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 (C5Me5 = 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(AlMe4)(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)(CH2)(Me)(8)] and metallocene dimers [Cp*(2)Ln(AlMe4)](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)(AlMe4)](4) or monolanthanum complex [Cp*La(AlMe4){Me3Al(CH2)OtBu}] and monoyttrium complex [Cp*Y(AlMe4)(Me(3)AlOtBu)], respectively. Complexes [Cp*Ln(AlMe4)(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 [Ph3C][B(C6F5)(4)] and [PhNHMe2][B(C6F5)(4)], as well as borane B(C6F5)(3). The trans-directing effect of AlMe3 in the binary systems [Cp*Ln(AlMe4)(2)]/borate is revealed and further corroborated by the fabrication of high-cis-1,4 polybutadiene (97%) with "aluminum-free" [Cp*DyMe2](3)/[Ph3C][B(C6F5)(4)]. The formation of multimetallic active species is supported by the polymerization activity of pre-isolated cluster [(Cp*Ho)(3)Me-4(CH2)(thf)(2)].