The composition of grignard compounds : VIII. Alkylmagnesium fluorides

Abstract

Although alkylmagnesium chlorides, bromides and iodides are monomeric in tetrahydrofuran over a wide concentration range, ebullioscopic studies reported herein show that alkylmagnesium fluorides are dimeric in diethyl ether and tetrahydrofuran over the same concentration range. Low temperature NMR, IR, fractional crystallization and dioxane precipitation studies indicate that the mobile Schlenk equilibrium used to describe organomagnesium chlorides, bromides and iodides does not exist int he case of alkylmagnesium fluorides due to the unusual stability of MgFMg bridge bonds. Instead, the composition of alkylmagnesium fluorides in both diethyl ether and tetrahydrofuran is best described by the dimeric units *I) or (II), or (I) and (II). ▪ Some evidence is presented to support a mixed bridge system (I) for those cases where the RMgF compounds were prepared in the presence of by-product R 2Mg compounds. Alkylmagnesium fluorides react with H 2O, O 2, CO 2, PhCN and Ph 2CO in a manner similar to, but significantly different from, the corresponding alkyl-magnesium bromides.