Magnesium complexes containing biphenyl-based tridentate imino-phenolate ligands for ring-opening polymerization of rac-lactide and α-methyltrimethylene carbonate

Abstract

A series of racemic 2-[(2′-(dimethylamino)biphenyl-2-ylimino)methyl]-4-R²-6-R¹-phenols (L¹H–L⁴H) were reacted with {Mg[N(SiMe₃)₂]₂}₂ to provide four heteroleptic magnesium complexes (L^1–4)MgN(SiMe₃)₂·(THF)_n (1, R¹ = ^tBu, R² = Me, n = 1; 2, R¹ = R² = CMe₂Ph, n = 0; 3, R¹ = CPh₃, R² = ^tBu, n = 1; 4, R¹ = Br, R² = ^tBu, n = 0), which have been fully characterized. X-ray structural determination shows that complex 1 possesses a monomeric structure, but complex 4 is dimeric with C₂-symmetry where the two metal centers are bridged by two phenolate oxygen atoms of the ligands. The coordination geometry around the magnesium center in these complexes can be best described as a distorted tetrahedral geometry. The heteroleptic complexes 1–4 efficiently initiate the ring-opening polymerization of rac-lactide and α-methyltrimethylene carbonate (α-MeTMC) and the polymerizations are better controlled in the presence of 2-propanol. In general, the introduction of a bulky ortho-substituent on the phenoxy unit results in increases of both the catalytic activity and the stereo- or regioselectivity of the corresponding magnesium complex. Microstructure analyses of the resulting PLAs revealed that P_r values range from 0.46 to 0.81, depending on the catalyst and the polymerization conditions. For racemic α-MeTMC, detailed analyses using ¹H and ¹³C NMR spectroscopy indicated the preferential ring-opening of α-MeTMC at the most hindered oxygen–acyl bond (X_reg = 0.65–0.86).