Stabilizing reactive intermediates through site isolation

Abstract

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) .

Surface Organometallic and Coordination Chemistry toward Single-Site Heterogeneous Catalysts: Strategies, Methods, Structures, and Activities

“…Most late transition-metal species supported on SiO 2 , Al 2 O 3 , MgO, or TiO 2 react with H 2 to yield small metal nanoparticles (Scheme 32a). 213,218,247,381,468 Supported late or coinage metal trimethylsilylamide complexes, also form particles in the presence of H 2 and also passivate the regenerated OH groups of the support to yield supported nanoparticles on silica having no OH groups. 159,168,216 There are a few examples of stable late metal hydrides supported on oxides.…”

Surface ReOx Sites on Al2O3 and Their Molecular Structure–Reactivity Relationships for Olefin Metathesis

“…Table 1. 15,16,21,22 The in situ IR spectra of the surface hydroxyl region of the dehydrated supported ReO x /Al 2 O 3 catalysts as a function of rhenia loading are presented in Figure 7. At low surface ReO x coverage (3% ReO x /Al 2 O 3 ), mainly the μ 1 -Al IV , μ 1 -Al VI and μ 1 -Al V surface hydroxyls at 3787, 3768, and 3743 cm −1 , respectively, are consumed due to anchoring of the surface rhenia species on alumina.…”

Alkylidene and alkylidyne surface complexes: Precursors and intermediates in alkane conversion processes on supported single-site catalysts

“…We have shown that well-defined alkylidene and alkylidyne surface complexes have by and large the same structures as their molecular equivalents, and that their specific reactivity is strongly related to their higher stability through site isolation [49,57,123]. This translates into low-coordinated species and the possibility to work at higher temperatures without facing fast decomposition (mainly due to bimolecular deactivation), so that new reactivity such as alkane metathesis could be discovered.…”