Silica organometallic synthesis

The specific characteristics of a surface such as silica, if considered as a new reaction medium, were confirmed by the unusual selectivity and easy reactivity shown in so-called surface-mediated organometallic synthesis. 

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. 

Roberto, D., Cariati, E., Ugo, R., and Psaro, R., Surface-mediated organometallic synthesis High-yield preparations of neutral and anionic osmium carbonyl clusters by controlled reduction of silica-supported [Os(CO)3Cl2]2 and OsClj in the presence of Na2C03 or K2C03. Inorg. Chem. 35,2311 (1996). 

Saffarzadeh-Matin, S., Chuck, C.J., Kerton, F.M. and Rayner, C.M. (2004) Poly(dimethyl-siloxane)-derived phosphine and phosphinite ligands synthesis, characterization, solubility in supercritical carbon dioxide, and sequestration on silica. Organometallics, 23, 5176-81. 

In this chapter, we present an overview of our experience in the organometallic synthesis of ruthenium nanoparticles of controlled size and surface state. We also give insights on the study of their surface chemistry by using simple techniques, mainly IR and NMR both in solution and in solid state, hi addition, model hydrogenation reactions have been used. We also discuss the performances of these materials as catalysts in solution (organic and aqueous phases) and on a support (alumina, silica, or carbon materials). 

In this context, the organometallic synthesis of NPs led to interesting results for the preparation of composite nanomaterials, using alumina membranes [98], mesoporous silica, and carbon materials as supports for the deposition of NPs, mainly for hydrogenation,... 

The reaction of silica-supported bis(allyl)Rh complexes with PMe3 followed by hydrogen treatment leads to the synthesis of the surface SiO-RhH2(PMe3)4+ as the first example of a cationic organometallic complex attached to the silica surface by ion pairing. The counterion is presumed to be a siloxy group SiO- on the silica surface.276 277... 

Alkylidene complexes are generally considered to be reactive intermediates but the actual surface organometallic species have never been fully characterized. However, the synthesis of silica-supported tantalum(V) carbene complexes and their characterization have been reported.332... 

We then studied group 5 metals, especially tantalum-for which the laboratory already had great experience. Because of the studied reaction, alkyl or hydride-type compounds such as those developed in the laboratory could not be employed. Consequently, we became interested in alkoxo-type derivatives, either synthesized by reaction of the grafted complex with an alcohol or obtained by direct synthesis starting from an alkoxy-tantalum compound grafted on silica. In all cases, resulting complexes have been characterized by surface organometallic chemistry techniques, especially EXAFS and solid-state NMR (ID and 2D with C-labeled compounds). Indeed various compounds bonded by one, two or three surface bonds have been prepared and characterized. 

The tris-neopentyl Mo(VI) nitride, Mo(-CH2- Bu)3(=N) [134], reacts with surface silanols of silica to yield the tris-neopentyl derivative intermediate [(=SiO)Mo (-CH2- Bu)3(=NH)] followed by reductive elimination of neopentane, as indicated by labeling studies from labeled starting organometallic complex, to yield the final imido neopentylideneneopentyl monosiloxy complex [(=SiO)Mo(=CH- Bu)(-CH2 - Bu)(=NH)] [135]. The surface-bound neopentylidene Mo(VI) complex is an active olefin metathesis catalyst [135]. Improved synthesis of the same surface complex with higher catalytic activity by benzene impregnation rather than dichlorometh-ane on silica dehydroxylated at 700 °C has been reported [136],... 

Silicon carbides are generally synthesized by the pyrolysis of precursors, prepared by liquid phase methods. One possible way for precursor synthesis is the addition of carbon black or sucrose, to a gelling silica.8 In this method, the carbon is introduced from an external source. A more intimate contact between the carbon and silicon in the precursor is assured with the use of organometallic polymer precursors. The use of silane polymers for silicon carbide production was initiated by Yajima.9,10 Polymers having a -[Si-C]- backbone are crosslinked and pyrolysed to yield SiC." In the initial work, dimethyldichlorosilane was used as a starting monomer, which was subjected to a sodium catalyzed polymerization (reaction (C)). 

The serious disadvantage of the use of the reactions of the chlorinated silica with organometallic compounds is caused by the high sensitivity of the Grignard reagent and organolithium compounds to the action of water, side reactions with the participation of surface siloxane bonds, and problems of removing metallic impurities from the surface of modified matrices. The difficulties mentioned explain why this approach to the synthesis of surface chemical compounds with Si-C bonds finds limited application. 

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