Bis(allyl)aluminum Cation, Tris(allyl)aluminum, and Tetrakis(allyl)aluminate: Synthesis, Characterization, and Reactivity

Cationic, neutral, and anionic aluminum allyl compounds were synthesized,
and their reactivity toward electrophiles was studied. The THF adduct
of the previously elusive tris(allyl)aluminum, [Al(η1-C3H5)3(THF)] (1),
was isolated as an oil. Protonolysis of one allyl ligand in 1 using [NEt3H][BPh4] gave the cationic
bis(allyl)aluminum, a fragment of the crystalline [Al(η1-C3H5)2(THF)3−n]+[BPh4]−·(n+1)THF (n = 0, 1) (2). Single-crystal X-ray diffraction of [Al(η1-C3H5)2(THF)2]+[BPh4]− (2a) revealed a
tetrahedral aluminum center, while [Al(η1-C3H5)2(THF)3]+[BPh4]− (2b) contains a trigonal-bipyramidal
aluminum center with both allyl ligands in the equatorial plane. The
tetrakis(allyl)aluminate K+[Al(η1-C3H5)4]− (3) was also synthesized from the reaction of 1 with K(C3H5). Reactions of the allyl compounds 1−3 with (i) benzophenone, (ii) allyl halides
C3H5X (X = Cl, Br, I), and (iii) halogen X2 (X = Br, I) showed considerable difference with respect to
the ionic charge of the aluminum allyl.