Siloxide complexes

X-ray crystallographic analysis of Fe[0Si(0 Bu)3]3THF revealed a distorted tetrahedral geometry (toward a trigonal pyramid) at the Fe center. Related alkyl siloxide complexes of Fe(III) with dimeric structures, [Fe(OSiMe3)3]2 and [Fe(OSiEt3)3]2, have been reported by Schmidbaur and Richter [98]. 

The zinc tris(ferf-butoxy)siloxide complex Zn[0Si(0 Bu)3]2 2 was prepared from the reaction of ZnMe2 with HOSi(O Bu)3 [107]. This complex was structurally characterized as an asymmetric dimer with four - OSi(O Bu)3 ligands, each exhibiting a unique coordination mode ( fi - , r] ... 

Iridium(I) siloxide complexes-that is [ Ir( 4-OSiMe3)(cod) 2] and [Ir(cod)(PCy3)-(OSiMe3)]-have also been studied in the transformation of various vinylsilanes under pressure of syn-gas, giving mixtures of silylaldehydes accompanied by the hydrogenation product (Scheme 14.9) [57]. 

Iridium siloxide complexes show a similar activity. Catalytic tests performed in the presence of [ Ir( 4-OSiMe3)(cod) 2], with the use of trimethylvinylsilane and dimethylphenylsilane as reactants [59], gave the same type of silicon derivatives as those obtained by Murai and coworkers [58], but the siloxide iridium precursor used appeared to be more efficient under milder conditions. When the [Ir(cod)(PCy3)(OSiMe3)] was used rather than the binuclear iridium siloxide complex, Z-Me3SiCH2CH=CHOSiMe2Ph was obtained exclusively [59],... 

Well-Defined Surface Rhodium Siloxide Complexes and Their Application to Catalysis... 

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

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. 

Figure 7.3 Proposed structures of 7C-allyl rhodium siloxide complexes immobilized on silica.

Scheme 7.2 Reactions of the surface 7t-allyl rhodium siloxide complex with trimethylphosphine.

Scheme 7.3 Reaction of surface Jt-allyl rhodium siloxide complex with carbon monoxide.

Scheme 7.4 Formation of surface rhodium siloxide complexes containing phosphine and hydride ligands.

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

Scheme 7.5 Immobilization of rhodium(diene)siloxide complex.

I 7 Well-Defined Surface Rhodium Siloxide Complexes and Their Application to Catalysis Table 7.5 Hydrosilylation of 1-hexadecene and allyl ethers by polyhydrosiloxane (7.1) . 

Scheme 7.6 Oxidative addition of silicon hydride to surface rhodium(diene) siloxide complex 1.

Catalysis of hydrosilylation by dimeric or by monomeric rhodium (and iridium) siloxide complexes occurs via preliminary oxidative addition of siUcon hydride followed by elimination of disiloxane (detected by GC/MS) to generate the square planar 16e hydride complex with an already coordinated molecule of alkene (Scheme 7.7). 

Mechanism of HYdrosilylation Catalyzed by Surface versus Soluble Rhodium Siloxide Complexes j 309... 

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-... 

Scheme 7.8 Mechanism of heterogeneous catalysis of hydrosilylation by a surface rhodium (diene) siloxide complex.

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