Bis(permethylcyclopentadienyl)aluminum Compounds: Precursors to [Cp*2Al]+ but Not to Cp*3Al

Replacement of only two chloride ligands occurs when AlCl 3 is reacted with 3 equiv of either Cp*Na or Cp*K, forming Cp* 2 AlCl ( 1 ) [Cp* = C 5 Me 5 ]. This compound dissociates a chloride ion in polar solvents such as methylene chloride. Clean formation of decamethylaluminocenium salts could not be accomplished by abstraction of a chloride ligand from 2 with silver salts, LiB(C 6 F 5 ) 4, or NaB(C 6 F 5 ) 4 . However, reaction of Cp* 2 AlMe ( 2 ) with B(C 6 F 5 ) 3 cleanly affords the salt [Cp* 2 Al] + [B(C 6 F 5 ) 3 Me] - ( 3 ). X-ray crystal structures of both 2 and 3 are reported. In the solid state, 2 exhibits a bis-dihapto ring coordination similar to that of (η 2 -C 5 H 5 ) 2 AlMe. The molecular structure of 3 shows complete methyl anion abstraction from the aluminum. 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.