Siloxides, alkylated

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. 

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 polarity of the Al-C bond allows easy derivatization of the five-coordinate aluminum alkyls by alkane elimination (Figure 9). For example, Salen aluminum alkyls LAlMe could be converted to dimeric or polymeric Salen aluminum phosphinates [LAl 02P(H)Ph ] 98 (n = 2 or oo, depending on the Salen ligand backbone)98,99 by reaction with phenyl phosphinic acid, Salen aluminum siloxides LA10SiPh3 by reaction with triphenyl silanol,96 or Salen aluminum alkoxides LAIOR by reaction with an alcohol.100... 

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]. 

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. 

Structural evidence of the formation of alkylated siloxide complexes was obtained from the reaction of homoleptic tetramethylaluminates Ln(AlMe4)3 with one equivalent of tris(ferf.butoxy)silanol HOSi(Of-Bu)3. This fast protonolysis reaction results in heterobimetallic Ln/Al complexes Ln[OSi(Of-Bu)3](AlMe4)2(AlMe3) (Ln = Y, Ce, Pr, Nd, La) via quantitative... 

X-ray structure analysis revealed a 7-coordinate rare-earth metal center with two asymmetrically / -coordinating tetramethylaluminate ligands, an asymmetrically / -coordinating siloxide ligand and one methyl group of a trimethylaluminum donor molecule (Fig. 28). Such heteroleptic complexes can be regarded as molecular models of covalently bonded alkylated silica surface species. Moreover, isoprene was polymerized in the presence of 1-3 equivalents of diethylaluminum chloride, with highest activities observed for (Cl) (Ln) ratios of 2 1 (Table 12) (Fischbach et al., 2006, personal communication) [150]. 

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... 

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... 

Catalytic activity of synthesised Rh(I) siloxide complexes has been demonstrated in some reactions, i.e. in the hydrosilylation of alkenes [17] and allyl alkyl ethers [14, 18, 19] and in the silylative coupling of vinylsilanes with alkenes [20]. 

Imido complexes also react with organic electrophiles. For example, the molybdenum-imido complex in Equation 13.87 reacts with methyl bromide to generate the metal dibromide and trialkylaiulinium bromide. Simple alkylations of an 0x0 group to generate an aUcoxide product are uncommon, but one example of alkylation of a zironocene-oxo complex is shown in Equation 13.88. One example of a related silylation to generate a siloxide complex is shown in Equation 13.89. 

The overall scheme to generate the supported catalysts is shown in Equation 21.3a. Partially dehydrated silica is treated with a solution of Schrock s tungsten catalyst in pentane. The acidic hydroxyl groups protonate an alkyl group to form the siloxide-supported catalyst. In a similar fashion, partially dehydrated silica is treated with a solution of a bispyr-rolyl version of Schrock s molybdenum catalyst to release pyrrole and generate a catalyst... 

Siloxide complexes of rhodium and iridium are much more effective than the respective chloro-complexes in hydrosilylation of a variety of olefins such as 1-hexene, vinylsilanes, and allyl alkyl ethers (81). [ Rh(cod)()u-OSiMe3) 2] and [Rh(cod)(PCy3)3(OSilV[e3)] in particular are very active and selective catalysts for the hydrosilylation of glycidyl ether by triethoxysilane yielding commercially available silane coupling agent. 

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