetics of DMSO Ligand
Exchange in EtOAc and THF-d8. The interconversion
are calculated to be 3.3 s−1 and 116 s−1, in EtOAc and THF-d8,
between 2a and 3a (eq 8) resulted in line broadening of the
respectively, on the basis of the 19F NMR spectra under these
separated peaks in EtOAc and coalescence of the peaks in
conditions.30 These results, showing that ligand exchange in
THF-d8. The rate of this process can be estimated by the peak
width at half height, when the intensities of both peaks are
equal.29 At −60 °C, the concentrations of 2a and 3a are
approximately equal when 1.3 and 1.4 equiv of DMSO are
present in EtOAc and THF-d8, respectively. The exchange rates
THF-d8 is substantially faster than in EtOAc, can be
rationalized by a solvent-catalyzed ligand exchange mechanism,
in which the better coordination ability of THF31 relative to
EtOAc leads to faster exchange.32
Figure 12. 1H NMR spectra of Pd(TFA)2 in AcOH-d4 with various quantities of DMSO at 24 °C. Conditions: [Pd(TFA)2] = 15 mM (3.3 mg, 0.01
mmol), AcOH-d4 = 0.65 mL, 24 °C, [DMSO] = 0, 15, 30, 90, 300, and 600 mM.
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The exchange between bound and unbound DMSO was
analyzed by one-dimensional (1D) NOESY saturation-transfer
experiments, commonly called 1D EXSY (exchange spectros-
copy).33 When 3 equiv of DMSO are added to Pd(TFA)2 in
EtOAc or THF-d8, the solutions contain a mixture of 3a, 3b,
and unbound DMSO. The EXSY experiments performed on
these solution at −60 °C reveal that saturation of the signal of
3b does not affect other peaks. In contrast, excitation of 3a
causes inversion of the signal for the unbound DMSO (Figures
S14 and S15, SI).34 The lack of saturation transfer of 3b to
other species indicates that the DMSO ligands on 3b are not
exchanging with the unbound DMSO on the time scale of this
experiment (eq 12). In contrast, the saturation transfer from 3a
to the unbound DMSO suggests that the DMSO ligand on 3a
undergoes fast exchange with the unbound DMSO (eq
13).These results have shown that the O-bound DMSO ligands
are kinetically more labile than S-bound DMSO ligands. The
exchange of both O- and S-bound DMSO ligands with free
DMSO has been previously observed in a solution of
[Pd(bpy)(DMSO)Cl][PF6] in CD3NO2 at 35 °C.12
suspension reveals a broad resonance in the region of free
DMSO as the major species. A mixture of broad DMSO peaks
approximately 0.6 ppm downfield of free DMSO supports the
presence of S-bound DMSO ligands coordinated to PdII.
Further characterization of this system was not pursued because
of complications associated with the heterogeneity of this
system.
Coordination of the Bidentate Sulfoxide 1,2-Bis-
(phenylsulfinyl)ethane to Pd(OAc)2 and Pd(TFA)2. Recent
studies by White and co-workers have highlighted the utility of
bidentate-sulfoxide ligands in Pd(OAc)2-catalyzed oxidation
reactions, such as allylic acetoxylation.6c,f,g In light of these
results, we briefly investigated the coordination of 1,2-
bis(phenylsulfinyl)ethane to Pd(OAc)2 and Pd(TFA)2 in
1
1
EtOAc by H NMR spectroscopy. The H resonances for the
phenyl group of the free 1,2-bis(phenylsulfinyl)ethane ligand
appear as multiplets between 7.73 and 7.57 ppm (Figure 14).
Solution Structures of Pd(TFA)2/DMSO in AcOH-d4
and Toluene-d8. Pd-catalyzed aerobic oxidation reactions
with Pd(TFA)2/DMSO catalyst systems have also been carried
out in AcOH and toluene (eqs 1 and 4). NMR spectroscopic
analyses of mixtures of Pd(TFA)2 and DMSO suggest that
DMSO coordination to PdII is not as favorable in these solvents
as in EtOAc and THF (Figure 12). Upon addition of 1 equiv of
DMSO to Pd(TFA)2 in AcOH-d4, unbound DMSO (2.77
1
ppm) is the major DMSO species, evident in the H NMR
spectrum. Smaller broad peaks present at 3.27−3.56 ppm
suggest the presence of a mixture of minor S-bound DMSO-
ligated PdII species (cf. Table 1). 19F NMR spectra obtained
from the titration experiments similarly exhibit a mixture of
different species (Figure S17, SI). Integration of the bound
DMSO peaks approaches a 1:1 DMSO:Pd stoichiometry, but
only after addition of 20 equiv of DMSO (Figure 13). Possible
Figure 14. 1H NMR spectroscopic analysis of the mixture of 1,2-
bis(phenylsulfinyl)ethane with Pd(TFA)2 and Pd(OAc)2. Conditions:
[1,2-bis(phenylsulfinyl)ethane] = 15 mM (2.7 mg, 0.01 mmol), EtOAc
= 0.65 mL, 24 °C, ([Pd(OAc)2] = 15 mM), ([Pd(TFA)2] = 15 mM).
These resonances are essentially unperturbed upon addition of
one equivalent of Pd(OAc)2 to the solution of ligand. In
contrast, substantial changes are evident when Pd(TFA)2 is
combined with the ligand (1:1 ratio) in EtOAc. FTIR spectra
were acquired for both Pd−ligand samples upon removing the
EtOAc solvent under vacuum. The IR spectra of 1,2-
bis(phenylsulfinyl)ethane and of a 1:1 Pd(OAc)2/1,2-bis-
(phenylsulfinyl)ethane mixture exhibit strong absorption
bands at 1035 cm−1, corresponding to the S−O stretching
frequency (Figures S3 and S4, SI). These observations indicate
that 1,2-bis(phenylsulfinyl)ethane does not coordinate to
Pd(OAc)2 in EtOAc (eq 14), and this conclusion is consistent
Figure 13. Titration curves of DMSO added into the solution of
Pd(TFA)2 in AcOH-d4 at 24 °C. Conditions: [Pd(TFA)2] = 15 mM
(3.3 mg, 0.01 mmol), AcOH-d4 = 0.65 mL, 24 °C, [DMSO] = 0, 15,
30, 90, 300, and 600 mM.
structures consistent with a 1:1 DMSO:Pd stoichiometry
include the monomeric and dimeric species 2a and 2b (see
above). The high freezing point of AcOH-d4 limits the utility of
variable-temperature studies to gain further insights into this
system.
with previous studies in dichloromethane and chloroform
performed by White and co-workers.6g In contrast, the 1:1
Pd(TFA)2/1,2-bis(phenylsulfinyl)ethane mixture exhibits ab-
sorption bands at 1182 and 1148 cm−1 (Figure S5, SI). These
blue-shifted bands are consistent with S-bound sulfoxide
ligation.35 No significant absorption bands are observed in
the region corresponding to O-bound sulfoxides (1100−800
Pd(TFA)2 does not dissolve readily in toluene-d8. Addition
of 2 equiv of DMSO to the suspension of Pd(TFA)2 in toluene-
d8 leads to a bright-yellow solution but does not fully dissolve
Pd(TFA)2. These observations indicate that DMSO coordi-
1
1
nates to Pd(TFA)2; however, a H NMR spectrum of this
cm−1). These IR data, together with the H NMR data, show
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that coordination of 1,2-bis(phenylsulfinyl)ethane to Pd(TFA)2
is quite favorable (eq 15).
O-Bound DMSO ligands are considerably more labile than S-
bound DMSO in Pd(TFA)2/DMSO complexes. It seems
reasonable that O- and S-bound DMSO ligands may work
cooperatively in successful catalytic reactions. For example, a
labile O-DMSO ligand might be important to enable weakly
coordinating L-type ligands such as carbonyl oxygen atoms,
alkenes, sulfonamide nitrogen atoms, and arene C−H bonds
(cf. eqs 1−4) to access the coordination sphere of PdII. This
insight might explain the lack of catalytic activity exhibited by
Pd(TFA)2 coordinated with a bidentate bis(phenylsulfinyl)-
ethane ligand in dehydrogenation and oxidative amination.7,8
This ligand chelates to PdII via the sulfur atom of the two
sulfoxides.35 On the other hand, the more-strongly coordinat-
ing S-DMSO ligand might be important in other steps of the
catalytic mechanism. For example, the present studies do not
address DMSO coordination to Pd0; however, the “softer”
character of Pd0 relative to PdII suggests that DMSO will
coordinate to Pd0 preferentially via the sulfur atom.5 Such
coordination of DMSO to Pd0 should stabilize the Pd0
intermediate by inhibiting its aggregation into Pd black and
facilitating oxidation of the catalyst by O2.We speculate that the
beneficial effect of DMSO in catalytic reactions carried out in
AcOH and toluene might reflect this interaction of DMSO with
Pd0, despite the poor coordinating ability of DMSO to PdII in
these solvents.
SUMMARY AND ANALYSIS
■
The coordination chemistry of the Pd(TFA)2/DMSO catalyst
system in various solvents highlights the complexity of DMSO
coordination to PdII and a subtle, but potentially important,
difference between the linkage−isomer coordination modes of
DMSO in solution relative to the solid state. The solid-state
structure of trans-Pd(DMSO)2(TFA)2 exhibits one S- and one
O-bound DMSO ligand, whereas our solution-state studies
suggest that the structure with two S-bound DMSO ligands is
nearly isoenergetic in EtOAc and THF (cf. Figures 6 and 13).
When only 1 equiv of DMSO is present, both the crystallo-
graphic and solution-phase spectroscopic data show that
DMSO coordinates to PdII via the sulfur atom.
The solvent identity has a significant impact on the
Pd(TFA)2/DMSO coordination chemistry. In Scheme 2, the
Scheme 2. Summary of Solution Structures of Pd(TFA)2/
DMSO in Various Solvents
EXPERIMENTAL SECTION
■
All commercially available compounds were ordered from Sigma-
Aldrich except for Pd(TFA)2, which was obtained from Strem. EtOAc
was purified by fractional distillation under N2. All samples were
prepared and experiments carried out in air.
1H and 19F NMR spectra were acquired on a Varian INOVA-500
1
MHz spectrometer. The chemical shifts (δ) of H NMR spectra are
given in parts per million and referenced to solvent, nondeuterated
CH3CO2Et (2.05 ppm) and the residual C3/C4 ethylene protons of
THF-d8 (1.73 ppm). The chemical shifts (δ) in the 19F NMR spectra
were referenced relative to the corresponding 1H spectra. Spectra were
processed with MestReNova software. Infrared spectra were obtained
with a Bruker Tensor 27 spectrometer equipped with a single
reflection MIRacle Horizontal ATR ZnSe crystal by Pike Technolo-
gies.
Preparation of Pd(S-DMSO)(O-DMSO)(TFA)2. Crystals of Pd(S-
DMSO)(O-DMSO)(TFA)2 were obtained by vapor diffusion of
hexane into an EtOAc solution of Pd(TFA)2 and 2 equiv of DMSO at
room temperature. Pd(TFA)2 (8.4 mg, 0.25 mmol) was dissolved in
0.5 mL of EtOAc, forming a deep-red solution. The solution turned
bright-yellow upon addition of DMSO (3.6 μL, 0.5 mmol). This
solution was filtered through glass wool into a 4 mL vial and then
transferred into a 15 mL vial containing 4 mL hexane. The vial was
sealed with a Teflon cap and maintained at room temperature
overnight. Deep-orange crystalline needles formed.
Preparation of the Pd(S-DMSO)(OH2)(TFA)2 Complex. Crys-
talline Pd(S-DMSO)(OH2)(TFA)2 was obtained by vapor diffusion of
pentane into an EtOAc solution of Pd(TFA)2 and 1 equiv of DMSO at
low temperature. Pd(TFA)2 (8.4 mg, 0.25 mmol) was dissolved in 0.5
mL of EtOAc to form a deep-red solution. Upon addition of DMSO
(1.8 μL, 0.25 mmol), the solution turned bright-yellow. The solution
was filtered through glass wool into a 4 mL vial, and then transferred
into a 15 mL vial containing 4 mL of pentane. The vial was sealed with
a Teflon cap and placed in refrigerator at −15 °C. After 4 days, orange
crystalline needles formed.
NMR Spectroscopy Study of Pd(TFA)2/DMSO in EtOAc.
Pd(TFA)2 (6.6 mg, 0.02 mmol) was weighed into a vial, followed by
injection of EtOAc (0.64 mL). The suspension transformed into a
deep-red solution upon sonicating for 10 min. Fluorobenzene (4 μL,
0.041 mmol) was injected as an internal standard. The solution was
transferred into an NMR tube. The spectrometer probe was precooled
major PdII complexes formed in the presence of 2 equiv of
DMSO in different solvents are highlighted in boxes. In EtOAc
and THF-d8, DMSO coordination to Pd affords an equilibrium
mixture of monomeric Pd(DMSO)2(TFA)2 linkage isomers.
DMSO coordinates less effectively to Pd(OAc)2, relative to
Pd(TFA)2, and also less effectively to Pd(TFA)2 in AcOH and
toluene, relative to EtOAc and THF-d8. In these cases, the
substoichiometric coordination of DMSO to PdII is evident
from the spectroscopic data, probably reflecting the presence of
bi- or trinuclear PdII-carboxylate species that are not fully
cleaved by DMSO.
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to the desired temperature and allowed to equilibrate for 30 min. The
sample was unlocked, and the 1H channel was used to perform
gradient shimming. A H NMR spectrum was acquired followed by
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tuning the probe for 19F and acquiring a 19F NMR spectrum.
Additional quantities of DMSO were then added from a stock solution
into the same sample. The sample was allowed to mix at room
temperature for 5 min and cooled to the desired temperature, and the
spectrum was recorded. A longer mixing time was tested and resulted
in no change of the spectrum. The volume change caused by titration
was controlled to be less than 5% over the course of the experiments.
NMR Spectroscopy Study of Pd(TFA)2/DMSO in THF-d8.
Pd(TFA)2 (3.3 mg, 0.01 mmol) was weighed into a vial. THF-d8 (0.64
mL) was injected to form a deep-red solution, and then fluorobenzene
(4 μL, 0.041 mmol) was added as an internal standard. The solution
was transferred into an NMR tube. The spectrometer probe was
precooled to the desired temperature and allowed to equilibrate for 30
min. A 1H NMR spectrum was acquired, followed by tuning the probe
for 19F and acquiring a 19F NMR spectrum. Additional quantities of
DMSO were then added from a stock solution into the same sample.
The volume change caused by titration was controlled to be less than
5% over the course of the experiments.
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This procedure was also used for the NMR spectroscopy studies of
Pd(TFA)2/DMSO in AcOH-d4 and Pd(OAc)2/DMSO in EtOAc.
NMR Spectroscopy Study of Pd(TFA)2/DMSO in Toluene-d8.
Pd(TFA)2 (3.3 mg, 0.01 mmol) was weighed out in a vial and formed
a suspension with addition of toluene-d8 (0.64 mL). Pd(TFA)2
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DMSO. Fluorobenzene (4 μL, 0.041 mmol) was injected as the
internal standard. The NMR spectra were acquired the same way as
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ASSOCIATED CONTENT
* Supporting Information
■
S
Additional spectroscopic data (IR, NMR), description of
dynamic NMR analysis, and an X-ray crystallographic data
file. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*stahl@chem.wisc.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Lara Spencer for performing X-ray structure
determinations of Figure 1. We are grateful to Dr. Charlie
Fry for training and assistance with NMR experiments.
Financial support of this work was provided by the NIH
(R01-GM100143). Instrumentation was partially funded by the
NSF (CHE-9974839, CHE-9629688, CHE-9629688, and
CHE-8813550) and the NIH (1 S10 RR13866-010). Computa-
tional resources were funded, in part, by the National Science
Foundation (CHE-0840494)
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[Pd(TFA)2] = 30 mM (6.6 mg, 0.02 mmol), [DMSO] = 39 mM (1.3
equiv), −60 °C. In THF-d8, [Pd(TFA)2] = 15 mM (3.3 mg, 0.01
mmol), [DMSO] = 21 mM (1.4 equiv), −60 °C.
(31) For studies of the coordinating ability of THF and the resulting
effect on reactions, see: Lucht, B. L.; Collum, D. B. Acc. Chem. Res.
1999, 32, 1035−1042.
(32) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals, 4th ed.; John Wiley & Sons: Hoboken, NJ, 2005.
(33) Claridge, T. D. W. High-Resolution NMR Techniques in Organic
Chemistry, 2nd ed.; Elsevier Science: Oxford, 2008.
(34) See the Supporting Information for the spectra.
(35) Crystal structures of bis-sulfoxide-ligated transition-metal
complexes exhibit S-coordination for both sulfoxides: (a) Pettinari,
C.; Pellei, M.; Cavicchio, G.; Crucianelli, M.; Panzeri, W.; Colapietro,
M.; Cassetta, A. Organometallics 1999, 18, 555−563. (b) Schaub, T.;
Diskin-Posner, Y.; Radius, U.; Milstein, D. Inorg. Chem. 2008, 47,
6502−6512.
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