New palladium nanoclusters. Synthesis, structure, and catalytic properties

Article abstract of DOI:10.1023/B:RUCB.0000042273.76439.7b

The reduction of palladium(ii) carboxylates Pd₃(OCOR) ₆ (R = Me, Et, CHMe₂, CMe₃) with hydrogen in alcohol solutions containing 1,10-phenanthroline (phen) and subsequent oxidation with oxygen gave new palladium nanoclusters, mainly particles with a nearly spherical metal core and an average size of 18 . Based on elemental analysis, NMR, X-ray photoelectron spectroscopy, and EXAFS, nanoclusters were described by the idealized formula Pd₁₄₇phen₃₂O₆₀(OCOR) ₃₀. The specimens contained up to ～25% smaller 55-atomic Pd clusters with a ～10 metal core. New nanoclusters catalyze hydrogenation of alkynes and alkenes, reduction of nitriles with formic acid, oxidation of aliphatic and benzylic alcohols, oxidative esterification of ethylene and propylene, and disproportionation of benzyl alcohol into toluene and benzaldehyde.

Full text of DOI:10.1023/B:RUCB.0000042273.76439.7b

1194
Russian Chemical Bulletin, International Edition, Vol. 53, No. 6, pp. 1194—1199, June, 2004
New palladium nanoclusters.
Synthesis, structure, and catalytic properties ✾
I. P. Stolyarov,a Yu. V. Gaugash,a G. N. Kryukova,b D. I. Kochubei,b M. N. Vargaftik,aꢀ and I. I. Moiseeva
aN. S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
31 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (095) 952 1274. Eꢀmail: mvar@igic.ras.ru
bG. K. Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences,
5 prosp. Akad. Lavrent´eva, 630090 Novosibirsk, Russian Federation.
Fax: +7 (383) 234 3056. Eꢀmail: kochub@catalysis.nsk.su
The reduction of palladium(II) carboxylates Pd3(OCOR)6 (R = Me, Et, CHMe2, CMe3)
with hydrogen in alcohol solutions containing 1,10ꢀphenanthroline (phen) and subsequent
oxidation with oxygen gave new palladium nanoclusters, mainly particles with a nearly spheriꢀ
cal metal core and an average size of 18 Å. Based on elemental analysis, NMR, Xꢀray photoꢀ
electron spectroscopy, and EXAFS, nanoclusters were described by the idealized formula
Pd147phen32O60(OCOR)30. The specimens contained up to ~25% smaller 55ꢀatomic Pd clusꢀ
ters with a ∼10 Å metal core. New nanoclusters catalyze hydrogenation of alkynes and alkenes,
reduction of nitriles with formic acid, oxidation of aliphatic and benzylic alcohols, oxidative
esterification of ethylene and propylene, and disproportionation of benzyl alcohol into toluene
and benzaldehyde.
Key words: palladium, nanoclusters, synthesis, catalytic properties.
Previously,1,2 we obtained giant palladium clusters with
the idealized formula Pd561L60(OAc)180 (1, is
Experimental
L
Solvents and reagents. nꢀPentane, nꢀhexane, and benzene
(all analytically pure grade) were purified by standard proceꢀ
dures.8 Diethyl ether (OST 84ꢀ2006ꢀ88) was kept over solid
КOH, passed through Al2O3, and distilled under argon. Ethanol
(rectified, 96%), propionic and trimethylacetic acids (Merck,
Germany), and glacial acetic acid (chemically pure grade) were
used as received.
Benzyl alcohol (analytically pure grade) and cyclohexene
(pure) were purified by fractional distillation under argon.
αꢀMethylbenzyl alcohol was prepared by a known procedure.9
оꢀPhenanthroline monohydrate C12H8N2•H2O (analytically
pure grade) was recrystallized from benzene to give an anhydrous
specimen. To remove nitrate and nitrite impurities, palladium(II)
acetate (pure grade, TU 6ꢀ09ꢀ40ꢀ2472ꢀ87) was refluxed with
freshly prepared Pd black in glacial AcOH until evolution of
nitrogen oxides ceased and recrystallized from the same solvent.
Palladium(II) propionate, isobutyrate, and pivalate were preꢀ
pared from the acetate Pd3(OAc)6 by ligand exchange with the
corresponding acids. The giant Pd561phen60(OAc)180 cluster was
synthesized from Pd3(OAc)6 and оꢀphenanthroline in AcOH
using a previously described procedure.3 The 10% Pd/C catalyst
was prepared by a known procedure.10
1,10ꢀphenanthroline (phen); 2, L is 2,2´ꢀdipyridine
(dipy)) by the reduction of palladium(II) acetate
Pd3(OAc)6 with gaseous H2 in acetic acid in the presence
of chelating heteroaromatic ligands L (phen or dipy) and
subsequent treatment with oxygen. Clusters 1 and 2 effiꢀ
ciently catalyze diverse liquidꢀphase reactions of organic
compounds (alkenes, alkyarenes, alcohols, etc.) under
mild conditions (20—60 °C, 1 atm).3
The OAc– ligands are readily replaced by other
X– anions (PF6–, O2–, BF4–, CF3COO–, etc.) upon treatꢀ
ment with the corresponding salts or acids.3—6 This proꢀ
duces no change in the [Pd561L60]180+ metal core but a
rearrangement of the icosahedral packing mode of metal
atoms to a cubooctahedral one.7
We found that the reduction of Pd3(OAc)6 and its
analogs in lower aliphatic alcohols can be used to prepare
nanoclusters with a much smaller metal core in high yields.
This communication describes the synthesis of new palꢀ
ladium nanoclusters by the reduction of palladium(II)
carboxylates Pd3(OCOR)6 (R = Me, Et, Pri, But) with
hydrogen followed by treatment with oxygen in alcohol
solutions (MeOH, EtOH, or PriOH). The chemical comꢀ
position, morphology, some structural characteristics,
and the catalytic properties of the new clusrers were
studied.
Synthesis of the cluster Pdꢀ147. Pd3(OCOBut)6 (0.772 g,
2.5 mmol) and оꢀphenanthroline (0.2253 g, 1.25 mmol) in 60 mL
of 96% EtOH were placed in a 250ꢀmL threeꢀnecked flask conꢀ
nected to two 100ꢀmL gas burettes (one filled with H2 and the
other filled with air or O2), and the mixture was magnetically
stirred at ~20 °C. The flask with the suspension was evacuated,
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1147—1152, June, 2004.
1066ꢀ5285/04/5306ꢀ1194 © 2004 Plenum Publishing Corporation

New palladium nanoclusters
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 6, June, 2004
1195
filled with gaseous H2, and saturated with hydrogen from a gas
burette with intense stirring. The absorption of H2 was first slow
and then it substantially accelerated, and the reaction mixture
turned black. After 85—90 mL of H2 (1.4—1.5 mol per mol
of Pd) had been absorbed, stirring was stopped and the flask was
evacuated and filled with air or O2. The solution was poured in
240 mL of Et2O and the black precipitate was separated by
centrifugation at 3000 rpm, washed with ether (3×50 mL, cenꢀ
trifugation), and dried in vacuo. The yield of the Pdꢀ147
nanocluster was 0.34 g (64% based on Pd). The Pdꢀ147 clusters
containing other ligands (MeCOO, EtCOO, and PriCOO) were
prepared in a similar way starting from the corresponding
palladium(II) salt.*
mole of H2/mole of Pd
2.0
1.5
1.0
0.5
1
3
2
4
0
Catalytic experiments were carried out under static condiꢀ
tions. A 5ꢀmL twoꢀnecked flask provided with a jacket for circuꢀ
lating a thermostating liquid, equipped with a magnetic stirrer
and a sampler, and connected to a 100 mL gas burette was used
as the reactor. In a typical experiment, the reactor was charged
with 3 mL of the solvent, 5—30 mg of the cluster or 15 mg of
Pd/C, and 50 mg of a substrate. The experiments were carried
out at temperatures of 20 to 70 °C. To monitor the course of the
reactions, samples of the reaction solution were taken out at
certain intervals with a 1ꢀµL microsyringe and analyzed by GLC.
The reaction products were identified by GC/MS analysis.
Methods of analysis. The elemental C,H,Nꢀanalysis of the
complexes was carried out on a Carlo Erba Strumenzione autoꢀ
mated C,H,Nꢀanalyzer (Italy). The palladium content in the comꢀ
plexes was determined by gravimetry after calcination at 900 °C.
The GC/MS analysis of the reaction products was carried
out on an Automass 150 instrument (Delsi Nermag, France),
and GLC analysis was done on a Varian 3600 chromatograph
(USA) (OVꢀ1 universal capillary column). NMR spectra were
recorded on a Bruker WPꢀ200 spectrometer, and IR spectra
were measured on a Specord M80 instrument (Carl Zeiss, Jena)
in KBr pellets. The electron micrographs of the nanoclusters
were obtained on a Jeol JEM 2010 electron microscope (Japan)
in the transmission mode with a magnification of (0.5—2.0)•106
using the lowest electron beam current (≤10 µA) to minimize
heating of the sample during recording. The samples were preꢀ
pared by deposition of a solution of the cluster on an amorphous
carbon support with a standard copper grid. The palladium
Кꢀedge EXAFS spectra were recorded on an EXAFS spectromꢀ
eter at the Siberian Center for the Synchrotron Radiation
(Novosibirsk) in the transmission mode, at an electron energy in
the storage ring of 2 GeV, and an average current of 80 mA. The
oscillating part of the Xꢀray photoelectron spectrum was exꢀ
tracted using the VIPER program.11 The spectra were processed
according to the EXCURV92 program.12 XꢀRay photoelectron
spectra were recorded on a photoelectron analyzer included in a
Riber LAS 3000 instrument set (France).
20
40
60
80
t/min
Fig. 1. Curves for H2 absorption during the reduction of
palladium(II) carboxylates Pd3(OCOR)6 (R = But (1), Me (2),
Et (3), Pri (4)) in the presence of 1,10ꢀphenanthroline in EtOH.
(R = Pri), trimethylacetate (R = But)) react with gaseous
H2 in alcohol solutions containing 1,10ꢀphenanthroline
with an initial PdII : phen ratio from 1 : 1 to 0.25 : 1, the
solution acquires a dark brown color but remains homoꢀ
geneous, and the amount of absorbed H2 markedly exꢀ
ceeds 1 mole per mole of Pd (Fig. 1), i.e., the value
needed to reduce PdII to Pd0 according to the reaction
Pd3(OCOR)6 + 3 H2 = 3 Pd0 + 6 RCOOH.
(1)
This implies that the reaction of phenanthroline carbꢀ
oxylate complexes of palladium(II) with H2 is not limꢀ
ited to the reduction of PdII to Pd0 but is accompanied by
the formation of hydride complexes of lowꢀvalence pallaꢀ
dium, similar to the polynuclear hydride complexes
[Pd4phen(OAc)2Hx]n (n ≈ 100), detected previously13
upon the reduction of Pd3(OAc)6 with hydrogen in AcOH.
The reaction rates and the amounts of absorbed hydrogen
are different for different carboxylates (see Fig. 1).
On contact with air or O2, oxygen absorption (Fig. 2)
and oxidation of the hydride complexes take place; durꢀ
ing this period, the solution remains homogeneous and
the darkꢀbrown, nearly black color virtually does not
change. The amount of absorbed oxygen markedly exꢀ
ceeds that necessary for oxidation of all the hydride ligands
mole of O2/mole of Pd
0.8
0.6
0.4
0.2
Results and Discussion
Synthesis of nanoclusters
When palladium carboxylates Pd3(OCOR)6 (ROCO is
acetate (R = Me), propionate (R = Et), isobutyrate
0
5
10
t/min
* The H2 absorption during the synthesis should not exceed
1.5 moles per mole of Pd; when 2 moles per mole of Pd are
absorbed, the yields of nanoclusters sharply decrease.
Fig. 2. Curve for O2 absorption by a Pd3(OAc)6—phen (2 : 1)
solution in EtOH after preliminary treatment with hydrogen.

1196
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 6, June, 2004
Stolyarov et al.
Table 1. Elemental analysis data for phenanthrolineꢀcontaining nanoclusters prepared from Pd3(OAc)6
and Pd3(OCOBut)6 in EtOH at different ratios of component concentrations ([phen]0 : [Pd]0)
Starting
complex
Experiment
Found (%)
Calculation
[phen]0 : [Pd]0
Calculated (%)
Pd
С
N
H.
Pd
С
N
H
Pd3(OAc)6
1
44.16 33.29 6.13 1.27
50.15 28.73 5.10 0.90
59.20 23.61 4.19 0.51
69.95 17.55 3.73 0.14
40.20 34.56 5.29 2.25
50.75 27.98 4.58 1.66
56.03 24.67 4.23 1.37
61.67 21.06 3.83 1.05
Pd561phen60(OAc)180
15.99 2.07
Pd147phen32(OAc)30O60
19.34 3.71 1.43
Pd147phen32(OCOBut)30O60
0.75
0.50
0.25
1.25
0.75
0.50
0.25
73.59
64.80
1.26
Pd3(OCOBut)6
61.58
25.23
3.53
2.07
(and/or coordinated molecular hydrogen) to give water.
According to GLC analysis, some of the oxygen is conꢀ
sumed for palladiumꢀcatalyzed oxidation of the solvent
(EtOH) to acetaldehyde.
Upon the addition of nonpolar solvents (Et2O, 1,4ꢀdiꢀ
oxane), the Pd nanoclusters thus formed precipitate, and
the precipitate is separated by centrifugation. The elemenꢀ
tal composition of the resulting black amorphous powder
changes depending on the initial Pd : phen ratio, the
nature of the carboxylate ligand, and the amount of H2
absorbed at the first stage of the synthesis.
100 Å
According to elemental analysis data (Table 1), the
content of palladium in any of the obtained specimens is
substantially lower, while that of organic ligands is much
higher than those in the giant Pdꢀ561 clusters obtained
previously.
Fig. 3. TEM image of a specimen of the phenanthrolineꢀconꢀ
taining palladium nanocluster prepared from Pd3(OCOBut)6 in
an alcohol solution (transmission mode, magnification ×650000).
The radial distribution curves of atoms calculated from
the EXAFS data show one palladium—light atom interꢀ
atomic distance (2.01 Å) and only one metal—metal inꢀ
teratomic distance (2.70 Å), which virtually coincides
with the Pd—Pd distance in the bulk metal and in the Pdꢀ
561 clusters with the closeꢀpacked metal core.4 This proꢀ
vides the conclusion that the specimens obtained are charꢀ
acterized by close packing of palladium atoms whose oxiꢀ
dation state is close to zero.
Highꢀresolution transmission electron microscopy
showed that the specimens contain metal particles of
nearly spherical shape (Fig. 3). The distribution curves of
particles over the metal core sizes have two local maxima
at ∼10 and ∼18 Å (Fig. 4).
%
Pd147
30
25
20
15
10
5
Pd55
On the basis of close packing of metal atoms and the
Pd—Pd interatomic distance (2.70 Å), one can estimate
the numbers of palladium atoms in the metal core of the
cluster particles corresponding to both maxima in the size
distribution curve (see Fig. 4). In the case of icosahedral
or faceꢀcentered cubic packing typical of palladium
nanoclusters, these values nearly coincide with those exꢀ
pected for the 12ꢀvertex Chini clusters14 with "magic"
numbers of palladium atoms, 55 and 147.15,16
0
0.5
1.0
1.5
2.0
2.5
d/nm
Fig. 4. Diameter distribution (d) of metal particles of the
phenanthrolineꢀcontaining palladium nanocluster prepared from
Pd3(OCOBut)6 in an alcohol solution according to transmission
electron microscopy.
Since the mass of a 147ꢀatomic cluster is almost three
times as great as that of a 55ꢀatomic species, one can
conclude that the obtained samples contain ≥75% of larger

New palladium nanoclusters
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 6, June, 2004
1197
Table 2. XꢀRay photoelectron spectroscopy data of palladium
nanoclusters
tive in the hydrogenation of phenylacetylene and styrene
than the giant Pdꢀ561 clusters or the traditional Pd/C
catalysts (Table 3). This may be due to the fact that the
activity of metallic catalysts increases with an increase in
the metal dispersity and with a decrease in the nanoꢀ
particle size.
Reduction of nitriles with formic acid. Previously,20 it
has been found that in the presence of giant Pdꢀ561 clusꢀ
ters, MeCN is reduced to Et3N and NH3 on treatment
with HCOOH under mild conditions. Our experiments
showed that the activity of Pdꢀ147 is much lower (see
Table 3). Unlike the reaction with Pdꢀ561, that in soluꢀ
tions of Pdꢀ147 starts after an induction period. Apparꢀ
ently, the Pdꢀ147 nanocluster is only the precursor of a
catalyst whose nature is still unknown. Heterogeneous
catalysts, Pd/C and Pd black, are virtually inactive in this
reaction.
Sample
Еb (Pd 3d5/2)
Pd0
Pd2+
Pdꢀfoil
335.8
336.5
335.8
336.0
—
—
337.5
337.5
Pd561phen60(OAc)180
Pd561phen60О60(PF6)60
Pd147
337.4—337.8
337.6
PdO
(18 5 Å) clusters, while the fraction of smaller (10 3 Å)
clusters does not exceed 25%. As in the case of giant
Pdꢀ561 clusters,17 individual components cannot be isoꢀ
lated from a polydisperse specimen of nanoclusters by
either fractional precipitation or column chromatography
on silica gel.
It follows from the ratio of integral intensities of the
proton signals for the phen and ButCOO groups in the
1H NMR spectrum of new nanoclusters that the number
of the ButCOO groups it contains is close to the number
of the phen ligands. On the basis of the elemental analyꢀ
sis (see Table 1), electron microscopy (see Figs. 3
and 4), EXAFS, and NMR data, the most representative
(∼147ꢀatomic) cluster molecules can be roughly described
by the idealized formula Pd147phen32(OCOR)30O60. Note
that a palladium nanocluster with an icosahedral metal
core, Pd145(CO)x(PEt3)30, and with a similar size has reꢀ
cently18 been prepared and characterized by singleꢀcrysꢀ
tal Xꢀray diffraction.
XꢀRay photoelectron spectra (Table 2) showed that
the energy of the main maximum, Pd 3d5/2, is virtually
the same for specimens of the giant and new clusters and
also for the massive metal (335.8—336 eV) and correꢀ
sponds to palladium in a zero oxidation state. In addition,
the spectra of new clusters also exhibit a small shoulder
with the binding energy Eb = 337.4—337.8 eV, which
corresponds to Pd atoms in a higher oxidation state close
to +2. Since the nanoclusters contain negatively charged
RCOO– (and, probably, O2–) ligands, the metal core has
a positive charge, which should be concentrated in a surꢀ
face layer of the metallic particle.19 Due to the relatively
high positive charge of palladium atoms located in the
surface layer of the metal polyhedron, nanoclusters can
behave as electrophiles and catalyze reactions that are
usually carried out in the presence of Lewis acids.
Oxidative dehydrogenation of cyclohexene. Under anꢀ
aerobic conditions, metallic palladium and Pd/C efficiently
catalyze the redox disproportionation of cyclohexene and
cyclohexaꢀ1,3ꢀdiene to give cyclohexane and benzene:21
3 С6H10
3 С6Н8
2 C6H12 + C6H6,
С6Н12 + 2 С6Н6.
The giant Pdꢀ561 cluster exhibits a very low activity,
the yield of benzene being somewhat higher than the yield
of cyclohexane. In our experiments with Pdꢀ147, disproꢀ
portionation of cyclohexene did not take place under
anaerobic conditions. The reaction products were found
to contain no cyclohexane and only a small amount of
benzene, not exceeding the amount of palladium introꢀ
duced in the solution. Apparently, the reaction represents
stoichiometric oxidation of cyclohexene with the pallaꢀ
dium atoms of the Pdꢀ147 cluster whose formal oxidation
state is close to +1. According to calculations,19 the posiꢀ
tive charge of the cluster metal core is mainly concenꢀ
trated in its outer layer (in the idealized structure, it conꢀ
tains 92 of the 147 atoms), so that the formal charge of
these atoms is about +1.5.
Redox disproportionation of benzyl alcohol. Recently,
it has been found that giant Pdꢀ561 clusters 22,23 and colꢀ
loidal palladium24 catalyze a rather unusual reaction, reꢀ
dox disproportionation of benzyl alcohol to give toluene,
benzaldehyde, and water:
2 PhCH2OH
PhMe + PhCHO + H2O.
Our experiments showed that the Pdꢀ147 nanoclusters
efficiently catalyze this reaction at 60 °C under argon (see
Table 3). We found that similar redox disproportionation
readily occurs also in the case of αꢀalkylbenzyl alcohols to
give the corresponding ketones and alkylbenzenes.
Catalytic activity of nanoclusters
To estimate the catalytic properties of new nanoꢀ
clusters, we studied their activity in reactions that are
known to proceed efficiently with catalysts based on meꢀ
tallic palladium and giant clusters.
Hydrogenation of alkenes and alkynes. Our experiments
showed that the Pdꢀ147 nanoclusters are much more acꢀ
2 ArCH(R)OH
R = H, Me, Et
ArCH2R + ArC(O)R + H2O,

1198
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 6, June, 2004
Stolyarov et al.
Table 3. Catalytic activity of palladium nanoclusters
Reaction
Catalyst
T/°C
w0 /mol (mol Pd h)–1
a
Hydrogenation of alkenes and alkynes
PhCH=CH2 + H2 → PhCH2Me
PhC≡CH + 2 H2 → PhCH2Me
Pdꢀ147
Pdꢀ561
Pd/C
Pdꢀ147
Pdꢀ561
Pd/C
20
20
20
20
20
20
242
155
25
360
180
55
Reduction of nitriles with formic acid
3 MeCN + 6 HCOOH → Et3N + 6 CO2 + 2 NH3
Pdꢀ147
Pdꢀ561
20
20
1.3
4.7
Redox disproportionation of benzyl alcohol b
2 PhCH2OH → PhCHO + PhCH3 + H2O
Pdꢀ147
Pdꢀ561
Pd/C
60
60
60
24
21
128
Oxidation of alcohols c
Pdꢀ147
PriOH + O2 → Me2CO + H2O
70
70
4
8
Pdꢀ561
Oxidative esterification of alkenes d
C2H4 + 1/2 O2 + AcOH → CH2=CHOAc + H2O
Pdꢀ147
Pdꢀ561
Pdꢀ147
Pdꢀ561
60
60
60
60
0.33
1.10
2.77
2.30
C3H6 + 1/2 O2 + AcOH → C3H5OAc + H2O
a
The rate of accumulation of the reaction products.
b The solvent was PhCH2OH, argon atmosphere.
c The solvent was PriOH, 1 atm of O2.
d The solvent was AcOH, 0.33 atm of O2.
However, an αꢀphenylꢀsubstituted benzyl alcohol,
benzhydrol, does not enter into this reaction, which may
be due to steric reasons.
Oxidative acetoxylation of ethene and propene. Our exꢀ
periments showed that in solutions of the Pdꢀ147 nanoꢀ
clusters in AcOH, diglyme, or MeCN (in the last two
solvents, 10% AcOH additive was used), ethene and proꢀ
pene undergo oxidative esterification at 20—60 °C and at
1 atm of O2 to give the corresponding alkenyl esters; howꢀ
ever, the reaction rate is ∼3 times lower in solutions of the
Pdꢀ147 clusters than in the presence of Pdꢀ561 (Table 4).
In solutions of the Pdꢀ561 and Pdꢀ147 clusters in benꢀ
zyl alcohols, disproportionation occurs without an inducꢀ
tion period; however, in the initial period, an amount of
carbonyl compound excessive with respect to the reaction
stoichiometry is produced. This can be explained by asꢀ
suming that redox disproportionation is accompanied by
parallel oxidation of benzyl alcohols due to the reduction
of Pd+1 atoms in the cluster to Pd0, similarly to that obꢀ
served in the reaction involving cyclohexene (see above).
Oxidation of aliphatic alcohols. The giant Pdꢀ561 clusꢀ
ter efficiently catalyzes partial oxidation of alcohols with
oxygen under mild conditions:3
C2H4 + O2 + AcOH
H2C=CHOAc + H2O
H2C=CHCH2OAc + H2O
C3H6 + O2 + AcOH
H2C=C(Me)OAc + H2O
Apart from allyl acetate, the transformation of proꢀ
pene gives isopropenyl acetate, whose yield changes deꢀ
MeOH + O2
EtOH + O2
PriOH + O2
HCOOMe + CO2,
pending on the partial pressure of O2. As PO increases
2
MeCHO + MeCH(OEt)2 + AcOEt,
Me2CO + Me2C(OPri)2.
from 0.09 to 0.33 atm, the selectivity of the reaction with
respect to allyl acetate decreases from 94 to 53—60% in a
solution of the Pdꢀ147 cluster and from 98 to 77% in
solutions of Pdꢀ561. Another reaction product is isoꢀ
propenyl acetate. This change in the reaction selectivity
can be explained by the destruction of nanoclusters under
the action of O2 to give PdII complexes, which oxidize
propene to isopropenyl acetate:25,26
The oxidation of PriOH in solutions of the Pdꢀ147
cluster at 50 °C starts after a long (∼1 h) induction period.
In the transient section of the kinetic curve, the Pdꢀ147
cluster is much more active than Pdꢀ561, but the steadyꢀ
state rates of the catalytic reaction are equal in both cases.
Palladium supported on the BAU carbon or Al2O3 exhibꢀ
its no catalytic activity under these conditions.
C3H6 + PdOAc2
H2C=CH(Me)OAc + AcOH + Pd0.

New palladium nanoclusters
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 6, June, 2004
1199
Table 4. Competitive oxidative esterification of ethene and proꢀ
pene with acetic acid (MeCN + 10% AcOH as the solvent,
0.33 atm of O2, 60 °C)
4. M. N. Vargaftik, I. I. Moiseev, D. I. Kochubey, and K. I.
Zamaraev, Faraday Discuss., 1991, 92, 13.
5. I. I. Moiseev and M. N. Vargaftik, in Catalysis by Diꢀ and
Polynuclear Metal Cluster Complexes, Ed. R. D. Adams and
F. A. Cotton, WileyꢀVCH, New York, 1998, 395.
6. I. I. Moiseev and M. N. Vargaftik, Zh. Obshch. Khim., 2002,
72, 550 [Russ. Chem. Bull., 2002, 72 (Engl. Transl.)].
7. V. P. Zagorodnikov, M. N. Vargaftik, D. I. Kochubei, A. L.
Chuvilin, S. G. Sakharov, and M. A. Maifat, Izv. Akad.
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Substrate
Catalyst
w0*/mol (at Pd h)–1
Vinyl Isoproꢀ Allyl
acetate penyl acetate
acetate
Propene
Propene
Pdꢀ147
Pdꢀ561
—
—
0.30
0.29
0.84
0.22
0.02
0.02
0.82
2.95
0.50
1.72
Ethene—propene (1 : 1) Pdꢀ147
Ethene—propene (1 : 1) Pdꢀ561
* The rate of accumulation of the reaction products.
The oxidation of ethene with oxygen in solutions of
the Pdꢀ147 cluster is slower than that of propene by alꢀ
most an order of magnitude (see Table 4). However, in the
presence of ethene, both the rate and the regioselectivity
of propene oxidation sharply change. The introduction of
an equimolar amount of ethene terminates almost comꢀ
pletely the formation of isopropenyl acetate, but does not
affect the oxidation of propene into allyl acetate.
The competitive oxidation of propene and ethene gives
allyl acetate (91%) and vinyl acetate (9%) as the major
products. This result can by no means be attributed to the
formation of PdII complexes due to the O2ꢀinduced cluster
destruction. Our experiments showed that isopropyl acꢀ
etate is almost the only product of the stoichiometric reacꢀ
tion of PdII acetate with an equimolar mixture of C2H4 and
C3H6. Therefore, it can be concluded that inhibition of the
vinylic oxidation of propene by ethene is due to the fact
that oxidative replacements of the H atoms at the vinyl and
allyl positions of the olefin molecules occur on different
active sites of the cluster and follow different mechanisms.
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Vargaftik, Mendeleev Commun., 1997, 7, 1.
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Chichester, 1995.
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Starchevsky, Yu. A. Pazdersky, I. P. Stolarov, M. N.
Vargaftik, and I. I. Moiseev, Adv. Synth. Catal., 2002, 344, 957.
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This study was supported by the Russian Foundation
for Basic Research (Project No. 02ꢀ03ꢀ32853), President
of the Russian Federation (Program for the Support of
Leading Scientific Schools, NSh 1764.2003.03), and the
Presidium of the RAS (Program "Targeted Synthesis of
Compounds with Specified Properties and Development
of Functional Materials Based on Them")
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Received December 17, 2003