Hafnium alkoxides

Zrrconium(IV) and hafnium(IV) complexes have also been employed as catalysts for the epoxidation of olefins. The general trend is that with TBHP as oxidant, lower yields of the epoxides are obtained compared to titanium(IV) catalyst and therefore these catalysts will not be discussed iu detail. For example, zirconium(IV) alkoxide catalyzes the epoxidation of cyclohexene with TBHP yielding less than 10% of cyclohexene oxide but 60% of (fert-butylperoxo)cyclohexene °. The zirconium and hafnium alkoxides iu combiuatiou with dicyclohexyltartramide and TBHP have been reported by Yamaguchi and coworkers to catalyze the asymmetric epoxidation of homoallylic alcohols . The most active one was the zirconium catalyst (equation 43), giving the corresponding epoxides in yields of 4-38% and enantiomeric excesses of <5-77%. This catalyst showed the same sense of asymmetric induction as titanium. Also, polymer-attached zirconocene and hafnocene chlorides (polymer-Cp2MCl2, polymer-CpMCls M = Zr, Hf) have been developed and investigated for their catalytic activity in the epoxidation of cyclohexene with TBHP as oxidant, which turned out to be lower than that of the immobilized titanocene chlorides . ... 

It was quite unexpectedly found that the amorphous samples of zirconium and hafnium alkoxides M(OR)4 contain several types of oxocomplexes, particularly, M3O(OR)i0 and M40(0R)14 [1612], The trinuclear Zr3([l3-0)()i3-OBu XOBu ), was isolated in a crystalline form and turned out to be a structural analog of the known isopropoxide clusters of Th, Mo, and U(IV) -MjOCOPr ),) [1520] (see also Sections 4.3 and 12.12). The inclusion of the solvent molecules inside the cavities of the structures and formation of alcohol solvates in many cases leads to microanalysis data that does not deviate much from those calculated for M(OR)n. 

Thermolysis of tin and lead alkoxozirconates leads to the formation of metals. The mass-spectral data indicate the presence ofbarium and aluminium derivatives in the gas phase, but no preparative data are accessible for them. The major application of zirconium and hafnium alkoxides lies now in the sol-gel technology of zirconate-titanate and solid solutions Zr02-Y203 (see Section 10.3), Except in the synthesis of oxide materials, the alkoxides of zirconium and hafnium are traditionally used in the polymer chemistry, where they are applied as the components in catalysts [1278, 1269] and as additives to polymers, improving their characteristics [825, 1403] and so on. Already in 1930s Meerwein has proposed the use of zirconium alkoxides for the reduction of aldehydes intoprimary alcohols (Meerwein-Schmidt reaction) [1420],... 

Saha et reported zirconium and hafnium alkoxide complexes bearing a... 

Table 2.9 indicates that the boiling points of zirconium " " and hafnium alkoxides are quite similar but differ from those of titanium analogues. " " An important feature of Table 2.9 is the fact that ferf-alkoxides of hafnium are more volatile than the analogous zirconium derivatives. The entropies of vaporization of zirconium and hafnium fert-amyloxides are also nearly the same (39.5 and 39.8 cal deg mol respectively) and these results are in satisfactory accordance with the chemical similarities of zirconium and hafnium. 

Table 2.9 Thermodynamic properties and molecular complexities of titanium, zirconium, and hafnium alkoxides...

Papendorf, B., Nonnenmacher, K., lonescu, E., Kleebe, H. J., Riedel, R. (2011). Strong influence of polymer architecture on the microstructural evolution of hafnium-alkoxide-modified silazanes uponceramization. Small, 7(7), 970-978. doi 10.1002/ smll.201001938 PMID 21381195. 

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