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

The synthesis and characterization of aluminum alkyl and alkoxide complexes bearing racemic 6,6'-di-methylbiphenyl-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-R-1-4-R-2-phenol): (LH2)-H-1, R-1 = R-2 = Me; (LH2)-H-2, R-1 = Bu-t, R-2 = Me; (LH2)-H-3, R-1 = R-2 = cumyl; (LH2)-H-4, R-1 = Br, R-2 = Bu-t) afforded the corresponding mono-and dinuclear aluminum methyl complexes [L1-3AlMe (1-3), L1-4Al2Me4 (4-7)]. Aluminum alkoxide complexes (LAlOPr)-Al-2-Pr-i (8), (LAlOBn)-Al-2 (9), and alpha-alkoxy ester complexes (LAl)-Al-2(OCMe2CO2Me) (10), (LAl)-Al-2[(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 alpha-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 Al-27 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 alpha-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 degrees 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.