Magnesium Complexes Supported by 6,6'-Dimethylbiphenyl-Bridged Salen Ligands: Synthesis, Characterization and Catalysis for rac-Lactide Polymerization

A series of racemic 6,6'-[(6,6'-dimethyl-[1,1'-biphenyl]-2,2'-diyl)bis(nitrylomethilidyne)]-bis(2-R-1-4-R-2-phenol) proligands ((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=CPh3, R-2=Me) were reacted with {Mg[N(SiMe3)(2)](2)}(2) to provide mononuclear and dinuclear magnesium complexes [(L2Mg2)-Mg-1] (1), L2-4Mg (2-4), {L-1(-3)[MgN(SiMe3)(2)](2)} (5-7). Complexes 3* and 4* in which each metal center is coordinated with a THF molecule were obtained when the corresponding crude complexes were recrystallized with a mixture of THF and n-hexane. Similarly, the formation of THF coordinated structure 7* of the heteroleptic dinuclear complex 7 was identified. The molecular structures of complexes 3*, 6 and 7* were established by X-ray single crystal diffraction studies, which show that mononuclear complex 3* possesses a five-coordinated metal center adopting a distorted square pyramid configuration, the two metal centers of the dinuclear complex 6 are bridged by two phenoxy oxygen atoms and each has a four-coordinated distorted tetrahedral configuration, and each metal center of the dinuclear complex 7* is still four-coordinated upon the coordination of THF but without bridging to each other. All complexes were investigated for the ring-opening polymerization (ROP) of rac-lactide (rac-LA) at 60 degrees C in toluene or tetrahydrofuran. Compared with the mononuclear counterparts, the dinuclear magnesium silylamido complexes showed significantly higher activities.

Automated summary

A series of chiral dinuclear magnesium complexes and heterobimetallic dinuclear magnesium complexes were synthesized through reactions with {Mg[N(SiMe3)(2)](2)}(2). The heterobimetallic dinuclear magnesium complexes showed higher activities in the ring-opening polymerization of rac-lactide.