Aluminum methyl, alkoxide and α-alkoxy ester complexes supported by 6,6′-dimethylbiphenyl-bridged salen ligands: synthesis, characterization and catalysis for rac-lactide polymerization

Abstract

The synthesis and characterization of aluminum alkyl and alkoxide complexes bearing racemic 6,6′-dimethylbiphenyl-bridged salen-type ligands, and their catalysis in the ring-opening polymerization (ROP) of rac-lactide are reported. Reactions of AlMe₃ with various amounts of the proligands L¹⁻⁴H₂ (6,6′-[(6,6′-dimethyl-[1,1′-biphenyl]-2,2′-diyl)bis(nitrylomethilidyne)]-bis(2-R¹-4-R²-phenol): L¹H₂, R¹ = R² = Me; L²H₂, R¹ = ^tBu, R² = Me; L³H₂, R¹ = R² = cumyl; L⁴H₂, R¹ = Br, R² = ^tBu) afforded the corresponding mono- and dinuclear aluminum methyl complexes [L¹⁻³AlMe (1–3), L¹⁻⁴Al₂Me₄ (4–7)]. Aluminum alkoxide complexes L²AlOⁱPr (8), L²AlOBn (9), and α-alkoxy ester complexes L²Al(OCMe₂CO₂Me) (10), L²Al[(S)-OCHMeCO₂Me] (11) were prepared via in situ alcoholysis of the parent aluminum methyl complex 2 with the corresponding alcohols. The molecular structures of mononuclear complexes 1–3, dinuclear complex 6, alkoxide complexes 8–9 and α-alkoxy ester complexes 10–11 were established by single-crystal X-ray diffraction studies. Two broad resonances at about 69–70 ppm and 25–41 ppm were observed in the ²⁷Al NMR spectra of complexes 10 and 11, indicating the existence of both four- and five-coordinate aluminum centers in solution, which results from the dissociation of one N donor of the salen ligand, accompanied by an association and dissociation equilibrium of the carbonyl group of the α-alkoxy ester ligand to the aluminum center. Complex 11 is also a rare example of an O-lactate model complex that mimics the first insertion of L-LA. All complexes were investigated for the ROP of rac-LA at 110 °C in toluene. The polymerization initiated by complexes 1–3 in the presence of ⁱPrOH showed living features, affording PLAs with narrow molecular weight distributions (PDIs = 1.03–1.05) and 65–73% isotacticities. Particularly, complex 8 showed an “immortal” behavior for the polymerization of rac-LA in the presence of excess alcohol. Compared with the mononuclear counterparts, the tetra-coordinate dinuclear aluminum complexes enabled a few fold boosts in activity, but gave atactic PLAs with broadened PDIs.