Modification of silica surfaces by metallasiloxanes containing Mo and Ti: Evidence for concurrent metal and ligand chemisorption

The molecular metallasiloxanes Mo((NBu)-Bu-t)(2)(OSiMe3)(2) 1, and Ti(OSiMe 3)(4), 2, both react with ' the hydroxyl groups on partially dehydroxylated silica surfaces. The amount of chemisorbed metal corresponds, in both case s, to one-half the number of available surface hydroxyl groups. Unlike graf ting reactions of metal alkoxides, the chemisorptions are not accompanied b y release of protonated ligands. The siloxide ligands are transferred to th e silica where they are bound via siloxane or "glass bonds", as in drop SiO SiMe3, 3. By C-13 and Si-29 CP/MAS NMR, two distinct signals for trimethyls iloxide groups were observed. One is assigned to 3 by comparison to an auth entic sample, while the second corresponds to trimethylsiloxide ligands coo rdinated to grafted metal complexes. The latter are susceptible to protonol ysis, 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 a nd HOSiMe3, followed by rapid condensation of the silanol with a second sur face hydroxyl to generate 3. Molecular metallasiloxanes are suggested as si ngle-source precursors to metal catalysts supported on alkylsilane-capped s urfaces.