Siloxides properties

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]. 

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]. 

An important area in the chemistry of titanium alkoxides is with the siloxides, Ti(OSiR3)4and their derivatives containing Hal, OR, oxoligands and the related polymers. The latter find rather broad applications as dielectric and isolation materials. They are briefly considered in the book by Feld and Cowe [560], a detailed review being included in the monograph by Andrianov [48]. Table 12.12 presents the properties of only certain simplest derivatives. 

Alkoxides and aryloxides have been intensively studied because of their interest, especially for Ba, as precursors to oxide materials having desirable electronic properties this development was stimulated by the discovery of superconducting materials such as YBajG CVx. The same interest applies to other volatile complexes like /3-diketonates. The alkyl or aryl oxides (also siloxides) are readily obtained by a variety of methods, notably (a) direct interaction of the metals with ROH in THF, toluene saturated with NH3,22 or other ether solvents (b) the interaction of... 

Ca, Sr, AND Ba Aryl Oxide, Alkoxide, and Siloxide Compounds and Their Physical Properties"... 

While the earliest work in this area was from Bradley, Chisholm and Caulton, and their group, have recently prepared both cluster alkoxides and siloxides that have some increased solubility properties, for example, HBa5(0)(0Ph)g(THF)8, H3Ba6(0)(0-r-Bu)(0CEt2CH20)(THF)3, H4Ba6(/u.6-0)(0CH2CH20... 

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. 

A comparison of the Ln-0 distances of rare earth siloxides and alkoxides is complicated by a lack of structural information on alkoxides that have similar steric properties to triphenylsiloxides. However, according to a search in the Cambridge Database, the Ln-0 distances of aliphatic rare earth alkoxides appear to be somewhat smaller, consistent with the increased overall electron-releasing properties of alkoxides relative to siloxides. 

The molecular structure of the isolated product could unambiguously be identified by X-ray analysis as the heterotrimetallic lithimn-scandium aluminum siloxide (31). The selective formation of (31) is promising insofar as this synthetic strategy may open the door to the preparation of a wide variety of heterotrimetallic siloxides with perhaps interesting electronic and magnetic properties, from readily available salt adducts of rare earth siloxides. 

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