The molecular metallasiloxanes Mo(NtBu)2(OSiMe3)2
1, and Ti(OSiMe3)4, 2, both react with
the hydroxyl groups on partially dehydroxylated silica surfaces. The amount of chemisorbed
metal corresponds, in both cases, to one-half the number of available surface hydroxyl groups.
Unlike grafting reactions of metal alkoxides, the chemisorptions are not accompanied by
release of protonated ligands. The siloxide ligands are transferred to the silica where they
are bound via siloxane or “glass bonds”, as in ⋮SiOSiMe3, 3. By 13C and 29Si CP/MAS NMR,
two distinct signals for trimethylsiloxide groups were observed. One is assigned to 3 by
comparison to an authentic sample, while the second corresponds to trimethylsiloxide ligands
coordinated to grafted metal complexes. The latter are susceptible to protonolysis, while 3
is not. The proposed grafting mechanism involves reaction of 1 or 2 with a single surface
hydroxyl to form a chemisorbed metal complex and HOSiMe3, followed by rapid condensation
of the silanol with a second surface hydroxyl to generate 3. Molecular metallasiloxanes are
suggested as single-source precursors to metal catalysts supported on alkylsilane-capped
surfaces.
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