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

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A series of mononuclear and dinuclear magnesium complexes bearing racemic 2,2’-diamino-6,6’-dimethylbiphenyl-bridged salen ligands were synthesized. The various structures of these complexes were fully characterized and their catalytic behaviors toward the ring-opening polymerization of rac-lactide were explored.


A series of racemic 6,6’-[(6,6’-dimethyl-[1,1’-biphenyl]-2,2’-diyl)bis(nitrylomethilidyne)]-bis(2-R1-4-R2-phenol) proligands (L1H2 , R1=R2=Me; L2H2 , R1= t Bu, R2=Me; L3H2 , R1=R2=cumyl; L4H2 , R1=CPh3, R2=Me) were reacted with {Mg[N(SiMe3)2]2}2 to provide mononuclear and dinuclear magnesium complexes [L1 2Mg2] (1), L2–4 Mg (24), {L1 3 [MgN(SiMe3)2]2} (57). 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 °C in toluene or tetrahydrofuran. Compared with the mononuclear counterparts, the dinuclear magnesium silylamido complexes showed significantly higher activities.

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