Metal 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. 

Although organosilanes appear to react slowly (if at all) with water alone, in the presence of acids or bases (e.g., alkali metal hydroxides), reactions to give a silanol and H2 are rapid, with bases being particularly powerful catalysts. The evolution of H2 in this type of reaction may be used as both a qualitative and a quantitative test for Si-H bonds, and the mechanism of the acid and the base hydrolysis has been discussed in detail (30,31). This hydrolytic method is not very common for the preparation of silanols that are to be isolated, because both acids and bases catalyze the condensation of silanols to siloxanes, and therefore, only compounds containing large substituents are conveniently made in this way. If an anhydrous alkali metal salt is used, a metal siloxide may be isolated and subsequently hydrolyzed to give the silanol [Eq. (10)] (32). 

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

Characterization of 6 by solid-state NMR spectroscopy as well as elemental analysis (only 28% decrease of rhodium content in samples after 20 cycles in the hydrosilylation) are convincing evidence of the high efficiency of surface rhodium siloxide complexes in the hydrosilylation of carbon-carbon multiple bonds as well as, presumably, in other reactions catalyzed by late transition metal siloxides syn-... 

A mixed metal siloxide complex YCu(OSiPh3)4(PMe2Ph) was obtained by a simple addition reaction (Eq. 3) [48], The geometry around the yttrium atom is distorted tetrahedral. The molecule is cleaved in solution by Lewis basic solvents like THF. 

Heterogeneous diene polymerization catalysts based on modified and unmodified silica-supported lanthanide complexes are known as efficient gas-phase polymerization catalysts for a variety of support materials and activation procedures (see Sect. 9). Metal siloxide complexes M(()SiR3 )x are routinely employed as molecular model systems of such silica-immobilized/ grafted metal centers [196-199]. Structurally authenticated alkylated rare-earth metal siloxide derivatives are scarce, which is surprising given that structural data on a considerable number of alkylated lanthanide alkoxide and aryloxide complexes with a variety of substitution patterns is meanwhile available. 

Metal siloxides can often be prepared using silanols as precursors, but it is also possible to hydrolyse a siloxide to give a silanol as shown for the spirotitanasiloxane 37,... 

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. 

The use of large, nitrogen-centered substituents at a silanetriol center does allow the isolation and characterization of stable silanetriols as shown in equation (24). These compounds can be used to prepare transition metal siloxide complexes (Scheme 34), still containing potentially moisture-sensitive Si-N bonds that may be precursors to metal-containing ceramics for reviews of these compounds, see References 193-195. 

Keywords titanium, zirconium, oligosilanes, silanols, metal siloxides... 

The chemistry of transition metal siloxide complexes has continued to attract considerable attention in the field of material science [1] and catalysis [2], particularly since 1980. The synthesis, reactivity, and bonding of such complexes in a wide variety of supporting ligand environments continues to be explored. In this regard numerous silanediols, disilanols and silanetriols have been described in the literature [3,4], which could be used as building blocks for the preparation of novel titanium-containing heterocubanes and titanasiloxanes [4-6]. 

Like alcohols, silanols have been found to react with metal alkoxides to form the corresponding homo- and heteroleptic metal siloxides (Eq. 2.199). 

Metal siloxides are more resistant to hydrolysis than their alkoxide analogues this salient difference in hydrolytic stability of metal siloxides may be ascribed to the water-repelling property of trialkylsiloxo groups. 

Reactions of anhydrous rare earth halides with aUcah metal siloxides, RsSiOM, yield hetero- and homoleptic rare earth siloxides via salt metathesis (equation 2). The aforementioned alkah metal siloxides can be conveniently generated from deprotonation of silanols with Bu"Li or MH. However, these reactions require donor solvents such as THF and Et20 and are often accompanied by solvent coordination to the final product or lead to the formation of salt adducts and ate complexes. " ... 

Marciniec B, Maciejewski H (2001) Transition metal-siloxide complexes synthesis, structure and application to catalysis. Coord Chem Rev 223 301... 

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