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

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.

Abstract

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), L^2–4 Mg (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 °C in toluene or tetrahydrofuran. Compared with the mononuclear counterparts, the dinuclear magnesium silylamido complexes showed significantly higher activities.

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