Transitions 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... 

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

According to the general idea presented by Wolczanski [4], alkoxide and siloxide (alternative to cyclopentadienyl complexes) of transition metals are bonded through a o-type orbital such as the sp hybrid and via Jt-donation of two pjt orbitals perpendicular to the M-O direction (Figure 7.1). 

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]. 

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-... 

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. 

Siloxides, like alkoxides, have been employed as ancillary ligands of transition metal complexes, markedly influencing the reactivity of a metal center by electronic and steric effects of the substituents at the silicon [15, 16]. 

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. 

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]. 

There are a number of synthetic pathways to trivalent rare earth siloxides and their Lewis base adducts such as salt metathesis, transesterification, acid-base chemistry, and silanolysis. Less frequently used are insertions of organo rare earth complexes into cychc and linear sUoxanes and CO2 insertion into rare earth silylamides. Transesterification is a common route for the preparation of stericaUy less-hindered early transition metal trialkyl siloxides and involves... 

O Hair and coworkers have investigated the gas-phase chemistry of siloxide and silamide ions in flowing afterglow experiments. The reaction of siloxide anions, HsSiO" (as well as substituted analogues), with CS2 was found to lead to the exclusive formation of COS and the corresponding HsSiS" anion (equation 42). As suggested by ab initio calculations carried out in the same smdy, the reaction quite likely proceeds via a fivefold coordinated cyclic transition state (93). 

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

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