Synthesis of Well-defined Solid Catalysts by Surface Organometallic Chemistry - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 15

surface, as the silica-metal bond had been created previously. One typical
case is that of Ta(OMe) 5 which is a dimer. When treating the alkyl-alkylidene
surface complex with methanol a monomer is obtained while reaction of the
alkoxy derivative leads to the formation of a dimer, as shown by EXAFS.
This can be of interest for applications in catalysis as in some cases dimers
are expected to be the active sites. Depending on the molecule a lot of
functions can be introduced around the metal. If the reaction is made on the
complex prepared by reaction with a silica dehydroxylated at high tem-
perature, this new species will have only one bond with the surface. If the
starting silica had been dehydroxylated at low temperature, the main species
will have two bonds with the surface and in the case of group 4 elements if
the reaction is made with the hydride the main resulting species will have
three bonds with the surface. However, and as for the direct grafting
reaction of the alkoxy derivatives, alkoxy groups are also formed on the
surface (see above). This point can be important in catalysis as it can modify
the sorption properties of the solid and so can affect the kinetics of the
reaction.

d n 9 r 4 n g | 1

1.5 Formation of Cationic Complexes on the Surface
All the above synthesis methods allow only the preparation of neutral surface
complexes linked to the surface by a covalent bond. However, in some cases,
the catalytic species is a cation, for example metallocene species in the case
of olefin polymerization or chromium cations for the trimerization of
ethylene. Classically, this cation is formed by reaction of a neutral complex
with a Lewis acid which must have a non-coordinating ability after its
transformation into the corresponding anion. With regard to these con-
ditions B(C 6 F 5 ) 3 was found to be a very good candidate.
When applying this methodology to the design of single site hetero-
geneous catalysts on a surface such as silica, two different strategies may be
used:

.

1. To graft first the Lewis acid co-catalyst directly to the surface in order to
create a covalent bond with the surface and then to react this supported
Lewis acid with the molecular complex in order to achieve the prep-
aration of a 'floating' cationic complex.
2. To graft first a precursor complex directly with the silica surface in
order to create a covalently bonded complex such as described above
and in a second step react this surface complex with a Lewis co-catalyst
which will abstract an alkyl group from the surface complex, leading to
a cationic species covalently bonded to the surface.

These two methods will lead to different surface species the coordination
sphere around the metal containing or not a covalent bond with the surface.
We will first describe one example of the first method with B(C 6 F 5 ) 3 as a
Lewis acid (Scheme 1.8) but this strategy can be applied to other compounds.