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

Abstract 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 ( L 1 H 2 , R 1 =R 2 =Me; L 2 H 2 , R 1 = t Bu, R 2 =Me; L 3 H 2 , R 1 =R 2 =cumyl; L 4 H 2 , R 1 =CPh 3 , R 2 =Me) were reacted with {Mg[N(SiMe 3 ) 2 ] 2 } 2 to provide mononuclear and dinuclear magnesium complexes [ L 1 2 Mg 2 ] ( 1 ), L 2–4 Mg ( 2 – 4 ), { L 1 – 3 [MgN(SiMe 3 ) 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.