Unusual case of catalysis by palladium clusters

Article abstract of DOI:10.1007/s11172-005-0324-2

An unusual for Pd catalysts dehydration of α-alkyl and α, α′-dialkylbenzyl alcohols PhCR′R″OH (R′ = H, Me, Et, Bu; R″ = H, Me) occurs in the presence of the palladium(I) cluster [Pd₄(CO)₄(OAc)₄] (1) in an inert atmosphere to form ethers PhCR′R″-O-CR′ R″ and water. The catalyst is an intermediate of cluster 1 reduction to Pd black, while neither the starting cluster 1, nor Pd black, which is the decomposition product, are active in the catalysis of this reaction.

Full text of DOI:10.1007/s11172-005-0324-2

Russian Chemical Bulletin, International Edition, Vol. 54, No. 3, pp. 803—806, March, 2005
803
Brief Communications
Unusual case of catalysis by palladium clusters
I. P. Stolarov,a Zh. V. Dobrokhotova,a G. N. Kryukova,b N. Yu. Kozitsyna,a
a
A. E. Gekhman,a M. N. Vargaftik,a and I. I. Moiseev
ꢀ
аN. 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, Russian Academy of Sciences,
5 prosp. Akad. Lavrent´eva, 630090 Novosibirsk, Russian Federation.
Fax: +7 (383) 234 3056. Eꢀmail: kochub@catalysis.nsk.su
An unusual for Pd catalysts dehydration of αꢀalkylꢀ and α,α´ꢀdialkylbenzyl alcohols
PhCR´R″OH (R´ = H, Me, Et, Bu; R″ = H, Me) occurs in the presence of the palladium(I)
cluster [Pd4(CO)4(OAc)4] (1) in an inert atmosphere to form ethers PhCR´R″—O—CR´R″
and water. The catalyst is an intermediate of cluster 1 reduction to Pd black, while neither the
starting cluster 1, nor Pd black, which is the decomposition product, are active in the catalysis
of this reaction.
Key words: palladium, clusters, benzyl alcohol, αꢀalkylbenzyl alcohol, α,α´ꢀdialkylbenzyl
alcohol, dibenzyl ethers, catalysis.
Aliphatic alcohols (MeOH, EtOH, PriOH) have preꢀ
viously1 been shown to be oxidized with the palladium(I)
carbonyl acetate cluster [Pd4(CO)4(OAc)4] (1) according
to the stoichiometric reaction affording Pd black, CO2,
and a set of organic products: aldehydes or ketones, acꢀ
etals, and esters (Scheme 1).
Scheme 1
In this work, we studied the reaction of cluster 1 with
alkylaromatic alcohols.
Results and Discussion
We found that the reaction of cluster 1 with benzyl
alcohol and its αꢀalkylꢀ and α,α´ꢀdialkyl derivatives
PhCR´R″OH (R´ = H, Me, Et, Bu; R″ = H, Me) in an
inert atmosphere (argon) involves two types of catalytic
R´ = H, Me; R″ = H, Me, Et
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 788—791, March, 2005.
1066ꢀ5285/05/5403ꢀ0803 © 2005 Springer Science+Business Media, Inc.

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Russ.Chem.Bull., Int.Ed., Vol. 54, No. 3, March, 2005
Stolarov et al.
Y (%)
reactions along with the stoichiometric reduction of clusꢀ
ter 1: (I) unusual for Pd catalysts alcohol dehydration (in
acidꢀfree medium) to form ether (Scheme 2, reaction (1))
or arylalkene (reaction (2)) and (II) alcohol redox disproꢀ
portionation to form aldehyde (ketone) and alkylarene
(reaction (3)).
90
80
70
60
50
40
30
20
10
1
2
Scheme 2
2 PhCR´R″OH = PhCR´R″OCR´R″Ph + H2O,
R´ = H, Me, Et, Bu; R″ = Me, Et, Bu,
(1)
(2)
(3)
3
PhCR´R″OH = PhCR=CH2 + H2O,
R´ = Me, R″ = Me,
0
20
40
60
80
100 120 t/min
Fig. 1. Curves of diꢀαꢀmethyl benzyl ether accumulation (Y )
during the Pdꢀcatalyzed dehydration of αꢀmethylbenzyl alcohol
at different temperatures: 60 (1), 45 (2), and 30 °C (3). The
solvent is αꢀmethylbenzyl alcohol, and the catalyst precursor is
cluster 1 (0.01 M).
2 PhCH(R´)OH = PhC(O)R´ + PhCH2R´ + H2O,
R´ = H, Me, Et, Bu.
The transformations of benzyl alcohol and its α,α´ꢀdiꢀ
alkyl derivatives via reactions (1)—(3) occur with a high
conversion of the starting alcohol (60—90%) when a susꢀ
pension of cluster 1 is stirred in a medium of the initial
alcohol without additives of any acid under argon at
40—60 °С (Table 1, Fig. 1). Unlike benzyl alcohols,
βꢀphenylethanol does not undergo reactions (1)—(3) unꢀ
der the conditions under study.
viously been observed under anaerobic conditions in
the presence of giant palladium clusters Pdꢀ561 and
Pdꢀ147 2—4 and Pd black.5 However, catalysis by pallaꢀ
dium compounds of aromatic alcohol dehydration to form
ethers or olefins in a medium containing no strong acid
(reactions (1) and (2)) is rather unusual and, as far as we
know, is unprecedent.
Remarkably, neither the starting cluster 1 itself, nor
Pd black, the final product of its reduction catalyze reacꢀ
tions (1) and (2). After a sample of asꢀprepared cluster 1
was added to αꢀmethylbenzyl alcohol, its light yellow
color changes rapidly to black; however, neither a conꢀ
sumption of the starting alcohol, nor a formation of the
products of its transformation were observed within the
first 5—10 min at 45 °С. At a higher temperature (60 °С)
the induction period shortens to 3—5 min, and at 35 °С
the accumulation of the reaction product becomes noꢀ
ticeable only after ~20 min; however, after ~1 h the cataꢀ
lyst completely loses its activity (see Fig. 1).
It is of interest that acetic acid, which can be formed
in small amounts as a result of cluster 1 decomposition,
exhibits no catalytic effect in reactions (1) and (2) when
the cluster is absent. Nevertheless, the addition of AcOH
(~0.1 mol L–1) to the catalyst, which had lost its activity,
again induces catalysis of the reactions in question but
with a lower rate.
Virtually the same catalytic properties but without an
induction period exhibit strongly darkened, almost black
samples of cluster 1 after short (15—20 min) heating in
dry argon to 100—110 °С or after storage in air (20 °С,
natural humidity 70—80%) for 1—3 days. Palladium black
formed after complete decomposition of cluster 1 is cataꢀ
The catalytic redox disproportionation of benzyl alcoꢀ
hol to benzaldehyde and toluene (reaction (3)) has preꢀ
Table 1. Products of transformation of benzyl alcohols in the
presence of cluster 1
Substrate
Products
Yield (%)*
PhCH2OH
PhCH2OCH2Ph
PhCHO
PhCH3
PhCH(Me)OCH(Me)Ph
PhCOMe
5
50
45
~90
~5
1
25
75
—
PhCH(Me)OH
PhC(Me)2OH
PhCH=CH2
PhC(Me)2OC(Me)2Ph
PhC(Me)=CH2
**
PhCH(Et)OCH(Et)Ph
PhCOEt
PhCH2CH2OH
PhCH(Et)OH
~90
5
PhСH2Et
1
PhCH(Bu)OH
PhCH(Bu)OCH(Bu)Ph
PhCOBu
~90
5
PhCH2Bu
1
Note. Suspension of cluster 1 (0.1 mol L–1) in the substrate (2 h,
60 °С), and the 60—90% substrate conversion.
* Of the sum of organic reaction products.
** No reaction.

Unusual case of catalysis by palladium clusters
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 3, March, 2005
805
Relative intensity (%)
100
80
60
40
20
2
1
10
20
30
40
50
60
70
80
2θ/deg
Fig. 2. Diffraction patterns of the samples of cluster 1: 1, initial sample; and 2, after heating to 200 °С under argon. Vertical lines show
reflexes characteristic of an fcc lattice of metallic palladium.
lytically inactive in reactions (1) and (2) but cataꢀ
lyzes redox disproportion of benzyl alcohol (reaction (3)).
The DTA—TGA data showed that the weight loss of
cluster 1 accompanied by a considerable endotherm beꢀ
gins at 110—120 °С, and the endotherm maximum lies
at 175 °С.
a
The Xꢀray diffraction data (Fig. 2) indicate that after
twoꢀstep heating to 110 and 140 °С (i.e., until the weight
loss was completed) the diffraction patterns of the samples
differ slightly from that of the starting cluster 1, whereas
the detected phase of metallic palladium appears only
after heating to 200 °С. This sample does not catalyze
reactions (1) and (2) but is active in redox disproportionꢀ
ation of benzyl alcohol (reaction (3)).
500 nm
The IR spectra of cluster 1 after its occurring in the
reaction mixture (30 min at 60 °С) indicate a complete
disappearance of the νCO bands (1940 and 1970 cm–1),
while the νCOO bands are considerably shifted (1418→1448
and 1550→1630 cm–1) and substantially decreased in inꢀ
tensity.
b
The electronic microphotographs of the catalytically
active cluster samples show that immediately after the
induction period the microcrystal surface of the starting
cluster 1 begins to cover with fine metallic particles
(Fig. 3, a). Thirty min after the beginning of the catalytic
reaction, only formless structures and fine nanoparticles
of metallic palladium are seen instead of microcrystals of
cluster 1 (Fig. 3, b).
The data obtained are insufficient to draw a certain
conclusion about the chemical nature of a palladium comꢀ
pound responsible for catalysis of reactions (1) and (2).
Nevertheless, it is clear that this is neither the starting
cluster 1 nor metallic palladium. It can be assumed that
500 nm
Fig. 3. Electron microphotographs of cluster 1 drawn from the
reaction mixture: a, 5 min after the beginning of the reaction at
60 °С (immediately after the end of the induction period); and
b, 30 min after the beginning of the reaction at 60 °С.
the catalyst of the reaction is a lowꢀstability palladium
compound formed due to the removal of the bridging CO

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Stolarov et al.
ligands from cluster 1, while the OAc– ligands are probꢀ
ably retained
KBr pellets or Nujol. Xꢀray diffraction analyses of catalyst
samples were carried out using an FRꢀ552 monochromator
chamber (CuKα1 radiation, germanium as internal standard).
DTA—TGA analyses were carried out on DSCꢀ20 and TGꢀ50
modules of a TAꢀ3000 thermoanalyzer (Mettler) in dry argon
with a rate of 5 deg min–1. Electron microphotographs of
nanoclusters were obtained on a JEM 2010 electron microꢀ
scope (JEOL, Japan) in a transmission mode (amplification
(0.1—2.0)•106, beam current 2—10 µA). Samples were preꢀ
pared by mechanical powder sputtering or supporting of a cataꢀ
lyst suspension in MeCN on an amorphous carbon support with
a standard copper grid.
By analogy to the kinetic scheme of positional butene
isomerization in the presence of the chloropalladium(I)
complex [Pd2Cl4]2–, which proceeds via the nonradical
chain mechanism,6,7 this reaction can be considered as a
step of generation of the catalytically active complex (cataꢀ
lytic chain initiation). Products of the catalytic reaction
appear and the catalytically active complex is generated
in the stages of catalytic chain propagation
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 05ꢀ03ꢀ
32683), the Foundation of the President of the Russian
Federation (Program for Support of Leading Scientific
Schools, Grant NShꢀ1764.2003.03), and the Russian
Academy of Sciences (Program of the Presidium of the
Russian Academy of Sciences "Target Synthesis of Subꢀ
stances with Specified Properties and Creation of Related
Functional Materials").
References
The decomposition of the catalytically active complex
[Pdn(OAc)n] terminates the chain and deactivates the
catalyst.
1. T. V. Chernysheva, T. A. Stromnova, M. N. Vargaftik, and
I. I. Moiseev, Izv. Akad. Nauk, Ser. Khim., 1996, 780 [Russ.
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2. S. L. Gladii, M. K. Starchevskii, Yu. A. Pazderskii, M. N.
Vargaftik, and I. I. Moiseev, Izv. Akad. Nauk, Ser. Khim.,
2001, 881 [Russ. Chem. Bull., Int. Ed., 2001, 50, 921].
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Vargaftik, and I. I. Moiseev, Mendeleev Commun., 2002, 1.
4. I. P. Stolarov, Yu. B. Gaugash, G. N. Kryukova, D. I.
Kochubey, M. N. Vargaftik, and I. I. Moiseev, Izv. Akad.
Nauk. Ser. Khim., 2004, 1147 [Russ. Chem. Bull., Int. Ed.,
2004, 1194].
Experimental
Benzyl, βꢀphenylethyl, and αꢀmethylꢀ, αꢀethylꢀ, αꢀbutylꢀ,
and α,α´ꢀdimethylbenzyl alcohols (all reagent grade, Aldrich)
were distilled in vacuo (1 Torr) before use. Palladium(I) carboꢀ
nyl acetate was synthesized by carbonylation of palladium(II)
acetate with carbon monoxide in acetic acid using a known
procedure.8
5. V. V. Potekhin, V. A. Matsura, and V. B. Ukraintsev,
Zh. Obshch. Khim., 2000, 70, 886 [Russ. J. Gen. Chem., 2000,
70 (Engl. Transl.)].
6. S. V. Pestrikov and I. I. Moiseev, Izv. Akad. Nauk SSSR. Ser.
Khim., 1964, 349; 1965, 1717 [Bull. Acad. Sci. USSR, Div.
Chem. Sci., 1964, 13; 1965, 14 (Engl. Transl.)].
7. I. I. Moiseev, A. A. Grigor´ev, and S. V. Pestrikov, Zh. Obshch.
Khim., 1968, 4, 354 [J. Gen. Chem. USSR, 1968, 4 (Engl.
Transl.)].
8. T. A. Stromnova, L. G. Kuz´mina, M. N. Vargaftik, G. Ya.
Mazo, Yu. T. Struchkov, and I. I. Moiseev, Izv. Akad. Nauk,
Ser. Khim., 1978, 720 [Bull. Acad. Sci. USSR, Div. Chem. Sci.,
1978, 27 (Engl. Transl.)].
αꢀMethylbenzyl alcohol (2 mL) and cluster 1 (20 mg) were
magnetically stirred in a 5ꢀmL glass flask, whose temperature
was maintained constant. Experiments were carried out at temꢀ
peratures from 40 to 60 °С. Reaction products were identified by
GCꢀMS on an Automassꢀ150 GCꢀMS spectrometer (Delsi
Nermag, France). Concentrations were determined by GLC on
a Shimadzu GCꢀ17A chromatograph.
Elemental analyses of catalyst samples were carried out on
an automated C,H,Nꢀanalyzer (Carlo Erba Strumentazione,
Italy). The palladium content was determined by the gravimetꢀ
ric method after calcination at 900 °С. IR spectra were recorded
on a Specord Mꢀ80 spectrophotometer (Carl Zeiss, Jena) in
Received January 12, 2005