C–H-Bond Activation and Isoprene Polymerization Studies Applying Pentamethylcyclopentadienyl-Supported Rare-Earth-Metal Bis(Tetramethylaluminate) and Dimethyl Complexes

As previously shown for lutetium and yttrium, 1,2,3,4,5-pentamethylcyclopentadienyl (C(5)Me(5) = Cp*)-bearing rare-earth metal dimethyl half-sandwich complexes [Cp*LnMe(2)](3) are now also accessible for holmium, dysprosium, and terbium via tetramethylaluminato cleavage of [Cp*Ln(AlMe(4))(2)] with diethyl ether (Ho, Dy) and tert-butyl methyl ether (TBME) (Tb). C–H-bond activation and ligand redistribution reactions are observed in case of terbium and are dominant for the next larger-sized gadolinium, as evidenced by the formation of mixed methyl/methylidene clusters [(Cp*Ln)(5)(CH(2))(Me)(8)] and metallocene dimers [Cp*(2)Ln(AlMe(4))](2) (Ln = Tb, Gd). Applying TBME as a “cleaving” reagent can result in both TBME deprotonation and ether cleavage, as shown for the formation of the 24-membered macrocycle [(Cp*Gd)(2)(Me)(CH(2)OtBu)(2)(AlMe(4))](4) or monolanthanum complex [Cp*La(AlMe(4)){Me(3)Al(CH(2))OtBu}] and monoyttrium complex [Cp*Y(AlMe(4))(Me(3)AlOtBu)], respectively. Complexes [Cp*Ln(AlMe(4))(2)] (Ln = Ho, Dy, Tb, Gd) and [Cp*LnMe(2)](3) (Ln = Ho, Dy) are applied in isoprene and 1,3-butadiene polymerization, upon activation with borates [Ph(3)C][B(C(6)F(5))(4)] and [PhNHMe(2)][B(C(6)F(5))(4)], as well as borane B(C(6)F(5))(3). The trans-directing effect of AlMe(3) in the binary systems [Cp*Ln(AlMe(4))(2)]/borate is revealed and further corroborated by the fabrication of high-cis-1,4 polybutadiene (97%) with “aluminum-free” [Cp*DyMe(2)](3)/[Ph(3)C][B(C(6)F(5))(4)]. The formation of multimetallic active species is supported by the polymerization activity of pre-isolated cluster [(Cp*Ho)(3)Me(4)(CH(2))(thf)(2)].