Mechanochemical Synthesis of [1,3-(SiMe3)2C3H3]3(Al,Sc), a Base-Free Tris(allyl)aluminum Complex and Its Scandium Analogue

Unsolvated tris­(allyl)­aluminum, (C3H5)3Al, has never been isolated, although adducts with THF, OPPh3, and pyridine are known. Attempts to make a base-free derivative with the bulky 1,3-bis­(trimethylsilyl)­allyl anion (A′) from the reaction of aluminum halides and K­[A′] in ethers or by deprotonation of HA′ with R3Al (R = Me, Et) were not successful. However, grinding AlX3 and K­[A′] in the solid state, followed by extraction of the mixture with hexanes, produces A′3Al in high yield. A single-crystal X-ray structure demonstrated that the monomeric molecule contains σ-bonded allyl ligands (average Al–C 1.964(3) Å) and, like the known gallium analogue, is fluxional in solution. A DFT study of the parent (C3H5)3Al indicates that the ligands would be of mixed hapticity, i.e., (η1-C3H5)2(η3-C3H5)­Al, although the trimethylsilylated ligands evidently prevent that conformation in A′3Al. A′3Al reacts immediately with benzophenone in hexanes to produce Al­(OCPh2A′)3 but negligibly with the bulky alcohol HOCPh3. Grinding A′3Al with ScCl3 yields no observable product, although the same reaction with K­[A′] produces the hexane-soluble A′3Sc, whose X-ray crystal structure reveals three π-bound A′ ligands.