The Versatility of Aluminum Systems: Ligand Transfer Agents and Polymerization Catalysts

Abstract

Aluminum complexes, specifically those employing bulky ligand frameworks such as sal (sal = 2-[CH═N(2,6-ⁱPr₂-C₆H₃)]-4,6 - ͭ Bu₂-phenoxide) and α-diimine (α-diimine = [(2,6-ⁱPr₂-C₆H₃)N═C(Me)]₂) derivatives are studied in various contexts. During ethylene polymerization with LCu(II) catalysts in the presence of methylaluminoxane (MAO), ligand (L) transfer is observed from the copper centre to the aluminum centre present in MAO. In the α-diimine case, an (imino-amido)AlMe2 complex is formed by α-diimine ligand transfer to aluminum followed by alkylation of one imino moiety in the ligand backbone. These ligand transfer products are then shown to be active as ethylene polymerization catalysts, bringing into question the role of the copper species. The (sal)AlMe₂, (sal)AlMeCl and (imino-amido)AlMe₂ complexes were also used as initiators in the ring-opening polymerization of ε-caprolactone. Polymerization was studied with and without addition of tert-butanol as a co-initiator to determine its role and necessity in the catalytic cycle. Finally, the (imino-amido)AlMe₂ complex was also used as the starting complex in attempts at forming a mononuclear aluminum(I) target species. Reaction of (imino-amido)AlMe₂ with excess I₂ proved successful in forming the isolable precursor, (imino-amido)AlI₂. Attempts at reducing (imino-amido)AlI₂ with excess potassium were carried out in hopes of forming a very rare example of a mononuclear aluminum(I) species.