Synthesis of Well-defined Solid Catalysts by Surface Organometallic Chemistry - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 6

1.3 Formation of Grafted Organometallic Complexes
by Reaction with One Hydroxyl Group
As we have seen above, the active sites of the support are mainly (if carbon is
excluded) hydroxyl groups; only their distribution and strength depend on
the treatment and on the oxide under study. The formation of a chemical
bond between the organometallic fragment and the solid will then pass, in
most cases, through a reaction with these hydroxyl groups. We will describe
here what will happen and by comparison of various supports and organo-
metallic compounds how the reaction proceeds.
First of all, it is necessary to choose an organometallic compound with a
M 0 -X bond for which the reaction M-OH þ [M 0 ]-X

d n 9 r 4 n g | 1

M-O-[M 0 ] þ HX will be
favored thermodynamically, M-OH being a hydroxyl group of the support.
Many complexes can be chosen, such as chlorides or alkoxy derivatives.
However, in these two cases the evolved hydrogen chloride or alcohols can
further react with hydroxyl groups or M-O-M bridges of the support and so
modify its properties. A typical example is the reaction at room temperature
of tantalum methoxide Ta(OMe) 5 with silica dehydroxylated at high tem-
perature. 45 13 C CP-MAS NMR of the resulting material shows clearly two
signals for methoxy species. One of them can be attributed to a methoxy
group on tantalum as expected and a second to a methoxy group on the silica
support (such species can be synthesized by treatment at relatively high
temperature of silica with methanol). These silica methoxy groups are
formed by reaction of evolved methanol with a siloxane bridge or a silanol
group. As this reaction does not proceed at room temperature in the absence
of the tantalum complex, the metal plays the role of catalyst for this reaction.
The consequence is that the grafting reaction will not be clean and the
starting treatment of the support for the creation of isolated grafting sites
will not be ecient as new potential grafting sites will be created during the
reaction. It is for this reason that the organometallic complexes which will
be chosen must lead to inert X-H species. The best choice is to have evo-
lution of alkanes which cannot be activated by these supports. For this
purpose the ligands around the metal will be alkyl, alkylidene or alkylidyne
groups.

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1.3.1 Reaction of Metal Alkyl Complexes
Very often homoleptic organometallic complexes are chosen as they will lead
to only one surface complex, all ligands being equivalent. One problem is
that, kinetically, the reaction will be slow, compared for example with that
achieved with alkoxy compounds, due to the fact that the first step, the
physisorption on the support, will not be favored, the interaction between
hydroxyl groups and alkyl groups not being strong. To overcome this
problem the complex can be sublimed on the support, avoiding the use of a
solvent, but this method can be used only when it has a sucient
vapor pressure and sometimes the sublimation is accompanied by a