Molecular transition metal siloxide complex

Molecular versus Immobilized Transition Metal Siloxide Complexes in Catalysis... 

The simplest metal siloxide species, MOSi(O Bu)3 (M = Na, K, Li), are readily synthesized by reaction of HOSi(O Bu)3 with the desired alkah metal in pentane followed by crystalUzation at low temperature [65] (Eq. 2). These alkali metal siloxide salts have primarily been used as siloxide transfer reagents in the synthesis of transition metal siloxide complexes. The magnesium siloxide Mg[OSi(O Bu)3]2 was synthesized by the reaction of dibutylmagnesium with HOSi(O Bu)3 in THE at room temperature (83% isolated yield) [66]. Although crystals sufficient for an X-ray crystallographic analysis were not obtained, solution molecular weight determination found that Mg[0Si(0 Bu)3]2 is monomeric in benzene solution. This complex hydrolyzes immediately when exposed to the atmosphere. 

Two principle strategies have been employed for the synthesis of siloxide-containing molecular precursors. The first involves a silanolysis, or condensation, reaction of the Si - OH groups with a metal amido, alkyl, hahde, or alkoxide complex. The second method involves salt metathesis reactions of an alkali metal siloxide with a metal hahde. Much of our work has been focused on formation of tris(tert-butoxy)siloxide derivatives of the early transition metals and main group elements. The largely imexplored regions of the periodic table include the lanthanides and later transition metals. 

The properties of siloxide as ancillary ligand in the system TM-O-SiRs can be effectively utilized in molecular catalysis, but predominantly by early transition metal complexes. Mono- and di-substituted branched siloxy ligands (e.g., incompletely condensed silsesquioxanes) have been employed as more advanced models of the silanol sites on silica surface for catalytically active centers of early TM (Ti, W, V) that could be effectively used in polymerization [5], metathesis [6] and epoxidation [7] of alkenes as well as dehydrogenative coupling of silanes [8]. 

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