Ligand-Directed Assembly of Multimetallic Aluminum Complexes: Synthesis, Structure and ROP Catalysis

Multinuclear aluminum complexes are useful catalysts for the ring opening polymerization (ROP) of ϵ-caprolactone (ϵ-CL), often outperforming monometallic analogues in the production of biodegradable polycaprolactone (PCL). These complexes require control over metal-metal distance, ligand structure, and substituents. Herein, we report on the preparation of mono-, bi-, and octametallic aluminum complexes through direct chelation of multidentate bis(phosphino)-2,2-dimethyl-1,3-diamino ligands Me₂C[CH₂NHR₂P(O)]₂; R=Ph(1), ⁱPr(2), ^tBu(3). Ligands 1–3 yielded bimetallic complexes with two types of coordination pockets, as well as bimetallic systems with identical coordination environments. In addition to the diversity of structural patterns within the present series, the catalytic performances of [AlR₂{κ²-N,N’,–(Me₂C{CH₂NPh₂P(O)}₂}-(κ²-O,O’-AlR₂)] (R=Me(4), Et(5), ⁱBu(6)) outperformed that of the monometallic [AlEt₂{κ²-O,O’-(Me₂C(CH₂N^tBu₂P(O))₂H} (10), showing controlled and immortal ROP of ϵ-CL in the presence of BnOH as co-initiator. DFT calculations suggest that the primary active site for 4–6 is the N−Al–N center. Gaining insight into the underlying principles that lead to high activity and controlled production of PCL in bimetallic systems can help design tailor-made complexes with enhanced performance.