Siloxide

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. 

Tris(ferf-butoxy)siloxide molecular precursors of V(IV) and V(V) can be prepared via simple silanolysis reactions. For example, OV[OSi(O Bu)3]3 was obtained in 85% yield by reaction of OVCI3 with excess HOSi(O Bu)3 in the presence of pyridine [79]. Although crystals of sufficient quality for an X-ray structural analysis of 0V[0Si(0 Bu)3]3 were not obtained, its identity was confirmed by various spectroscopic and analytical techniques. Additionally, ( BuO)3VOSi(O Bu)3 and ( BuO)2V[OSi(O Bu)3]2 were obtained via reaction of V(0 Bu)4 with 1 and 2 equiv of HOSi(O Bu)3, respectively, in toluene at 80 °C [80] (Eq. 5). Both (fBu0)3V0Si(0 Bu)3 and CBu0)2V[0Si(0 Bu)3]2 are monomeric in the solid state, and possess only monodentate siloxide ligands... 

OSi Bus) ligands [84], However, Bradley has reported the less sterically hindered, homoleptic siloxide Ta(OSiMe3)5 [85]. 

The only group 8 tris(ferf-butoxy)siloxide molecular precursor that has been reported thus far is the iron(III) complex Fe[OSi(O Bu)3]3 THE, synthesized via the reaction of FeCls with NaOSi(0 Bu)3 in THF (74%) [71,97] (Eq. 6). An... 

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] ... 

Fig. 5 The molecular structure of Bu0B[0Si(0 Bu)3]2 generated from crystallographic data, with all hydrogen atoms and the siloxide ligand methyl groups omitted for clarity...

Successful strategies for generating complexes of the di(terf-butyl)phosphate ligand primarily focus on the use of H0P(0)(0 Bu)2 as a reagent. As with the related siloxide species, all synthetic manipulations must be performed under inert conditions to avoid hydrolysis of the M - O - P linkages. Complexes of the - 02P(0 Bu)2 ligand are useful precursors to M/P/0 oxide materials. 

The - 0B[0Si(0 Bu)3]2 ligand has provided species of the form L M OB[OSi (0 Bu)3]2 x that are viable molecular precursors to M/B/Si/0 materials. However, the chemistry of this ligand appears to be sensitive to the ancillary ligands on the associated metal, with ligand transfer to the boron center (sometimes accompanied by siloxide transfer from boron to the metal) being a primary pathway for decomposition [64,90]. 

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... 

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). 

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