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

Abstract

AbstractA 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=tBu, 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 [L12Mg2] (1), L2–4Mg (2–4), {L1–3[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 °C in toluene or tetrahydrofuran. Compared with the mononuclear counterparts, the dinuclear magnesium silylamido complexes showed significantly higher activities.