Activation of [Os3(CO)12] by the
silica surface via reaction with surface silanol groups
provides a new convenient general approach to the selective, high-yield
synthesis of various
osmium carbonyl clusters such as
[H4Os4(CO)12] and
[HOs3(CO)10Y] (Y = a
three-electron
donor such as OH, OR, Cl, Br, I, O2CR, and SCN).
These silica-mediated syntheses compare
favorably with the more conventional syntheses in solution because (i)
differently from other
related intermediates used in solution, silica-anchored
[HOs3(CO)10(OSi⋮)] is easily
obtained
in quantitative yield starting from
[Os3(CO)12] in one step and (ii) the
conversion of [HOs3(CO)10(OSi⋮)] occurs in one pot usually
in high yields and under mild reaction conditions.
In addition, the related species
[HOs3(CO)10(OH)], which can be
obtained almost quantitatively by hydrolysis of
[HOs3(CO)10(OSi⋮)], is even more
reactive than its precursor and
allows a three-step synthesis of various osmium clusters characterized
by excellent total
yields from [Os3(CO)12].
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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