Facile Hydrolysis and Alcoholysis of Palladium Acetate

Article abstract of DOI:10.1002/anie.201500620

Palladium(II) acetate is readily converted into [Pd₃(μ²-OH)(OAc)₅] (1) in the presence of water in a range of organic solvents and is also slowly converted in the solid state. Complex 1 can also be formed in nominally anhydrous solvents. Similarly, the analogous alkoxide complexes [Pd₃(μ²-OR)(OAc)₅] (3) are easily formed in solutions of palladium(II) acetate containing a range of alcohols. An examination of a representative Wacker-type oxidation shows that the Pd-OH complex 1 and a related Pd-oxo complex 4 can be excluded as potential catalytic intermediates in the absence of exogenous water. When is palladium acetate not palladium acetate? Anywhere near trace amounts of water or alcohols. Palladium acetate is extremely sensitive to hydrolysis in solution and moderately so in the solid state. The resulting hydroxide complex is likely to be the true starting point for many reactions and catalytic processes that are reported as using palladium acetate.

Full text of DOI:10.1002/anie.201500620

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201500620
German Edition: DOI: 10.1002/ange.201500620
Palladium Complexes
Facile Hydrolysis and Alcoholysis of Palladium Acetate**
Robin B. Bedford,* John G. Bowen, Russell B. Davidson, Mairi F. Haddow,
Annabelle E. Seymour-Julen, Hazel A. Sparkes, and Ruth L. Webster
Abstract: Palladium(II) acetate is readily converted into
2
[
Pd (m -OH)(OAc) ] (1) in the presence of water in a range
3
5
of organic solvents and is also slowly converted in the solid
state. Complex 1 can also be formed in nominally anhyd-
rous solvents. Similarly, the analogous alkoxide complexes
2
[
Pd (m -OR)(OAc) ] (3) are easily formed in solutions of
3
5
palladium(II) acetate containing a range of alcohols. An
examination of a representative Wacker-type oxidation shows
that the Pd-OH complex 1 and a related Pd-oxo complex 4 can
be excluded as potential catalytic intermediates in the absence
of exogenous water.
W
hen considering the mechanisms associated with a syn-
2
thetic or catalytic process, for instance the transformation of
a pre-catalyst into an active species, it is critically important to
be confident of the integrity of the starting material.
Palladium(II) acetate is a ubiquitous precursor in an
extremely wide range of synthetic and catalytic transforma-
tions, yet we show herein that in very many cases what is
Scheme 1. Equilibrium between [Pd (OAc) ] and [Pd (m -OH)(OAc) ]
3 6 3 5
(1).
1
[6]
k -acetate on an adjacent palladium center. Subsequently,
Nosova and co-workers postulated, again based solely on
NMR studies, that this species was actually the m -hydroxy
complex [Pd (m -OH)(OAc) ] (1). Intrigued by this appar-
ent dichotomy, we examined the reaction of palladium acetate
with a large excess of water (10 equiv per Pd) in acetone. This
gave essentially quantitative conversion into the m -OH
2
[
1]
2
[7]
assumed to be palladium acetate is in fact not. This is
because palladium acetate is highly sensitive to moisture in
solution, reacting readily with low concentrations of water in
a range of common organic solvents, including solvents that
have been “dried” over calcium hydride, to give the structur-
ally characterized complex [Pd (m -OH)(OAc) ] (1). Sim-
ilarly, reaction with alcohols generates the alkoxide
complexes [Pd (m -OR)(OAc) ] 3.
3
5
2
1
complex 1 as determined by a H NMR spectrum of the
2
[2]
solution. Complex 1 was isolated in 86% yield as an orange
solid containing a small amount (7%) of palladium acetate. A
single-crystal X-ray crystallographic analysis unequivocally
confirmed the formulation of 1, and the molecular structure is
3
5
2
3
5
The trimeric structure of palladium acetate in solution—
2
[3]
[8]
[
Pd (m -OAc) ]—was demonstrated by Wilkinson et al.
shown in Figure 1.
3
6
while a subsequent X-ray crystallographic analysis confirmed
The main structural perturbation on replacing a bridging
acetate of the palladium acetate trimer with a m -OH ligand is
a decrease in the inter-palladium distances. Thus, although
[
4]
2
the structure in the solid state. Cotton and co-workers were
the first to notice that the trimer reacts reversibly with water
1
to generate a new species that, on the basis of H NMR
spectroscopic analysis, they assigned as the trimetallic com-
[
5]
plex 2 (Scheme 1), in which water coordinates to one of the
2
palladium atoms, converting a m -acetate ligand into a
[
*] Prof. Dr. R. B. Bedford, J. G. Bowen, R. B. Davidson,
Dr. M. F. Haddow, A. E. Seymour-Julen, Dr. H. A. Sparkes,
Dr. R. L. Webster
School of Chemistry, University of Bristol
Cantock’s Close, Bristol, BS8 1TS (UK)
E-mail: r.bedford@bristol.ac.uk
Dr. R. L. Webster
Present address: Department of Chemistry, University of Bath
Bath BA2 7AY (UK)
[
**] We thank the EPSRC for a studentship (J.G.B.), the EPSRC and GSK
for funding under the collaborative EPSRC Programme for Synthetic
Organic Chemistry with AZ-GSK-Pfizer (R.L.W.) and the NSF (grant
number 966420 to R.B.D.).
Figure 1. X-ray crystal structure of complex 1. Thermal ellipsoids set at
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201500620.
50% probability, Me-H atoms omitted for clarity.
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2
[
3
Pd (m -OAc) ] has an average inter-Pd separation of around
agent, calcium hydride. While solid samples of palladium
acetate sealed under air show no increase in the amount of
1 after 3 months, a sample left open to air for only 8 days
showed 34% conversion to 1.
The surprisingly facile solvolytic reactivity of palladium
acetate in solution is not limited to hydrolysis. By NMR
spectroscopy, we find that the equivalent m-OEt complex 3a is
formed quantitatively on dissolving palladium acetate in
3
6
[
4]
.15 ꢀ, this is reduced slightly to 3.0113(2) ꢀ for adjacent
acetate-only-bridged palladium atoms in 1, while the
Pd1···Pd1 separation is only 2.8640(3) ꢀ.
The H NMR spectrum of 1 in [D ]acetone showed three
signals at d = 1.794, 1.860, and 1.887 ppm in a 2:1:2 ratio for
the three distinct methyl environments, consistent with the
C -symmetric structure being preserved in solution. This was
1
6
[
9]
s
supported by the detection of signals indicating three distinct
environments for acetate ligands in the C NMR spectrum.
a mixture of 10% (v/v) ethanol in [D ]dichloromethane
(Scheme 2). Similarly the formation of 3a from palladium
2
1
3
Further evidence that the Pd structure of 1 is maintained in
3
solution was provided by a DOSY NMR experiment. This
gave a diffusion coefficient for 1 in [D ]acetone of 1.7 ꢁ
6
À9
2
À1
1
0
m s , similar to that of [Pd OAc ] which gave a value
of 2.0 ꢁ 10 m s under the same conditions.
3 6
À9
2
À1
[10]
Addition of acetic acid (2.2 equiv) to a 93:7 mixture of
1
and [Pd (OAc) ] in [D ]acetone (44.7 mm [Pd]) led to the
3
6
6
regeneration of some palladium acetate (1:[Pd (OAc) ] =
3
6
2
Scheme 2. Formation of m -alkoxide complexes 3. [a] Spectroscopic
7
5:25), indicating that 1 and palladium acetate are in
1
yield ( H NMR), product contains residual phenol.
equilibrium (Scheme 1), possibly via the intermediacy of 2.
Figure 2 shows the effect of adding increasing amounts of
acetate also proceeded quantitatively within 5 min on reac-
tion with 10% mixtures of ethanol in [D ]acetone, dioxane,
6
[
13]
[
D ]THF, CDCl , and [D ]toluene. Interestingly a solution
8 3 8
of palladium acetate (14.9 mm) in reagent-grade dichloro-
methane also showed a significant amount of 3a (approx-
imately 90%), as a result of the 0.2% of ethanol used as
a stabilizer for the solvent.
In an analogous manner, excess propanol, isopropanol,
isobutanol, benzyl alcohol, and phenol all reacted with
palladium acetate in dichloromethane solution to give
spectroscopically the quantitative formation of alkoxides
3
b–f (Scheme 2). These reactions can be performed in air,
except for the formation of the phenoxide complex 3 f, which
is air-sensitive. Solid samples of 3a–e could be isolated in
[
14]
a greater than 95:5 ratio of 3:[Pd (OAc) ].
The single-
3
6
crystal X-ray structure of complex 3a is shown in Figure 3 and
[8]
is very similar to that of the m-hydroxo complex 1 (Figure 1).
Figure 2. Extent of formation of 1 in representative solvents with
1
increasing water concentration, determined by H NMR spectroscopy
after 1 h.
water to palladium acetate in various solvents. It is clear that
in solvents that are miscible with water (THF, acetone, 1,4-
dioxane) the equilibria can be pushed in favor of the
essentially quantitative formation of 1. By contrast the data
obtained in solvents in which water is only sparingly soluble
(
toluene, chloroform, dichloromethane) plateau at less than
[11]
quantitative conversion.
It is important to note that in each case, the first datum for
each plot in Figure 2 was obtained from the reaction of
residual water in the solvent after it had been dried over
[12]
calcium hydride.
With the exception of toluene, some
conversion into the hydroxide 1 was seen in all of the
nominally anhydrous solvents. It is clear that solutions of
palladium acetate are highly sensitive to traces of water,
reacting more extensively and more rapidly than the drying
Figure 3. X-ray crystal structure of complex 3a. Thermal ellipsoids set
at 50% probability, Me-H and Et-H atoms omitted for clarity.
2
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The m-hydroxo and m-ethoxide complexes 1 and 3a can be
interconverted by reaction with excess ethanol and water
respectively, although the reactions are slow. Thus, 1 reacts
with ethanol (30 equiv) to give a 1:3 mixture of 1 and 3a after
sistent with a 1,2-migratory insertion process involving the
species Pd-OH.
[20]
Interestingly, in one instance, on reaction of palladium
acetate with 5-isoquinoline sulfonic acid, we obtained crystals
4
4
1
8 h, while 3a reacts with H O (30 equiv) giving 1a quanti-
of the hexapalladium m -oxo complex [{Pd (OAc) } (m -O)]
3 5 2
2
1
[8]
tatively. A H NMR spectrum of a competition reaction
between palladium acetate and 10 equivalents per palladium
(4), the crystal structure of which is shown in Figure 4.
of both water and ethanol in [D ]acetone showed quantitative
6
conversion into a 1.89:1 mixture of 1 and 3a within 30 min,
indicating that the equilibria lie in favor of the m-hydroxo
[
15]
species.
Finally, with complex 1 in hand, we turned our attention to
trying to resolve an issue that remains somewhat contentious,
namely whether or not palladium hydroxide complexes play
any role in the Wacker oxidation of alkenes. There is still
some debate as to whether the oxygen is incorporated into the
product by anti addition of an external oxygen nucleophile
À
(
H O or OH ) to the coordinated alkene or by 1,2-migratory
2
insertion of the coordinated alkene into the PdÀO bond of
[16]
a hydroxide complex (Scheme 3).
4
Figure 4. X-ray crystal structure of [{Pd (OAc) } (m -O)] (4). Thermal
3
5 2
ellipsoids set at 50% probability, H atoms omitted for clarity.
Scheme 3. Potential mechanisms for the CÀO bond-forming step in
the Wacker oxidation of alkenes.
2
Complex 4 is clearly related to the m -hydroxo complex 1,
and indeed may well form via the intermediacy of 1. The
formation of 4 proved to be highly capricious, but its
appearance in at least one reaction in the presence of
a strong acid could potentially have a bearing on Wacker
Palladium acetate can be used to good effect in the
Wacker oxidation of alkenes to ketones, using benzoquinone
[
17,18]
as an oxidant in the presence of a strong acid.
We have
[21,22]
previously shown that p-toluene sulfonic acid reacts with
oxidation.
However, when this acid was added but water
palladium acetate to give unusual palladacycles in which the
excluded from the alkene oxidation reaction in Table 1
(entry 4), very little ketone formation was detected,
effectively precluding this possibility.
[19]
toluene ring has undergone orthometallation. By contrast,
1
H NMR spectra of reactions of 1-naphthyl, 5-isoquinoline, or
3
-pyridyl sulfonic acids in toluene reveal the formation of the
In conclusion, palladium acetate should be treated as
highly moisture-sensitive in solution and moderately sensitive
in the solid state. All reactions performed in aqueous or
alcoholic media should be considered to be based on
complexes 1 and 3 respectively, not palladium acetate. Does
this matter? If the material behaves the way you want it to in
a reaction, then it is tempting to say no. However, if you are
interested in the mechanism of a process, then it is of course
crucial that your considerations are based on what is actually
present in the reaction.
2
m -OH complex 1, indicating that it can be formed under the
acidic conditions often employed in Wacker chemistry with
palladium acetate.
[17,18]
Table 1 summarizes the results of stoichiometric reactions
of palladium acetate or complex 1 with the representative
alkene, 1-octene. In the presence of water, the reaction with
palladium acetate yields 64% of the product, 2-octanone. By
contrast, the m-hydroxo complex 1 gives no product under
anhydrous conditions, whereas the same reaction repeated in
the presence of added water yields 46% of the ketone. The
data from these reactions (entries 1–3) seem to be incon-
Keywords: acetate ligands · homogeneous catalysis ·
hydrolysis · oxidation · palladium
Table 1: Oxidation of 1-octene by [Pd (OAc) ] or 1 to form 2-octanone.
3
6
[
a]
Entry
Conditions
[Pd (OAc) ], acetone : H O (7:1)
Yield [%]
[1] Nitrite ion contamination can also play an important role in the
chemistry of palladium acetate, for example see: S. E. Bajwa,
T. E. Storr, L. E. Hatcher, T. J. Williams, C. G. Baumann, A. C.
Whitwood, D. R. Allan, S. J. Teat, P. R. Raithby, I. J. S. Fairlamb,
Chem. Sci. 2012, 3, 1656.
1
2
3
4
64
0
46
7
3
6
2
1, anhydrous acetone
1, acetone : H O (7:1)
2
1, 5-isoquinoline sulfonic acid (1 equiv)
anhydrous acetone
[
2] Palladium hydroxide complexes are believed to be important
intermediates in many Suzuki cross-coupling reactions. For
a recent example with key references, see: A. R. Kapdi, G.
[
a] Yield of octanone determined by GC (mesitylene internal standard).
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.
Angewandte
Communications
Dhangar, J. L. Serrano, J. Perez, L. Garcıa, I. J. S. Fairlamb,
Chem. Commun. 2014, 50, 9859.
3] T. A. Stephenson, S. M. Morehouse, A. R. Powell, J. P. Heffer, G.
Wilkinson, J. Chem. Soc. 1965, 3632.
4] A. C. Skapski, M. L. Smart, J. Chem. Soc. D 1970, 658.
5] V. I. Bakhmutov, J. F. Berry, F. A. Cotton, S. Ibragimov, C. A.
Murillo, Dalton Trans. 2005, 1989.
(PCP)Pd-OH in the presence of water. See: G. R. Fulmer,
A. N. Herndon, W. Kaminsky, R. A. Kemp, K. I. Goldberg, J.
Am. Chem. Soc. 2011, 133, 17713.
[
[16] For a discussion and key references, see: N. M. Weliange, P. R.
Sharp, Organometallics 2012, 31, 6823.
[
[
[17] D. G. Miller, D. D. M. Wayner, J. Org. Chem. 1990, 55, 2924.
[18] For representative recent examples, see: a) B. Morandi, Z. K.
Wickens, R. H. Grubbs, Angew. Chem. Int. Ed. 2013, 52, 2944;
Angew. Chem. 2013, 125, 3016; b) G. Zhang, X. Xie, Y. Wang, X.
Wen, Y. Zhao, C. Ding, Org. Biomol. Chem. 2013, 11, 2947; c) B.
Morandi, Z. K. Wickens, R. H. Grubbs, Angew. Chem. Int. Ed.
2013, 52, 9751; Angew. Chem. 2013, 125, 9933.
[19] R. B. Bedford, M. F. Haddow, C. J. Mitchell, R. L. Webster,
Angew. Chem. Int. Ed. 2011, 50, 5524; Angew. Chem. 2011, 123,
5638.
[20] At this stage we cannot entirely exclude the possibility that in the
presence of exogenous water, complex 1 dissociates to a lower
nuclearity palladium-hydroxide species that is itself able to react
with alkenes by 1,2-insertion, however in all of the NMR
experiments undertaken, including the DOSY diffusion experi-
ments, we see no evidence in support of this suggestion.
[21] The O atom of a Pd-oxo complex could in principle directly add
to the double bond of an alkene, mimicking “oxygenase”
metalloenzyme activity. For a discussion of oxygenase-like
versus oxidase-like activity of Pd catalysts, see: S. S. Stahl,
Angew. Chem. Int. Ed. 2004, 43, 3400; Angew. Chem. 2004, 116,
3480.
[
6] Complex 2 was also proposed as the intermediate in the
hydrolysis of [Pd (OAc) ] to monomeric palladium acetate,
3
6
see: L. A. Adrio, B. N. Nguyen, G. Guilera, A. G. Livingston,
K. K. Hii, Catal. Sci. Technol. 2012, 2, 316.
7] V. Nosova, Y. Ustynyuk, L. Bruk, Inorg. Chem. 2011, 50, 9300.
8] See Table S3 in the Supporting Information for crystallographic
data.
[
[
[
9] The OH signal is not detected in acetone; seen at d = À0.99 ppm
in CDCl .
3
[
[
10] The Stokes – Einstein equation predicts a hydrodynamic radius
of approximately 4.0 ꢀ for 1 (see the Supporting Information).
For the recent application of the Stokes – Einstein equation to
Pd clusters, see: F. Armbruster, J. Meyer, A. Baldes, P. O.
3
Burgos, I. Fernꢂndez, F. Breher, Chem. Commun. 2011, 47, 221.
11] The coordinating solvents acetonitrile and dimethylformamide
give more complicated results: although complex 1 is detected,
there is no straightforward correlation with a loss of [Pd (OAc) ]
3
6
over time, suggesting other reactions are occurring, most likely
the formation of lower nuclearity solvent-coordinated com-
plexes. [Pd (OAc) ] in [D ]pyridine (14.9 mm) with [H O] =
3
6
5
2
1
4
47 mm, gave no 1 after 1 h, instead H NMR revealed a single
[22] We cannot entirely exclude the possibility that in one or more of
the NMR experiments, the species assigned as 1 may in fact be 4.
However, in all cases, the OAc peaks match (within
Æ 0.009 ppm) those of isolated 1 dissolved in the anhydrous
solvent and the DOSY NMR experiments suggest that, in
acetone, the product is a Pd rather than a Pd cluster.
signal at d = 2.07 ppm. On standing, crystals of [Pd([D ]Py) ]-
5
4
[
HCO ] were deposited, and analyzed by X-ray crystallography
3 2
(
see Supporting Information).
[
12] Solvents were stirred over CaH overnight, distilled, and stored
2
over 4 ꢀ molecular sieves (except [D ]acetone). Residual water
6
3
6
determined by Karl Fischer titration (see the Supporting
Information).
[
[
13] 3a is not stable in EtOH and undergoes rapid decomposition.
14] See the Supporting Information. With PhOH, the product was
contaminated with excess alcohol.
Received: January 22, 2015
Revised: February 23, 2015
Published online: && &&, &&&&
[
15] A preference for a Pd-OH species was detected in the
equilibrium between a (PCP)Pd-OMe pincer complex and
4
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Palladium Complexes
R. B. Bedford,* J. G. Bowen,
R. B. Davidson, M. F. Haddow,
A. E. Seymour-Julen, H. A. Sparkes,
R. L. Webster
&&&& — &&&&
Facile Hydrolysis and Alcoholysis of
Palladium Acetate
When is palladium acetate not palladium
acetate? Anywhere near trace amounts of
water or alcohols. Palladium acetate is
extremely sensitive to hydrolysis in solu-
tion and moderately so in the solid state.
The resulting hydroxide complex is likely
to be the true starting point for many
reactions and catalytic processes that are
reported as using palladium acetate.
Angew. Chem. Int. Ed. 2015, 54, 1 – 5
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!