Synthesis siloxides

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. 

Besides supported (transition) metal catalysts, structure sensitivity can also be observed with bare (oxidic) support materials, too. In 2003, Hinrichsen et al. [39] investigated methanol synthesis at 30 bar and 300 °C over differently prepared zinc oxides, namely by precipitation, coprecipitation with alumina, and thermolysis of zinc siloxide precursor. Particle sizes, as determined by N2 physisorpt-ion and XRD, varied from 261 nm for a commercial material to 7.0 nm for the thermolytically obtained material. Plotting the areal rates against BET surface areas (Figure 3) reveals enhanced activity for the low surface area zinc... 

Our study on the synthesis, structure and catalytic properties of rhodium and iridium dimeric and monomeric siloxide complexes has indicated that these complexes can be very useful as catalysts and precursors of catalysts of various reactions involving olefins, in particular hydrosilylation [9], silylative couphng [10], silyl carbonylation [11] and hydroformylation [12]. Especially, rhodium siloxide complexes appeared to be much more effective than the respective chloro complexes in the hydrosilylation of various olefins such as 1-hexene [9a], (poly)vinylsiloxanes [9b] and allyl alkyl ethers [9c]. 

On the other hand, Tilley et al. have reported a synthesis of a well-defined tris(tert-butoxy)siloxy-iron(lll) complex [13] as well as respective molecular siloxide complexes of cobalt [14] and copper [15], which appear to become precursors for their grafting onto silica and application as catalysts for oxidation of alkanes, alkenes and arenes by hydrogen peroxide. 

Synthesis, Characterization and Catalytic Activity of Well-Defined Suface Rhodium Siloxide Complexes... 

Routes to monomeric , mononuclear , monolanthanide alkoxides, enolates, siloxides and aryloxides - an expanded title which will put the scope of the article in a more concrete form. The synthesis of mononuclear alkoxides, in particularly homoleptic derivatives [1], was decisively stimulated by the discovery of high temperature superconducting ceramics based on YBa2Cu307<, where yttrium represents the lanthanide elements [2]. The support of volatile and highly soluble molecular precursors is a prerequisite for synthesizing thin films of these materials by means of MOCVD [3] and sol gel processes [4], respectively. More recently, lanthanide alkoxide reagents became established in... 

Scheme 41 Synthesis of structurally characterized heterobimetallic Ln/Al siloxide complexes Ln[OSi(Ot-Bu)3](AlMe4)2(AlMe3) (Ln = Y, La, Ce, Pr, Nd) [150,200]...

The preparation and structures of siloxanediol derivatives has been reviewed29,184 but some representative examples of reactions are given below. Two general methods have been used to make metal siloxides from silanols one involves the direct reaction of a silanol with the metal derivative, usually an amide, alkoxide, halide or alkyl (to give amine, alcohol, HX or alkane byproducts respectively), while the second involves the preparation of a simple alkali metal derivative which can then be used as a siloxide transfer reagent for further synthesis. Only the direct route will be considered here. Two of the most useful siloxide transfer species, (MOPh2Si)20 (M = Li or Na), are... 

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. 

With the intent to investigate the ability of polyhydroxy silanes to act as supporting anionic ligands for metals, we focused our studies on the synthesis of novel Ti and Zr siloxide complexes from triol 8a (Scheme 2). When Ti(OEt)4 was reacted with one equivalent of 8a in heptane for 3 h, a yellow microcrystalline material could be isolated in a yield of 35%. An X-ray Structure analysis revealed the formation of the bimetallic Ti-siloxide 14, in which both Ti atoms are connected to each other... 

Synthesis of Glycidoxypropyl-Silanes and -Siloxanes via Rhodium Siloxide-Catalyzed Hydrosilylation... 

The aim of this work was to examine the catalytic activity of rhodium siloxide complexes in the hydrosilylation of allyl glycidyl ether by triethoxysilane and hydro(poly)siloxanes, leading to optimization of procedures for the synthesis of epoxy-functional silanes and siloxanes. 

Our contribution to this field was the synthesis, isolation, and full characterization a few new rhodium siloxide complexes, both dimeric [13] and monomeric (see Fig. 1) [14]. 

Hydrosilylation is also one of the basic methods for synthesis of modifying silicones. The rhodium siloxide complexes have been used in the reaction between heptamethyltrisiloxane and allyl glycidyl ether, which is a modeling reaction of the polymeric system (Eq. 2). 

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