This overview describes the reaction of organometallics with oxide surfaces and the formation of highly reactive species. In the case of silica, the surface can be seen as a large siloxy ligand, which helps to stabilize reactive intermediates through site isolations. This is translated into very highly reactive and stable well-defined alkene metathesis catalysts as well as the formation of hydrides species, which display unusual reactivities toward alkanes (e.g., low-temperature hydrogenolysis and metathesis of alkanes). In the case of alumina, it allows the formation of highly reactive, but stable cationic species or masked carbenic species whose structures are unusual by comparison with molecular chemistry. Scheme 2 (a) Grafting of [Re(≡CtBu)(=CHtBu)(CH 2 tBu) 2 ] on SiO 2-(700) . (b) Proposed model for the greater activity of asymmetric system. Scheme 3 Scheme 6
On alumina
GeneralitiesThe surface of alumina is more complex than this of silica. Indeed, in contrast to silica, in which Si is always tetracoordinated and mainly tetrahedral, the Al atoms of the bulk of transition aluminas (γ or δ) are found in two geometries, tetrahedral (Al Td ) or octahedral (Al Oh ) geometries, whose ratio (Al Td /Al Oh ) is ca. 1:3 for γ-Al 2 O 3 . Considering the surface Al atoms, they will therefore have different natures (tri-, tetra-, penta-, or hexacoordinated) depending on the origin of the Al atom (Al Td /Al Oh ) and the level of hydration of the surface. This will in turn induce a variety of Lewis acid (Al V , Al IV , and Al III ) and hydroxyl sites (Al IV -OH, Al V -OH, Al VI -OH as well as µ 2 -and µ 3 -OH; Scheme 8a). The combination of spectroscopic data and computational studies clearly shows that the (1,1,0) surface is the most abundant (ca. 70 %) and contains the OH groups [54,55]. Moreover, while the nature of the OH species is best described by a trihydrated alumina per unit cell, other studies clearly show that there C. COPÉRET © 2009 IUPAC, Pure and Applied Chemistry 81, 585-596 592 Scheme 7 Scheme 8 (a) IR spectrum of the OH region of Al 2 O 3-(500) and the associated OH group assignment. (b) Models of very reactive Lewis acid sites (defects). (c) Reactivity of defect sites with H 2 and CH 4 . Scheme 10 (a) Grafting of MeReO 3 on Al 2 O 3-(500) . (b) Proposed resting states and active sites of MeReO 3 /Al 2 O 3-(500) .