Al and Zn complexes bearing N,N,N-tridentate quinolinyl anilido-imine ligands: synthesis, characterization and catalysis in l-lactide polymerization

Abstract

Reactions of N,N,N-tridentate quinolinyl anilido-imine ligands with AlMe₃ afford mononuclear aluminum complexes {κ³-[{2-[ArNC(H)]C₆H₄}N(8-C₉H₆N)]}AlMe₂ (Ar = 2,6-Me₂C₆H₃ (1a), 2,6-Et₂C₆H₃ (1b), 2,6-ⁱPr₂C₆H₃ (1c)) or dinuclear complexes AlMe₃{κ¹-[{2-[ArNC(H)C₆H₄]N(8-C₉H₆N)}-κ²]AlMe₂ (R = 2,6-Me₂C₆H₃ (2a), 2,6-Et₂C₆H₃ (2b), 2,6-ⁱPr₂C₆H₃ (2c)) depending on the ratios of reactants used. Similar reactions of ZnEt₂ with these ligands give the monoligated ethyl zinc complexes {κ³-[{2-[ArNC(H)]C₆H₄}N(8-C₉H₆N)]}ZnEt (Ar = 2,6-Me₂C₆H₃ (3a), 2,6-Et₂C₆H₃ (3b), 2,6-ⁱPr₂C₆H₃ (3c)) or bisligated complexes {κ³-[{2-[ArNC(H)]C₆H₄}N(8-C₉H₆N)]}Zn{κ²-[{2-[ArNC(H)]C₆H₄}N(8-C₉H₆N)]} (Ar = 2,6-Me₂C₆H₃ (4a), 2,6-Et₂C₆H₃ (4b), 2,6-ⁱPr₂C₆H₃ (4c)). These complexes were well characterized by NMR and the structures of 1a, 2a, 2c, 3b and 4c were confirmed by X-ray diffraction analysis. The aluminum and zinc complexes were tested to initiate lactide polymerization in which the zinc complexes show moderate to high activities in the presence of benzyl alcohol.