Bis(permethylcyclopentadienyl)aluminum compounds: Precursors to [Cp*Al-2](+) but not to Cp*Al-3

Replacement of only two chloride ligands occurs when AlCl3 is reacted with 3 equiv of either Cp*Na or Cp*K, forming Cp*2AlCl (1) [Cp* = C5Me5]. This c ompound dissociates a chloride ion in polar solvents such as methylene chlo ride. Clean formation of decamethyl-aluminocenium salts could not be accomp lished by abstraction of a chloride ligand from 2 with silver salts, LiB(C6 F5)(4), or NaB(C6F5)(4) However, reaction of Cp*2AlMe (2) with B(C6F5)(3). cleanly affords the salt [CP*2Al](+) [B(C6F5)(3)Me](-) (3). X-ray crystal s tructures of both 2 and 3 are reported. In the solid state, 2 exhibits a bi s-dihapto ring coordination similar to that of (eta(2)-C5H5)(2)AlMe. The mo lecular structure of 3 shows complete methyl anion abstraction from the alu minum. The decamethylaluminocenium salt is more stable in solution than its nonmethylated analogue; however, it is also considerably less active as an initiator for the cationic polymerization of isobutene.