Non-trivial behavior of palladium(II) acetate

Article abstract of DOI:10.1039/b502122g

Reaction of activated palladium metal with a HNO₃/acetic acid mixture produces both orange Pd₃(OAc)₆, 1, and purple Pd₃(OAc)₅(NO₂), 2. Compound 2 has a trinuclear structure derived from that of the well-known triangular complex 1 in which one acetate group has been replaced by a nitrite group which is bonded to one palladium atom by the nitrogen atom and to another Pd atom using one of the oxygen atoms. Highly pure 1 can be made by continuous removal of the nitric oxides from the reaction mixture using a flow of N2. ¹H NMR spectra of solutions of 1 in CDCl₃ and C₆D₆ show several signals of various intensities when a small amount of water is present in the deuterated solvents but only one signal when the solvents are thoroughly dried. These results are consistent with the occurrence of one or more hydrolysis processes when the solvents contain water and suggest that hypotheses about various [Pd(OAc)₂]_n aggregates that have previously been brought forward in the literature to explain the complexity of the spectrum of 1 are unnecessary, especially for nonpolar solvents. Compound 2 does not hydrolyze, and in wet or dried solvents shows a ¹H NMR spectrum that consists of five equal-intensity signals due to the five nonequivalent acetate groups. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b502122g

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F U L L P A P E R
Non-trivial behavior of palladium(II) acetate†
Vladimir I. Bakhmutov,a John F. Berry,b F. Albert Cotton,*b Sergey Ibragimovb and
Carlos A. Murillo*b
a Laboratory for Nuclear Magnetic Resonance, Department of Chemistry, Texas A&M
University, P.O. Box 30012, College Station, TX 77842-3012, USA
b Laboratory for Molecular Structure and Bonding, Department of Chemistry, Texas A&M
University, P.O. Box 30012, College Station, TX 77842-3012, USA.
E-mail: cotton@tamu.edu, murillo@tamu.edu
Received 10th February 2005, Accepted 20th April 2005
First published as an Advance Article on the web 4th May 2005
Reaction of activated palladium metal with a HNO3/acetic acid mixture produces both orange Pd3(OAc)6, 1, and
purple Pd3(OAc)5(NO2), 2. Compound 2 has a trinuclear structure derived from that of the well-known triangular
complex 1 in which one acetate group has been replaced by a nitrite group which is bonded to one palladium atom by
the nitrogen atom and to another Pd atom using one of the oxygen atoms. Highly pure 1 can be made by continuous
removal of the nitric oxides from the reaction mixture using a flow of N2. 1H NMR spectra of solutions of 1 in CDCl3
and C6D6 show several signals of various intensities when a small amount of water is present in the deuterated solvents
but only one signal when the solvents are thoroughly dried. These results are consistent with the occurrence of one or
more hydrolysis processes when the solvents contain water and suggest that hypotheses about various [Pd(OAc)2]n
aggregates that have previously been brought forward in the literature to explain the complexity of the spectrum of
1 are unnecessary, especially for nonpolar solvents. Compound 2 does not hydrolyze, and in wet or dried solvents
shows a 1H NMR spectrum that consists of five equal-intensity signals due to the five nonequivalent acetate groups.
In solution, the structure of palladium acetate has remained
Introduction
somewhat controversial. Claims have been made supporting the
Palladium(II) acetate (1) is an important substance with many
persistence of the trinuclear molecules in solution,12 while other
significant uses. It catalyses many organic reactions,1 is easily
evidence is said13 to support the formation of various aggregates
reduced by light2 or heat3 to form thin films of Pd and
1
of [Pd(OAc)2]n (n = 1, 2, 3, etc.). The H NMR spectrum of
palladium acetate in methanol has been reported14 to show a
large number of signals in the range where a single acetate signal
would be expected for the symmetrical triangular structure. This
was said to be indicative of a variety of aggregates and possibly
even the occurrence of ionic species. More recently in a paper
describing the use of palladium acetate for the preparation of a
catalyst and entitled “Too many precautions in making a catalyst
is never a loss of time: A lesson we learned at our own expense”,
a discussion was provided for the importance of the nature of
the palladium acetate in solution and the solvent used for the
preparation of catalysts.15
can produce colloids4 and nanowires.5 The primary use of 1,
however, is as a starting material for the syntheses of other Pd(II)
compounds as well as for the preparation of palladium catalysts
and their precursors.6 It is usually prepared by reaction of a slight
excess of activated palladium metal, usually made by reduction
of palladium dichloride with formic acid,7 and a deficiency of
nitric acid in acetic acid to assure the entire consumption of
nitric acid.8 In the solid state, the structure is generally trinuclear
with nearly idealized D3h symmetry, in which each of the three
palladium atoms is in a square planar environment and there are
six bridging acetate groups,9 as shown in Chart 1. The structures
of four crystalline forms having five independent molecules
show that the D3h structure is persistent and there are only
small variations in the non-bonded Pd · · · Pd distances in the
Pd3(OAc)6 molecules. This unusual structure is known in only
one other case,10 viz. Cu3[O2C(2,4,6-(Me2CH)3C6H3]6. Recently,
a structure was reported of a polymeric, water-insoluble form
(1ꢀ).11 The chain structure was determined from X-ray powder
diffraction data of a pink substance formed upon prolonged
heating of a mixture prepared by adding acetic acid to a solution
of palladium dissolved in nitric acid.
Because of our interest in metal–metal bonded dipalladium
6+ 16
5+
compounds such as those containing Pd2
and Pd2 units,17
we have recently initiated a broad study of these compounds.
Although we had a considerable amount of palladium dichloride
available, its low solubility in common organic solvents limits the
utility of PdCl2 as a starting material for the preparation of such
complexes, and we preferred to use the soluble palladium acetate.
Therefore, we sought a good method for the preparation for 1
from PdCl2. What we expected to be a trivial synthetic exercise
has led us to believe that not all of the so-called preparations of
palladium acetate indeed lead to the desired compound in good
purity. Here we report the structure of Pd3(OAc)5NO2, 2, which
has a structure similar to that of Pd3(OAc)6 except that one
acetate ion has been replaced by a bridging nitrite ion. We can
now also shed some light on the solution behavior of Pd3(OAc)6
in both wet and carefully dried NMR solvents. We also provide
a reliable and reproducible synthesis of Pd3(OAc)6 using PdCl2
as starting material.
Chart 1
Results and discussion
Although palladium(II) acetate is commercially available, there
are several recent reports describing its preparation.18 We
† In memoriam Mervin J. Bazile, Jr. (1977–2005).
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
D a l t o n T r a n s . , 2 0 0 5 , 1 9 8 9 – 1 9 9 2
1 9 8 9
©

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originally selectedthereactionofactivated Pdmetalin nitric acid
described by Zhang and coworkers,18a because it appeared to be a
straightforward, high yield method. This method is summarized
by the following pair of reactions:
PdCl2 + NaHCO2 + NaOH → Pd + CO2 + 2 NaCl + H2O (1)
3Pd + 6HNO3 + 6HOAc → Pd3(OAc)6 + 6NO2 + 6H2O (2)
The first reaction occurs as reported with rapid formation of
finely divided palladium metal. The freshly formed powder is
subsequently oxidized by a mixture of HNO3 and acetic acid,
and the product is extracted in a 1 : 2 mixture of CH2Cl2 :
hexanes. Crystals are obtained by evaporation in a stream of N2.
However, this produced a mixture of purple and orange crystals
which can be separated manually. The 1H NMR spectrum
of the orange crystals in CDCl3 was identical to that of a
commercial sample of 1 (Aldrich) and shows five resonances
of various intensities at around 2 ppm as shown in Fig. 1a.
This is not consistent with the D3h symmetry of Pd3(OAc)6 in
the solid state but is consistent with literature reports.14 The
1H NMR spectrum of the purple crystals shows five resonances
of essentially equal intensities at 1.961, 2.004, 2.045, 2.073 and
2.082 ppm (Fig. 1b). The spectrum of the bulk product from the
reaction mixture, shown in Fig. 1c, is a superposition of the two
spectra shown in Figs. 1a and 1b which indicates that these are
the only soluble species produced in the reaction. The orange
crystals are a new crystalline form of the well-known triangular
Pd3(OAc)6 molecule, crystallized with one CH2Cl2 molecule per
formula unit as interstitial solvent (Fig. 2). The purple crystals,
which have the composition Pd3(OAc)5NO2, have the structure
shown in Fig. 3. In this compound, the trinuclear motif is
preserved but one of the acetate groups is replaced by a nitrite
group (NO2−) which is bound to the palladium atom by the N
atom and one O atom. The substitution of an acetate group
breaks the D3h symmetry of 1 causing the remaining acetate
groups to be nonequivalent, thus accounting for the five peaks of
equal intensity in the 1H NMR spectrum. The formation of 2 can
be attributed to the attack of nitrogen oxides on the Pd3(OAc)6
molecule. These oxides are generated during the oxidation of Pd
powder with HNO3. To avoid the formation of Pd3(OAc)5NO2,
the brown nitrogen oxide gases must be eliminated promptly by
bubbling N2 gas through the reaction mixture. In this way, pure
Pd3(OAc)6 forms as the sole product.
Fig. 2 Molecular structure of Pd3(OAc)6 with ellipsoids drawn at the
30% probability level and hydrogen atoms removed.
Fig. 3 Molecular structure of Pd3(OAc)5(NO2) with ellipsoids drawn
at the 30% probability level and hydrogen atoms removed.
The appearance of various signals in the 1H NMR spectrum
of freshly prepared and commercial samples of 1 was puzzling.
We studied this issue by examining several 1H NMR spectra of
Pd3(OAc)6 in commercial solvents, such as CD3OD, CDCl3 and
C6D6. It was noted that the number and relative intensities of
some peaks are extremely sensitive to the nature of the solvent
but more importantly the signal for water that was always present
in the solvents CDCl3 and C6D6 was absent in solutions of 1.
Furthermore, when rigorously dry CDCl3 or C6D6 was used
to dissolve either freshly synthesized or commercial samples of
1, each spectrum displayed only one sharp signal. In CDCl3
this signal is at 2.006 ppm and corresponds to the major peak
in Fig. 1c. The signal shifts to 1.622 ppm when the spectrum
is obtained in C6D6.19 We now believe that these observations
are consistent with the occurrence of one or more hydrolysis
processes when the nonpolar solvents contain a small amount
of water (Scheme 1). Because molecular weight determinations
Fig.
1
1H NMR spectra taken at 300 MHz in solutions of
CDCl3 (as supplied by the manufacturer). (a) commercial Pd3(OAc)6,
(b) Pd3(OAc)5(NO)2, (c) “Pd3(OAc)6” prepared according to ref. 12a.
Scheme 1
1 9 9 0
D a l t o n T r a n s . , 2 0 0 5 , 1 9 8 9 – 1 9 9 2

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in benzene and dichloromethane solutions indicate that the
molecule is trinuclear,7,12 it is likely that this trinuclear core is
retained.
a mixture of acetic acid and nitric acid, no N2 flow was applied
to eliminate the brown gases formed. Concentration of the
solution by a factor of 3 was accomplished by slow evaporation
under mild heating, and cooling to room temperature yielded
an orange powder. To this powder was added a mixture of
10 ml of CH2Cl2 and 10 ml of hexanes. A small amount of solid
was eliminated after filtration. Slow evaporation of the solution
was performed by passing a constant N2 flow over it. Two
types of crystals were obtained. Orange plates were Pd3(OAc)6
and purple cubes were Pd3(OAc)5(NO2). The products were
separated manually. Yield of 2: 0.2601 g (40.0%). 1H NMR
(CDCl3, 300 MHz, d, ppm): 1.960 (s, 1H), 2.004 (s, 1H), 2.043
(s, 1H), 2.072 (s, 1H), 2.081 (s, 1H); IR (KBr, cm−1): 1608 vs,
1560 s, 1541 s, 1517 s, 1420 vs, 1351 m, 1261 m, 1197 m, 1153 w,
1098 s, 1024 s, 949 w, 867 w, 800 m, 696 m, 622 w.
It is interesting to note that a similar hydrolysis process does
not occur in the nitrito compound 2, since the five expected
resonances for the acetate groups all have equal intensity, and
no extraneous peaks are present even in the presence of small
amounts of water. A structurally analogous complex having five
acetate ligands and one oxazolone ligand around the triangular
Pd3 core also shows only five acetate resonances in its 1H NMR
spectrum.20 Thus, it appears that once one acetate ligand is
replaced by a stronger bidentate ligand, the resulting Pd3(OAc)5-
(L) complex is kinetically less labile than the Pd3(OAc)6 pre-
cursor. This would also explain why a more complex mixture
of Pd3(OAc)n(NO2)6−n species is not observed in the reaction
mixture.
The occurrence of hydrolysis is also consistent with the
known reactivity of Pd3(OAc)6 toward ligands such as amines,
phosphines and arsines leading to mononuclear complexes
such as trans-[Pd(OAc)2(PR3)2], in which the acetate groups
are monodentate.7,21 The results we report here would seem to
provide an explanation for the previously reported “anomalous”
behavior of palladium(II) acetate. We note particularly that the
hypothesis about various [Pd(OAc)2]n species in solution with n
other than 3 seems unnecessary.
X-Ray quality crystals of 2 were obtained by recrystallization
of 50 mg of the purple solid from the reaction in a mixture of
10 ml of CH2Cl2 and 20 ml of hexanes.
X-Ray diffraction analyses
A suitable crystal of 1·CH2Cl2 was placed inside a loop using
a small amount of stopcock grease, and was mounted on the
goniometer of a Bruker SMART 1000 CCD area detector
diffractometer. The crystal was centered in the goniometer, and
cooled to −60 ◦C in a stream of cold nitrogen. Geometric and
intensity data were collected using SMART software.22 The data
were processed using SAINT software,23 and corrections for
absorption were applied using the program SADABS.24 Crystal
data are presented in Table 1. Table 2 lists pertinent bond
distances and angles for 1·CH2Cl2 and 2. The positions of the Pd
atoms were determined using the Patterson method available in
the SHELX-97 software package.25 This was followed by least
squares refinement and difference Fourier synthesis to determine
the positions of the remaining heavy atoms. Hydrogen atoms
were placed in calculated positions based on a riding model.
CCDC reference numbers 262707 and 262708 for 2 and
1·CH2Cl2, respectively.
Experimental
General procedures
Unless otherwise stated, operations were carried out in air. Sol-
vents were purified using a Glass Contour purification system.
Deuterated solvents were used as received, except for samples
that were carefully dried using freshly dried molecular sieves.
Pd3(OAc)6 was purchased from Aldrich and deuterated solvents
were purchased from Cambridge Isotope Laboratories, Inc.
Physical measurements
The IR spectra were recorded on a Perkin-Elmer 16PC FTIR
spectrometer using KBr pellets. Elemental analyses were carried
out by Canadian Microanalytical Services in British Columbia,
Canada. Samples were vacuum dried prior to elemental analyses
in order to remove the interstitial solvent molecules of the
Table 1 Crystal data and structure refinement for 1·CH2Cl2 and 2
1·CH2Cl2
2
1
crystals. H and 13C NMR spectra were obtained on a VXR-
Empirical formula
Formula weight
Space group
Pd3C13H20O12Cl2
758.39
Pbcm
8.139 (2)
15.891(4)
17.500 (5)
2263.5(11)
4
2.226
2.649
0.0214, 0.0563
0.0244, 0.0583
Pd3C10H15O12N
660.43
Pna21
15.854 (4)
11.608 (3)
10.109 (3)
1860.5(8)
4
2.358
2.927
0.0280, 0.0717
0.0309, 0.0736
300 NMR spectrometer.
Synthesis of Pd3(OAc)6 (1). A mixture of sodium hydroxide
(1.0 g, 25 mmol) and HCOONa (0.80 g, 11.7 mmol) was
added to a solution of PdCl2 (0.500 g, 2.86 mmol) in 50 ml of
water. Formation of a palladium powder occurred immediately.
The suspension was stirred for 30 min to allow the palladium
particles to coagulate. The palladium was separated by filtration,
washed with acetone and dried under vacuum. The solid was
then suspended in 20 ml of glacial acetic acid, and 0.3 ml of
concentrated nitric acid was slowly added with stirring. The
resulting solution was heated to reflux for 30 min while N2 was
bubbled through the reaction mixture and the volume of the
solution was then reduced to a third of the original volume
by slow evaporation using mild heating. After cooling to room
temperature, an orange powder was isolated by filtration. Yield:
0.605 g (94.4%). 1H NMR (very dry CDCl3, 300 MHz, d, ppm):
2.006 (s); IR (KBr, cm−1): 1600 vs, 1430 vs, 1350 w, 1157 vw,
1047 vw, 951 vw, 696 m, 625 vw.
˚
a/A
˚
b/A
˚
c/A
3
˚
V/A
Z
Density (calcd.)/g cm−3
l(Mo–Ka)/mm−1
R1a, wR2b [I > 2r(I)]
R1a, wR2b (all data)
1
2
2
2 2
a R1 = RꢁFo| − |Fcꢁ/R|Fo|. b wR2 = [R[w(Fo − Fc2)2]/R[w(Fo ) ]] ,
w = 1/r2(Fo2) + (aP)2 + bP, where P = [max(0 or Fo2) + 2(Fc2)]/3.
Table 2 Selected interatomic distancesa for 1·CH2Cl2 and 2
1·CH2Cl2
2
Pd · · · Pdb
Pd–O
Pd–N
3.1678[7]
1.995[2]
—
3.103 [1]
2.011 [5]
1.949(7)
Crystals of 1 were obtained by addition of 50 mg of the
Pd3(OAc)6 synthesized as just described to a mixture of 10 ml of
CH2Cl2 and 10 ml of hexanes. After filtration, slow evaporation
of the solvent under a N2 stream produced orange plates the
following day.
a
˚
Distances given in A. Numbers in square brackets correspond to
average values. b In trinuclear forms the reported Pd · · · Pd separations
˚
are in the range of 3.081(2) to 3.203(1) A (see ref. 9) while the unique
Synthesis of Pd3(OAc)5(NO2) (2). This was prepared simi-
larly to 1 but when the palladium metal was allowed to react with
˚
distance in the polymeric form is 2.9192(1) A (see ref. 11).
D a l t o n T r a n s . , 2 0 0 5 , 1 9 8 9 – 1 9 9 2
1 9 9 1

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See http://www.rsc.org/suppdata/dt/b5/b502122g/ for cry-
stallographic data in CIF or other electronic format.
10 R. Cle´rac, F. A. Cotton, K. R. Dunbar, E. A. Hillard, M. A.
Pretrukhina and B. W. Smucker, C. R. Acad. Sci. Paris, Chim., 2001,
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Acknowledgements
We thank the National Science Foundation and the Robert
A. Welch Foundation for support. J. F. B. thanks the National
Science Foundation for a predoctoral fellowship.
15 C. Bianchini, A. Meli and W. Oberhauser, Organometallics, 2003, 22,
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1 9 9 2
D a l t o n T r a n s . , 2 0 0 5 , 1 9 8 9 – 1 9 9 2