Precursor and Solvent Choice for Nonhydrolytic Sol-Gel Processes

Zr2(0 Pr)6(acac)2 (b)[66]. Introduction of a che- charge of the central atom [64]. lating ligand results in =0.02 A elongation of 

The reactions of nonhydrolytic removal of alkyl groups from metal alkoxides with generation of oxo-ligands and then, finally, oxides, have been discovered by Bradley et al. in the 1950s [127], and in the last decade, they have been named as Bradley reaction, especially for halide-free synthesis of oxides. A nimiber of recent reviews can be found on this topic [65,128,129]. A detailed description of the related processes can be found in Ref. [59]. 

Major processes in the Bradley reaction can be described by sum equations  

The driving force in the mechanism of these reactions is the formation of an alkyl cation R , attacking the neighboring nucleophilic group at the metal center either a halide or an alkoxide. The reaction is facilitated by the improved stability of involved alkyl cation CH2=CH—CH2 (allyl) C6H5CH2 (benzyl) (CH3)3C (t-but d) (CH3)2CH (i-prop)d) CH3CH2 (ethyl) CH3 (methyl) [130] and catalyzed by both acids (e.g., traces of water/humidity, especially when halides are applied [131]) and bases (alkali or alkaline earth alkoxides [132]). 

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