Phosphane-free palladium-catalyzed coupling reactions: The decisive role of Pd nanoparticles

Article abstract of DOI:10.1002/(SICI)1521-3773(20000103)39:1<165::AID-ANIE165>3.0.CO;2-B

Nanosized palladium colloids, generated in situ by reduction of Pd¹¹ to Pd⁰ [Eq. (a)], are involved in the catalysis of phosphane-free Heck and Suzuki reactions with simple palladium salts such as PdCl₂ or Pd(OAc)₂, as demonstrated by transmission electron microscopic investigations.

Full text of DOI:10.1002/(SICI)1521-3773(20000103)39:1<165::AID-ANIE165>3.0.CO;2-B

COMMUNICATIONS
polystyrene derivatives is problematic), immobilizing differ-
ent ligands, and immobilizing dendritically modified ligands in
the sol ± gel process.
Phosphane-Free Palladium-Catalyzed Coupling
Reactions: The Decisive Role of Pd
Nanoparticles**
Manfred T. Reetz* and Elke Westermann
Received: August 11, 1999 [Z13861]
The number of examples of Pd-catalyzed C�C bond
forming reactions continues to rise, for example, in the area
of Heck, Suzuki, and Stille coupling reactions.[ One of the
most common catalysts is [Pd(PPh ) ], but precatalysts such as
1]
[
[
1] a) W. Keim, B. Driessen-Hölscher in Handbook of Heterogeneous
Catalysis, Vol. 1 (Eds.: G. Ertl, H. Knözinger, J. Weitkamp), Wiley-
VCH, Weinheim, 1997, pp. 231 ± 240; b) M. T. Reetz, A. Zonta, J.
Simpelkamp, Angew. Chem. 1995, 107, 373 ± 376; Angew. Chem. Int.
Ed. Engl. 1995, 34, 301 ± 303.
3
4
II
PdX in the presence of excess PPh , which reduces Pd to
2
3
0
Pd , are also used. In addition to new types of catalysts or
[2]
precatalysts such as Pd ± carbene complexes or phospha-
palladacycles,[ simple Pd salts such as PdCl or Pd(OAc) in
2] H. U. Blaser, B. Pugin in Chiral Reactions in Heterogeneous Catalysis
3]
(Eds.: G. Jannes, V. Dubois), Plenum, New York, 1995, pp. 33 ± 57, and
2
2
references therein.
the absence of stabilizing phosphane ligands are increasingly
[
[
[
3] D. Seebach, R. E. Marti, T. Hintermann, Helv. Chim. Acta 1996, 79,
[1]
being used in Heck and Suzuki reactions. Preparatively
1710 ± 1740, and references therein.
significant examples are the Jeffery system[ in which
4]
4] H. Sellner, D. Seebach, Angew. Chem. 1999, 111, 2039 ± 2041; Angew.
Chem. Int. Ed. 1999, 38, 1918 ± 1920, and references therein.
5] J. F. Biernat, P. Konieczka, B. J. Tarbet, J. S. Bradshaw, R. M. Izatt in
Separation and Purification Methods, Vol. 23(2) (Eds.: P. C. Wankat,
C. J. van Oss, J. D. Henry), Marcel Dekker, New York, 1994, pp. 77 ±
Pd(OAc) in the presence of an excess of a phase-transfer
2
‡
�
catalyst such as nBu N Cl is used, or Pd(OAc)2 in an
4
[5]
aqueous medium. In both cases aryl iodides are usually
employed as the arylating agent. In the case of the industrially
348, and references therein.
more interesting aryl bromides, Pd(OAc) (or PdCl ) with or
[
[
6] A. Choplin, F. Quignard, Coord. Chem. Rev. 1998, 180, 1679 ± 1702.
7] The CPG used was provided by Grace GmbH (In der Hollerhecke 1,
D-67547 Worms, Germany (www.grace.com)) and has the following
batch numbers: SP2-8319.01 (200 Š), SP18-9042 (500 Š).
2
2
without N,N-dimethylglycine (DMG) as an additive is cur-
[6]
rently one of the cheapest and most active catalysts. How
can such simple phosphane-free catalyst systems be under-
stood, given that it is generally assumed that phosphanes exert
[
[
8] J. N. Kinkel, K. K. Unger, J. Chromatogr. 1984, 316, 193 ± 200.
9] K. D. McMurtrey, J. Liq. Chromatrogr. 1988, 11, 3375 ± 3384.
[1]
[
[
[
[
[
[
10] a) G. L. Ellman, Arch. Biochem. Biophys. 1959, 82, 70 ± 77; b) R. D.
a stabilizing and activating effect? We report here a
mechanistic study[ supported by transmission electron mi-
croscopy (TEM) which points to the involvement of interme-
diary Pd nanoparticles as colloids in such systems.
7]
Badley, W. T. Ford, J. Org. Chem. 1989, 54, 5437 ± 5443.
11] B. S. Sproat, M. J. Gait in Oligonucleotide Synthesis. A practical
approach (Ed.: M. J. Gait), IRL Press, Oxford, 1984, pp. 83 ± 115.
12] K. V. Gothelf, I. Thomsen, K. A. Jùrgensen, J. Am. Chem. Soc. 1996,
First the Heck reaction of styrene (1; 12 mmol) with
bromobenzene (2; 10 mmol) at 1308C in N-methylpyrrolid-
inone (NMP; 10 mL) was studied. Our previously reported
catalyst system with [PdCl (C H CN) ] (1.5 mol%)/DMG as
1
18, 59 ± 64.
13] B. Schmidt, D. Seebach, Angew. Chem. 1991, 103, 1383 ± 1385; Angew.
Chem. Int. Ed. Engl. 1991, 30, 1321 ± 1323.
14] C. Girard, H. B. Kagan, Angew. Chem. 1998, 110, 3088 ± 3127; Angew.
Chem. Int. Ed. 1998, 37, 2922 ± 2959.
2
6
5
2
the precatalyst and sodium acetate (20 mmol) as the base was
used.[ As expected, essentially complete conversion with
formation of the Heck products 3 ± 5 in a ratio of 96:1:3 was
observed over 6 h [Eq. (1)].
15] When TADDOL immobilized on CPG was used without hydro-
6]
phobization poor results were obtained in the addition of Et
PhCHO.
16] The treatment with aqueous acid clearly does not cause clogging of the
pores by TiO /ZnO, but, rather, appears to have a positive effect on
reproducibility (see right part of the curves in Figure 1).
17] Reviews: a) R. Dahinden, A. K. Beck, D. Seebach in Encyclopaedia of
Reagents for Organic Synthesis, Vol. 3 (Ed.: L. A. Paquette), Wiley,
Chichester, 1995, pp. 2167 ± 2171; b) H. Yamamoto, A. Yanagisawa, K.
Ishihara, S. Saito, Pure Appl. Chem. 1998, 70, 1507 ± 1512; c) M.
Shibasaki, H. Sasai, T. Arai, Angew. Chem. 1997, 109, 1290 ± 1310;
Angew. Chem. Int. Ed. Engl. 1997, 36, 1237 ± 1256; d) S. Kobayashi,
Pure Appl. Chem. 1998, 70, 1019 ± 1026; e) K. Mikami, Pure Appl.
Chem. 1996, 68, 639 ± 644.
2
Zn to
[
[
2
Ph
+
PhBr
1
2
(1)
Ph
Ph
Ph
Ph
Ph
+
+
Ph
3
4
5
The course of the reaction was monitored by periodically
taking samples and analyzing them by gas chromatography.
Parallel to these analyses, the samples were studied by TEM.
[
*] Prof. M. T. Reetz, Dipl.-Chem. E. Westermann
Max-Planck-Institut für Kohlenforschung
Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr (Germany)
Fax : (‡ 49)208-306-2985
E-mail: reetz@mpi-muelheim.mpg.de
[
**] E.W. thanks the Fonds der Chemischen Industrie for a doctoral
stipendium. We thank Dr. M. Maase for help in the analysis of TEM
images.
Angew. Chem. Int. Ed. 2000, 39, No. 1
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000
0570-0833/00/3901-0165 $ 17.50+.50/0
165

COMMUNICATIONS
The reaction begins after an induction period of about one
[
8]
hour (Figure 1). We found that only samples that were taken
after the induction period contained Pd colloids. They have an
average particle size of 1.6 nm (Figure 2). The fact that not all
samples contain Pd nanoparticles rules out the unlikely
possibility of colloid formation following sample withdrawal.
Since the samples were directly deposited on carbon film
Figure 3. Plot of yield y versus time for the Heck reaction of ethyl acrylate
(
6) with iodobenzene (7) under Jeffery conditions at 508C in DMA;
‡
�
2 4
catalyst: Pd(OAc) (5 mol%)/nBu N Br .
without an induction period and proceeds continuously
(
Figure 3).
The TEM images of the samples that were used for GC
product analysis all showed the presence of Pd colloids with
an average particle size of 1.6 nm. It has long been known that
chemical or electrochemical reduction of transition metal
Figure 1. Plot of yield y versus time for the Heck reaction of styrene (1)
with bromobenzene (2) at 1308C in NMP; catalyst: [PdCl
1.5 mol%)/DMG.
2 6 5 2
(C H CN) ]
(
‡
�
salts in the presence of ammonium salts leads to R N X -
4
stabilized colloids.[ Recently, we achieved the size-selective
11]
‡
�
preparation of R N (RCO )-stabilized Pd colloids simply by
4
2
heating Pd salts such as Pd(NO ) or PdCl in the presence of
3
2
2
‡
�
tetraalkylammonium carboxylates R N (RCO ) at 608C
4
2
without any additional reducing agents. In this redox-con-
�
trolled process, the carboxylate anions RCO induce electron
2
transfer to Pd with formation of Pd0 as a colloid, and
II
[
12]
methane, ethane, and CO as inert side products. Hence, it
2
‡
�
is likely that under Jeffery conditions, R N X -stabilized Pd
4
colloids are formed and then function as in situ catalysts.
Indeed, in previous studies it was demonstrated that pre-
Figure 2. TEM image (left) of a typical sample of a Pd colloid formed in
the reaction of 1 with 2 in the presence of [PdCl (C CN) ]/DMG as
precatalyst. The size distribution of the nanoparticles (right) appears
unsymmetrical because the TEM instrument does not detect particles
smaller than 1 nm; d ˆ average diameter; s ˆ standard deviation; n ˆ
number of particles.
2
6
H
5
2
‡
�
formed R N X -stabilized Pd colloids are catalytically ac-
4
[13]
tive. Differences in catalytic activity of in situ generated Pd
colloids and preformed analogues may be due to different
cluster sizes, impurities, or aging processes in the case of the
preformed colloids, a hypothesis that needs to be examined in
greater detail. For example, the Heck reaction of unactivated
coated TEM grids by dip-coating without workup, the process
can be viewed as ªin situº TEM analysis. We assume that
‡
�
aryl bromides with catalysis by preformed R N X -stabilized
4
[
PdCl (C H CN) ] undergoes rapid ligand exchange with
2 6 5 2
Pd colloids gives low yields,[ whereas the coupling of 2 with
13b]
NaOAc to form Pd(OAc) , which undergoes thermolytic
2
0
6 under Jeffery conditions proceeds smoothly (2 (1 mmol); 6
decomposition at 1308C with formation of Pd in the form of a
‡
�
[
9]
(2 mmol); NEt (2.5 mmol); nBu N Cl (1 mmol); NMP/
3 4
colloid. This is corroborated by our recent observation that
molecular sieve 4 Š; 1308C/15 h: 100% conversion (81%
yield of 8) or 86% conversion (70% yield of 8) with NaOAc
as base).
thermolysis of Pd(OAc) at 1008C (3 h) in the likewise polar
2
solvent propylene carbonate (PC) quantitatively affords PC-
[
10]
stabilized Pd colloidal particles 8 ± 10 nm in size.
Such
Although the appearance of Pd colloids in the present cases
occurs simultaneously with the initiation of the Heck reaction,
this cannot be viewed as definitive proof for the actual
participation of nanoparticles in the catalytic cycle. To shed
light on this question, the reaction of a preformed Pd colloid
with the stoichiometric amount of iodobenzene (7) was
studied in the absence of an olefin.
relatively large preformed thermostable colloids are capable
of catalyzing Heck reactions.
[
10]
A similar TEM study was carried out with ethyl acrylate (6;
0 mmol) and iodobenzene (7; 5 mmol) under Jeffery con-
1
[
4]
‡
�
ditions (12.5 mmol NaOAc; 5 mmol nBu N Br ; 0.25 mmol
4
Pd(OAc)2; 5 mL dimethylacetamide (DMA); 508C/1 h;
>
95% conversion) [Eq. (2)]. The reaction starts immediately
If the hypothetical role of Pd colloids in the catalytic cycle
were correct, then oxidative insertion of Pd into the C�I bond
CO2Et
2
CO Et
+
PhI
(2)
with formation of phenylpalladium species 9 (or 10) and
Ph
concomitant dissolution of the Pd colloid would be expected
6
7
8
‡
�
[12]
(
Scheme 1). An nOct N (HCO )-stabilized Pd colloid
4 2
166
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000
0570-0833/00/3901-0166 $ 17.50+.50/0
Angew. Chem. Int. Ed. 2000, 39, No. 1

COMMUNICATIONS
+NR4
were a little smaller (1.3 nm) than those in the Heck reactions.
Control experiments showed previously that preformed Pd
colloids catalyze the Suzuki reaction.
X
X
+NR4
+NR4
X
R4N+
X
NR4
[13a]
+
7
X
PdI
Our studies point to the involvement of in situ generated
nanosized Pd colloids in three preparatively significant
phosphane-free catalyst systems based on simple Pd salts,
X
NR4
X
R4N+
+
9
X
X
R4N+
+NR4
‡
�
X
2
namely, Pd(OAc) /DMG, Pd(OAc) /(nBu) N Cl
2
2
4
and
Pd(OAc) . It is therefore likely that other phosphane-free
1
2
PdX3
3
�
NaOAc
Heck reactions and related C C bond-forming processes in
the presence of Pd salts without special ligands are also
catalyzed by nanosized Pd colloids. Indeed, this suspicion was
10
Scheme 1.
previously voiced in the case of Pd(OAc) -catalyzed Ullmann
2
reactions.[
20]
(
average particle size 2.2 nm) was allowed to react with the
calculated amount of iodobenzene (7) at room temperature
25 h) in NMP/THF (1/1) and at 708C for 2.5 h with stirring.
Although the systems described here involve colloidal
solutions of Pd nanoparticles, processes of this kind do not
belong to the traditional area of homogeneous catalysis.[
(
21]
The black color typical of Pd colloids changed to dark red.
The reaction was monitored by UV/Vis spectroscopy, which
revealed the continuous decrease of the broad plasmon band
in the range 400 ± 650 nm typical of transition metal col-
Rather, catalysis is likely to occur at defect sites, steps, and
kinks on the surface of the colloidal metal particles,[
13a, 14]
a
process that is more closely related to heterogeneous cataly-
sis.[22, 23]
[
14]
loids, as well as the gradual appearance of a new band at
50 nm. The 13C NMR spectrum at the end of the reaction
shows a set of signals in the aromatic region at d ˆ 127.5, 127.8,
4
Received: August 11, 1999 [Z13860]
[
15]
1
29.2 and 141.1, which is clearly different from the signals of
iodobenzene (d ˆ 94.7, 128.1, 130.9, 139.1). The signal at d ˆ
41.1 appears in a range which is typical for Pd-bound C atoms
[
1] a) J. Tsuji, Palladium Reagents and Catalysts: Innovations in Organic
Synthesis, Wiley, Chichester, 1995; b) A. de Meijere, F. E. Meyer,
Angew. Chem. 1994, 106, 2473 ± 2506; Angew. Chem. Int. Ed. Engl.
1
[
16]
of arylpalladium compounds. Differentiating between 9 and
the more likely anionic species of type 10 (which are known to
occur as phosphane adducts)
1
994, 33, 2379 ± 2411; c) S. Bräse, A. de Meijere in Metal-catalyzed
[
1, 17]
is not trivial. Dimeric
Cross-coupling Reactions (Eds.: F. Diederich, P. J. Stang), Wiley-
VCH, Weinheim, 1998, pp. 99 ± 166; d) A. Suzuki, J. Organomet.
Chem. 1999, 576, 147 ± 168; e) N. Miyaura, A. Suzuki, Chem. Rev.
[
18]
structures are also possible. Decisive is the proof of the
participation of the species in the actual Heck reaction. Upon
adding styrene 1 and NaOAc to the red solution at 708C, the
Heck product 3 was indeed formed (Scheme 1). In the case of
the analogous stoichiometric reaction of the preformed Pd
colloid with chlorobenzene under the same conditions no
reaction took place, that is, no dissolution of the Pd colloid
occurred, as shown by UV/Vis and TEM studies.
1
995, 95, 2457 ± 2483; f) T. N. Mitchell in Metal-catalyzed Cross-
coupling Reactions (Eds.: F. Diederich, P. J. Stang), Wiley-VCH,
Weinheim, 1998, pp. 167 ± 202; g) V. Farina, V. Krishnamurthy, W. J.
Scott, Org. React. N. Y. 1997, 50, 1 ± 652; h) V. V. Grushin, H. Alper,
Chem. Rev. 1994, 94, 1047 ± 1062; i) R. C. Larock, J. Organomet.
Chem. 1999, 576, 111 ± 124; j) G. Dyker, Chem. Ber. 1997, 130, 1567 ±
1
578; k) A. F. Littke, G. C. Fu, J. Org. Chem. 1999, 64, 10 ± 11; l) M. S.
Stephan, A. J. J. M. Teunissen, G. K. M. Verzijl, J. G. de Vries, Angew.
Chem. 1998, 110, 688 ± 690; Angew. Chem. Int. Ed. 1998, 37, 662 ± 664.
Finally, the reaction of styrene (1) with bromobenzene (2)
was investigated with two other catalyst systems. According to
TEM analyses, the conventional reaction with Pd(OAc) /
[2] a) W. A. Herrmann, C. Köcher, Angew. Chem. 1997, 109, 2256 ± 2282;
Angew. Chem. Int. Ed. Engl. 1997, 36, 2162 ± 2187; b) W. A. Herrmann,
C.-P. Reisinger, M. Spiegler, J. Organomet. Chem. 1998, 557, 93 ± 96;
c) C. M. Zhang, J. K. Huang, M. L. Trudell, S. P. Nolan, J. Org. Chem.
2
[
1]
4
PPh as catalyst (2 mol%) in NMP as solvent at 1308C did
3
not lead at any time to Pd colloid formation. The same applies
to the reaction with the phosphapalladacycle trans-di(m-
aceto)-bis[o-(di-o-tolylphosphanyl)benzyl]dipalladium as the
1
999, 64, 3804 ± 3805; d) D. S. McGuinness, K. J. Cavell, B. W. Skelton,
A. H. White, Organometallics 1999, 18, 1596 ± 1605.
3] W. A. Herrmann, V. P. W. Böhm, C.-P. Reisinger, J. Organomet. Chem.
1999, 576, 23 ± 41.
4] a) T. Jeffery, M. David, Tetrahedron Lett. 1998, 39, 5751 ± 5754; b) T.
Jeffery, Tetrahedron 1996, 52, 10113 ± 10130.
[5] a) I. P. Beletskaya, Pure Appl. Chem. 1997, 69, 471 ± 476; b) W. A.
Herrmann, C.-P. Reisinger in Aqueous-Phase Organometallic Catal-
ysis: Concepts and Applications (Eds.: B. Cornils, W. A. Herrmann),
Wiley-VCH, Weinheim, 1998, pp. 383 ± 392; c) N. A. Bumagin, V. V.
Bykov, Tetrahedron 1997, 53, 14437 ± 14450; d) J. P. Genet, M.
Savignac, J. Organomet. Chem. 1999, 576, 305 ± 317.
[
[
[
3]
precatalyst.
The use of simple Pd salts as precatalysts in the Suzuki
[
1, 5c, 19]
coupling is also increasing.
We therefore studied the
Pd(OAc) -catalyzed reaction of phenylboronic acid (11) with
2
p-bromoacetophenone (12), which afforded the coupling
product 13 in 93% yield (1 mol% Pd(OAc) ; 2.5 equiv
2
NaHCO ; DMF; 10 h/1308C) [Eq. (3)]. The TEM images
3
showed the presence of Pd nanoparticles, which in this case
[6] M. T. Reetz, E. Westermann, R. Lohmer, G. Lohmer, Tetrahedron
Lett. 1998, 39, 8449 ± 8452.
[
1, 4]
[7] Other studies concerning the mechanism
of the Heck reaction:
B(OH)2
Br
a) C. Amatore, A. Jutand, J. Organomet. Chem. 1999, 576, 254 ± 278;
b) B. L. Shaw, New J. Chem. 1998, 77 ± 79; c) F.-G. Zhao, B. M.
Bhanage, M. Shirai, M. Arai in Reaction Kinetics and the Development
of Catalytic Processes (Eds.: G. F. Froment, K. C. Waugh), Stud. Surf.
Sci. Catal., Vol. 122, Elsevier, Amsterdam, 1999, pp. 427 ± 430; d) F.-G.
Zhao, B. M. Bhanage, M. Shirai, M. Arai, J. Mol. Catal. A 1999, 142,
O
+
(3)
O
11
12
13
3
83 ± 388; e) A. F. Shmidt, A. Khalaika, Kinet. Catal. 1998, 39,
Angew. Chem. Int. Ed. 2000, 39, No. 1
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000 0570-0833/00/3901-0167 $ 17.50+.50/0
167

COMMUNICATIONS
8
03 ± 809; Kinet. Katal. 1998, 39, 875 ± 881; f) A. F. Shmidt, A.
Khalaika, L. O. Nindakova, E. Y. Shmidt, Kinet. Catal. 1998, 39,
00 ± 206; Kinet. Katal. 1998, 39, 216 ± 222; g) W. A. Herrmann, C.
Broûmer, K. Öfele, M. Beller, H. Fischer, J. Organomet. Chem. 1995,
91, C1 ± 4; h) G. T. Crisp, Chem. Soc. Rev. 1998, 27, 427 ± 436.
8] In other cases the induction period is shorter.
The Tetrakis(carbonyl)dioxoosmium(vi)
2
‡
Cation: trans-[OsO (CO) ] **
2
2
4
Eduard Bernhardt,* Helge Willner,* Volker Jonas,
Walter Thiel, and Friedhelm Aubke*
4
[
[
9] In the thermolysis of Pd(OAc)
2
, the ligand (acetate) serves as the
0
Superacidic reaction media,[1, 2] in particular the Lewis
reductant, a process which generates Pd in addition to methane,
[10, 12]
2
ethane, and CO .
superacid SbF and the conjugate Brùnsted Lewis superacid
5
[
[
10] M. T. Reetz, G. Lohmer, Chem. Commun. 1996, 1921 ± 1922.
11] a) J. Kiwi, M. Grätzel, J. Am. Chem. Soc. 1979, 101, 7214 ± 7217; b) Y.
Sasson, A. Zoran, J. Blum, J. Mol. Catal. 1981, 11, 293 ± 300; c) M.
Boutonnet, J. Kizling, P. Stenius, G. Maire, Colloids Surf. 1982, 5, 209 ±
HF/SbF , have allowed in recent years the generation of a
5
substantial number of previously unknown homoleptic metal
[
3]
carbonyl cations,
for example the octahedral cations
[M(CO)6]2 (M ˆ Fe, Ru, Os ). Thermally stable salts are
‡
[4]
[5]
2
1
25; d) N. Toshima, T. Takahashi, H. Hirai, Chem. Lett. 1985, 1245 ±
248; e) M. Boutonnet, J. Kizling, R. Touroude, G. Maire, P. Stenius,
�
11
[6]
generally obtained with the superacid anion [Sb F ] .
2
Appl. Catal. A 1986, 20, 163 ± 177; f) K. Meguro, M. Torizuka, K.
Esumi, Bull. Chem. Soc. Jpn. 1988, 61, 341 ± 345; g) J. Wiesner, A.
Wokaun, H. Hoffmann, Prog. Colloid Polym. Sci. 1988, 76, 271 ± 277;
h) N. Satoh, K. Kimura, Bull. Chem. Soc. Jpn. 1989, 62, 1758 ± 1763;
i) H. Bönnemann, W. Brijoux, R. Brinkmann, E. Dinjus, T. Joussen, B.
Korall, Angew. Chem. 1991, 103, 1344 ± 1346; Angew. Chem. Int. Ed.
Engl. 1991, 30, 1312; j) N. Toshima, T. Takahashi, Bull. Chem. Soc.
Jpn. 1992, 65, 400 ± 409; k) M. T. Reetz, S. A. Quaiser, Angew. Chem.
Observations made during the synthesis of [Os(CO) ][Sb F ]
6
2
[7]
11
2
[5]
by reductive carbonylation of either Os(SO F) , OsF6, or
3
3
OsO4[ provide the motivation for this study: In the IR
8]
spectra of the products an additional band of varied intensity
� 1
is observed at 2253 cm in addition to the intense CO
2
‡
� 1
stretching band (F ) of [Os(CO) ] at 2190 cm . Since the
1u
6
�
1
1995, 107, 2461 ± 2463; Angew. Chem. Int. Ed. Engl. 1995, 34, 2240 ±
highest intensities of the band at 2253 cm are found in
samples prepared from OsO4,[ we have modified the
reaction conditions and intend to show that the high wave-
number band can be attributed to the new cation trans-
8]
2241; l) M. T. Reetz, W. Helbig, S. A. Quaiser, U. Stimming, N. Breuer,
R. Vogel, Science 1995, 267, 367 ± 369.
[
[
12] M. T. Reetz, M. Maase, Adv. Mater. 1999, 11, 773 ± 777.
13] a) M. T. Reetz, R. Breinbauer, K. Wanninger, Tetrahedron Lett. 1996,
2
‡
[
OsO (CO) ] .
37, 4499 ± 4502; b) M. Beller, H. Fischer, K. Kühlein, C.-P. Reisinger,
2 4
W. A. Herrmann, J. Organomet. Chem. 1996, 520, 257 ± 259.
14] J. S. Bradley in Clusters and Colloids (Ed.: G. Schmid), VCH,
Weinheim, 1994, pp. 459 ± 544.
15] In addition to these four signals, another small signal appears at d ˆ
When the reaction temperature is reduced from 60 ± 908C[5]
[
[
to room temperature and available CO is limited to a modest
excess (about 1.4-fold), a yellow product is formed after
1
0 days. This product is free of [Os(CO) ][Sb F ] according
6 2 11 2
128.8, which could not be assigned.
[
[
16] G. P. Cestaric, Dissertation, Universität Düsseldorf (Germany), 1999.
17] a) C. Amatore, A. Jutand, L. Mottier, Eur. J. Inorg. Chem. 1999, 7,
to the vibrational spectrum. The only volatile product, CO , is
2
formed in a molar ratio of approximately 1:1 (based on OsO ),
4
1081 ± 1085; b) F. Paul, J. Patt, J. F. Hartwig, Organometallics 1995, 14,
3030 ± 3039; c) K. J. Klabunde, J. Y. F. Low, J. Am. Chem. Soc. 1974,
96, 7674 ± 7680; d) F. E. Goodson, T. I. Wallow, B. M. Novak, J. Am.
as estimated from the intensity of n (CO ) in the gas-phase IR
as
2
spectrum.
The new compound, tentatively identified as [OsO (CO) ]-
Chem. Soc. 1997, 119, 12441 ± 12453.
2
4
[
[
18] a) G. P. F. van Strijdonck, M. D. K. Boele, P. C. J. Kamer, J. G. de V-
ries, P. W. N. M. van Leeuwen, Eur. J. Inorg. Chem. 1999, 1073 ± 1076;
b) M. T. Reetz, G. Lohmer, R. Schwickardi, Angew. Chem. 1998, 110,
[Sb F ] , is extremely sensitive to moisture and turns black in
2 11 2
air within seconds. Upon heating in a sealed tube above 808C,
decomposition to an inhomogeneous violet material and the
release of CO are noted. All attempts to obtain single crystals
of [OsO (CO) ][Sb F ] and affect a separation from any
492 ± 495; Angew. Chem. Int. Ed. 1998, 37, 481 ± 483, and references
therein.
19] a) T. I. Wallow, B. M. Novak, J. Org. Chem. 1994, 59, 5034 ± 5037;
b) M. Moreno-Ma nÄ as, F. Pajuelo, R. Pleixats, J. Org. Chem. 1995, 60,
2
4
2 11 2
2396 ± 2397; c) D. Badone, M. Baroni, R. Cardamone, A. Iellini, U.
Guzzi, J. Org. Chem. 1997, 62, 7170 ± 7173.
[
[
20] G. Dyker, A. Kellner, J. Organomet. Chem. 1998, 555, 141 ± 144.
21] Mechanistic discussion of homogeneous versus heterogeneous cataly-
sis: a) J. Schwartz, Acc. Chem. Res. 1985, 18, 302 ± 308; b) Y. Lin, R. G.
Finke, Inorg. Chem. 1994, 33, 4891 ± 4910.
22] J. M. Thomas, W. J. Thomas, Principles and Practice of Heterogeneous
Catalysis, Wiley-VCH, Weinheim, 1997.
23] By nature our studies say nothing about the question whether the
observed 1.3 ± 1.6 nm sized Pd colloids are the actual catalysts, or
whether they function as a ªreservoirº or ªdonorº of smaller and
possibly more reactive fragments.
[*] Dr. E. Bernhardt, Prof. Dr. H. Willner
Anorganische Chemie der Universität
Lotharstrasse 1, 47048 Duisburg (Germany)
Fax: (‡49)203-379-2231
[
E-mail: willner@uni-duisburg.de
Prof. Dr. F. Aubke
[
Department of Chemistry
The University of British Columbia
Vancouver B.C., V6T1Z1 (Canada)
Fax: (‡1) 604-8222847
E-mail: aubke@chem.ubc.ca
Dr. V. Jonas
San Diego Supercomputer Center MC 0505
9
500 Gilman Drive, San Diego, CA 92093 (USA)
Prof. Dr. W. Thiel
Max Planck Institut für Kohlenforschung
Kaiser-Wilhelm-Platz 1, 45470 Mülheim/Ruhr (Germany)
[
**] Financial support by the Deutsche Forschungsgemeinschaft, the
Schweizer Nationalfonds, the Fonds der Chemischen Industrie and
the Natural Sciences and Engineering Research Council of Canada is
gratefully acknowledged. We thank Dr. G. Balzer, Universität Hann-
over, for recording 13C MAS NMR spectra.
168
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000
0570-0833/00/3901-0168 $ 17.50+.50/0
Angew. Chem. Int. Ed. 2000, 39, No. 1