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(AlMe 4 ) bearing a [ N N N ] 2- post-metallocene-type ligand (H 2 BDPPpyr = 2,6-bis-(((2,6-diisopropylphenyl)amino)methyl)pyridine) were obtained by two different synthesis routes. Reaction of (BDPPpyr)Ln(NEt 2 )(THF) with trimethylaluminum afforded complexes (BDPPpyr)Ln(AlMe 4 ) 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(AlMe 4 ) 3 with H 2 BDPPpyr produced (BDPPpyr)Ln(AlMe 4 ), along with the byproduct (BDPPpyr)(AlMe 2 ) 2 . Dynamic NMR spectroscopy of (BDPPpyr)Ln(AlMe 4 ) revealed distinct fluxional behavior of the AlMe 4 - ligand depending on the metal size (Lu: associative via Lu(μ-Me) 3 AlMe; Sc: dissociative via Sc(μ-Me)AlMe 3 ). In the presence of trimethylaluminum, the yttrium derivative undergoes a ligand backbone metalation at the isopropyl methyl group yielding (BDPPpyr -H )Y[(μ-Me)AlMe 2 ] 2 featuring a [ N N NC ] 3- -type ligand. For the lutetium derivative, addition of THF caused cyclometallation products (BDPPpyr -H )Lu[(μ-Me)AlMe 2 ](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.