Abstract
Monoglyme forms a molecular 1:1 adduct with AlBr3 which dissociates into the ions AlBr2(DME)2 + and AlBr4 - in DME, CH2Br2 and CDCl3 as solvents. A hexacoordinated Al center is present in the solid adduct AlBr3·diglyme. The oxygen and bromine atoms are arranged in a meriodional configuration as shown by X-ray crystallography. However, in solution dissociation occurs into AlBr2(diglym)2 + and AlBr4 -. Not unexpectedly, AlBr3 forms the adduct AlBr3·2THF with tetrahydrofuran. The dioxane adduct AlBr3·diox is polymeric in the solid state and has a chain structure with pentacoordinated trigonal-bipyramidal Al centers. The Br atoms are arranged in a slightly distorted trigonal plane. In solution, the presence of tetracoordinated Al is indicated by 27AI NMR spectroscopy. AlBr3 cleaves one of the C-O bonds of 12-crown-4 leading to the dimer of ω-bromo-tetra(ethyleneoxy)aluminium dibromide. This dimer contains pentacoordinated Al centers as proven by X-ray structure analysis. In contrast, the adduct of AlBr3 with 15-crown-5 is most likely an ionic compound, and is best described as [AlBr2(15-crown-5)]AlBr4 based on 27Al NMR and IR data.
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