Chemistry Reference
In-Depth Information
R
s
X
-
N+
N+
X
-
N+
N+
R
R
X
-
N+
R
N+
colloid
particle
N+
N+
X
-
N+
N+
X
-
N+
Fig. 10.7
Visualisation of Palladium
Nanoparticles Stabilised with
Quaternary Salts.
Gurtler & Buchwald reported another reactive
phosphane-free system where PTC was combined
with dicyclohexylamine to generate a very active
Heck system that was utilised for the synthesis of
trisubstituted alkenes [359]. Using a polymer-
supported palladium(II) catalyst enhanced with
TBAB, Buchmeiser was able to vinylate aryl chlo-
rides in high yields [360].
that is based on nanoclusters of active metals sta-
bilised by lipophilic quaternary ammonium salts. The
role of the phase-transfer agents in these systems is
dual: to extract the precursor metal anion into the
organic phase; and to secure the stability of the metal-
lic nanocrystals formed upon reduction. One of the
earliest publications on the application of a stable
organosol in catalysis was published by Narasimhan,
who applied cetyltributylphosphonium bromide
and platinum in the hydrogenation of nitroben-
zene [371]. The formation of colloidal transition
metals stabilised in organic media by lipophilic qua-
ternary ammonium salts and its application in cataly-
sis was studied by Bonnemann [372]. Reetz has
visualised the nanostructure of quaternary ammo-
nium/Pd cluster complexes using scanning tunnelling
microscopy, high-resolution transmission electron
microscopy (TEM) [373], and surface Fourier trans-
form infrared techniques [374] (Fig. 10.7). The same
group applied tetra-
n
-octylammonium bromide
(TOAB)-stabilised Ni clusters in [3 + 2] cycloaddition
reactions [375] and entrapped Pd nanoclusters in
sol-gel materials [376].
Nanocrystalline yttrium oxide was prepared using
tetraalkylammonium hydroxides [377]. Stabilisation
of nanoparticles in the aqueous phase was proposed
by Schulz [378], who utilised water-soluble
N
-alkyl-
N
-(2-hydroxyethyl)ammonium salts to protect col-
loids of Rh(0). This system proved to be highly
effective for the biphasic hydrogenation of arenes at
room temperature and one atmosphere of hydrogen
gas. The preparation of transition metal nanoclusters
and their application in catalysis was reviewed
recently by Aiken & Finke [379].
8.2 Catalysis by onium-salt-stabilised
transition metal nanoclusters
One of the most versatile catalytic ion pairs,
[(C
8
H
17
)
3
NMe]
+
RhCl
4
-
, which was formed by extrac-
tion of RhCl
3
from aqueous to organic phase by
Aliquat 336, eventually was discovered to be a het-
erogeneous catalyst. This combination catalyst, devel-
oped by Blum [361,362], was applied in single- or
two-phase systems for the hydrogenation and trans-
fer hydrogenation [363] of olefins [364], acetylenes
and even arenes [365], and also for the oligomerisa-
tion [366], cyclooligomerisation [367] and carbonyl-
ation [368] of alkynes. The performance of this ion
pair was improved further by encapsulation into
sol-gel matrices [369]. The nature of the catalytic
system in benzene hydrogenation reactions was re-
investigated by Finke
et al
. [370], who presented
convincing evidence that the true catalysts in this pro-
cess are Rh(0) nanoclusters stabilised by Aliquat 336.
Apparently the RhCl
4
/Aliquat 336 ion pair was not
stable under reductive conditions and was reduced
spontaneously to form soluble Rh(0) nanoclusters.
This conclusion suggests a new vision for PTC—the
development of a 'soluble heterogeneous catalysis'
