Aluminum methyl, alkoxide and α-alkoxy ester complexes supported by 6, 6′-dimethylbiphenyl-bridged salen ligands: synthesis, characterization and catalysis for rac-lactide polymerization. Issue 15 (14th March 2016)

• Record Type: Journal Article
• Authors: Kan, Chao; Ge, Jilei; Ma, Haiyan
• Is Part Of: Dalton transactions. Volume 45:Issue 15(2016)
• Page Start: 6682
• Page End: 6695
• Publication Date: 2016-03-14
• Subjects: Chemistry, Inorganic -- Periodicals; Chemistry, Physical and theoretical -- Periodicals; Chemistry, Inorganic -- Periodicals; 546.05
• DOI: 10.1039/c5dt04633e
• Languages: English
• ISSNs: 1477-9226

Abstract

Abstract : A series of mono- and dinuclear salen aluminum complexes could efficiently catalyze the ROP of rac -LA, producing isotactic enriched and atactic PLA, respectively. In solution, the association/dissociation equilibrium of the carbonyl group was found in the model O -lactate complexes. 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 proligandsL 1−4 H2 (6, 6′-[(6, 6′-dimethyl-[1, 1′-biphenyl]-2, 2′-diyl)bis(nitrylomethilidyne)]-bis(2-R 1 -4-R 2 -phenol):L 1 H2, R 1 = R 2 = Me;L 2 H2, R 1 = t Bu, R 2 = Me;L 3 H2, R 1 = R 2 = cumyl;L 4 H2, R 1 = Br, R 2 = t Bu) afforded the corresponding mono- and dinuclear aluminum methyl complexes [L 1−3 AlMe (1–3 ), L 1−4 Al2 Me4 (4–7 )]. Aluminum alkoxide complexesL 2 AlO i Pr (8 ), L 2 AlOBn (9 ), and α-alkoxy ester complexesL 2 Al(OCMe2 CO2 Me) (10 ), L 2 Al[( S )-OCHMeCO2 Me] (11 ) were prepared via in situ alcoholysis of the parent aluminum methyl complex2 with the corresponding alcohols. The molecular structures of mononuclear complexes1–3, dinuclear complex6, alkoxide complexes8–9 and α-alkoxy ester complexes10–11 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 27 Al NMR spectra of complexes10 and11, 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. Complex11 is also a rare example of an O -lactate model complex that mimics the first insertion ofl -LA. All complexes were investigated for the ROP of rac -LA at 110 °C in toluene. The polymerization initiated by complexes1–3 in the presence of i PrOH showed living features, affording PLAs with narrow molecular weight distributions (PDIs = 1.03–1.05) and 65–73% isotacticities. Particularly, complex8 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.