Surface organometallic fragment silica

Scheme 55 Structures of surface organometallic fragments resulting from the reaction of [Rh(allyl)3] and various oxide supports (silica, alumina, and titania).

This is, to our knowledge, the first example of a selective preparation of a floating cationic organometallic fragment on an anionic and non-coordinating silica surface. 

For this complex, molecular chemistry does not adequately model the surface reactivity and the latter is strongly influenced by the presence of surface hydroxyl groups [22]. The organometallic fragments immobilized on silica have been reacted with trimethylphosphine to afford different silica-supported phosphine complexes of rhodium. The course of the reaction depends strongly on the hydroxyl content of the silica surface [23] (Scheme 7.2). 

The use of a silica-supported, tantalum alkyhdene as a precursor for alkane metathesis was found to result in a stoichiometric, alkane cross metathesis in which an initial pendant alkyl-alkylidene group was transformed to produce the desired, active species [76, 90]. This reaction was later observed to work with additional, well-defined systems supported on alumina [58] and sihca-alumma [53]. As mentioned previously, this transformation does not occur when the surface organometallic precursor bears no alkyl group. Exposing these supported, metal neopentyl, neopentylidene, and neopentyhdyne complexes to alkane at 150 °C produced alkanes containing a neopentyl fragment (CH jiBu) via cross metathesis. Propane metathesis with these alkyl-alkyhdene surface complexes typically generates stoichiometric amounts of dimethylpropane, 2,2-dimethylbutane, and 2,2-dimethylpentane (Scheme 2.11). 

The evidence, produced already in the early 1980s, that monometallic surface species like Os " or Rh carbonyl fragments, incapsulated into the surface of silica or alumina, may have the necessary mobility to react with each other, since they return quite easily under a CO atmosphere to the original clusters Os3(CO)i2 and Rh6(CO) respectively [26, 27, 31], was the origin of so-called surface mediated organometallic synthesis. 

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