Ligand-Assisted Formation of Heterobimetallic Adducts of AlMe3and ZnMe2with 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.