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

Abstract

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.