Distinct C−H Bond Activation Pathways in Diamido-Pyridine-Supported Rare-Earth Metal Hydrocarbyl Complexes

Transition metal precatalyst−organoaluminum cocatalyst interactions are of fundamental importance
in Ziegler−Natta polymerization catalysis. Rare-earth metal tetramethylaluminate complexes (BDPPpyr)Ln(AlMe4) bearing a [NNN]2- post-metallocene-type ligand (H2BDPPpyr = 2,6-bis-(((2,6-diisopropylphenyl)amino)methyl)pyridine) were obtained by two different synthesis routes. Reaction of (BDPPpyr)Ln(NEt2)(THF) with trimethylaluminum afforded complexes (BDPPpyr)Ln(AlMe4) of the small rare-earth metals scandium and lutetium. Corresponding compounds of the larger metals yttrium and lanthanum
were synthesized according to the tetramethylaluminate route, i.e., the reaction of Ln(AlMe4)3 with H2BDPPpyr produced (BDPPpyr)Ln(AlMe4), along with the byproduct (BDPPpyr)(AlMe2)2. Dynamic NMR
spectroscopy of (BDPPpyr)Ln(AlMe4) revealed distinct fluxional behavior of the AlMe4- ligand depending
on the metal size (Lu:  associative via Lu(μ-Me)3AlMe; Sc:  dissociative via Sc(μ-Me)AlMe3). In the
presence of trimethylaluminum, the yttrium derivative undergoes a ligand backbone metalation at the
isopropyl methyl group yielding (BDPPpyr-H)Y[(μ-Me)AlMe2]2 featuring a [NNNC]3--type ligand. For
the lutetium derivative, addition of THF caused cyclometallation products (BDPPpyr-H)Lu[(μ-Me)AlMe2](THF) and [Lu(BDPPpyr-H)]2 involving the isopropyl methine proton. Present studies not only clearly
show the enhanced reactivity of rare-earth metal methyl moieties [Ln−Me] but also that excessive use
of organoaluminum cocatalysts can result in gradual ligand degradation and concomitant catalyst
deactivation. The findings might contribute to a better understanding of activation/deactiviation sequences
in post-metallocene-promoted olefin polymerization.