Ligand-Assisted Formation of Heterobimetallic Adducts of AlMe ₃ and ZnMe ₂ with Zr and Hf Salan Catalysts for Olefin Polymerization

Cationic Salan Zr- and Hf-benzyl precatalysts for olefin polymerization were reacted with AlMe₃ and ZnMe₂ to probe their tendency to form stable heterobimetallic adducts and understand the role played by the ligand-based Lewis-basic functionalities in modulating the nature of such adducts. NMR studies in solution and DFT computations show that unlike typical metallocenes, Salan complexes bind AlMe₃ and ZnMe₂ to form only a single μ-Me interaction between the transition and the main-group metal. The latter is engaged in a further interaction with one oxygen atom of the ligand, leading to an unsymmetrical adduct. The formed heterobimetallic complexes are fluxional in solution, and their chemical exchange patterns were characterized by ¹H EXSY NMR. In the case of AlMe₃, exchange between bridging and terminal Al-Me moieties occurs preferentially via an intramolecular mechanism without the involvement of external AlMe₃. In stark contrast, both bridging and terminal Zn-Me groups undergo chemical exchange with external ZnMe₂ in the corresponding heterobimetallic adduct. Quantification of the activation parameters for Hf/ZnMe₂ systems suggests a dissociative mechanism for the exchange involving the bridging methyl and an associative mechanism for the terminal methyl.