Synthesis of Well-defined Solid Catalysts by Surface Organometallic Chemistry - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 8

d n 9 r 4 n g | 1

Scheme 1.3 Grafting reaction of tetramethyl tin on silica catalyzed by supported
heteropolyacid.

to graft metal alkyl complexes of the left of the periodic table on highly acidic
supports. Another consequence is that it is not possible to graft platinum
methyl complexes on silica via breaking of the Pt-Me bond as these com-
pounds are not stable thermally.
Another consequence of this mechanism is that it can be possible to graft
metal complexes of the right of the periodic table by use of acidic species as
catalysts: The grafting reaction of tetramethyl tin occurs at room tempera-
ture on H 3 PW 12 O 40 supported on silica but the amount of evolved methane
exceeds by at least one order of magnitude the number of protons of the
polyacid and can only be explained by a migration on the surface. The
mechanism (Scheme 1.3) passes through an attack of the tin complex by
the acidic proton of the heteropolyacid followed by a migration of the grafted
tin species on the surface and restoration of the acidic proton.

.

1.3.2 Reactivity of Metal Alkylidenes or Alkylidynes
If for the elements of group 4 (Ti, Zr, Hf) it is possible to prepare metal
complexes such as M(-CH 2 -C(CH 3 ) 3 ) 4 without hydrogen atoms in b position
of the metal (which can lead to side reactions, see below), it is not possible to
obtain the corresponding species for those of groups 5 and 6. Indeed the
steric hindrance around the metal is so high that a-H abstraction occurs,
leading to the evolution of alkane and the formation of a metal alkylidene
(for group 5) and even, after a second a-H abstraction, of a metal alkylidyne
(or a dialkylidene). The resulting complexes are now heteroleptic as they