Magnesium and Calcium Complexes Containing Biphenyl-Based Tridentate Iminophenolate Ligands for Ring-Opening Polymerization of <i>rac</i>-Lactide

A series of racemic 2-[(2′-methoxybiphenyl-2-ylimino)­methyl]-4-R²-6-R¹-phenols (<b>L</b>^<b>1</b><b>H</b>-<b>L</b>^<b>8</b><b>H</b>) were reacted with {Mg­[N­(SiMe₃)₂]₂}₂ and Ca­[N­(SiMe₃)₂]₂·2THF (THF = tetrahydrofuran), respectively, to provide nine heteroleptic magnesium complexes <b>L</b>^{<b>1</b>–<b>8</b>}MgN­(SiMe₃)₂ [R¹ = ^<i>i</i>Pr, R² = H (<b>1a</b>); R¹ = ^<i>t</i>Bu, R² = Me (<b>2a</b> and <b>2a</b>·THF); R¹ = R² = ^<i>t</i>Bu (<b>3a</b>); R¹ = R² = CMe₂Ph (<b>4a</b>); R¹ = CPh₃, R² = ^<i>t</i>Bu (<b>5a</b>); R¹ = 1-piperidinylmethyl, R² = ^<i>t</i>Bu (<b>6a</b>); R¹ = Cl, R² = ^<i>t</i>Bu (<b>7a</b>); R¹ = Br, R² = ^<i>t</i>Bu (<b>8a</b>)], two homoleptic calcium complexes (<b>L</b>^<b>2,5</b>)₂Ca (<b>2b</b> and <b>5b</b>), and one heteroleptic calcium complex [(<b>L</b>^<b>4</b>)­CaN­(SiMe₃)₂·THF] (<b>4b</b>), which have been fully characterized. In the solid state, magnesium complexes <b>2a</b> and <b>6a</b> are isostructural, and each possesses a monomeric structure, while magnesium complexes <b>7a</b> and <b>8a</b> are dimeric, 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. Although bearing the same iminophenoloate ligand, the molecular structures of complexes <b>2a</b> and <b>2a</b>·THF are different from each other. In complex <b>2a</b>·THF, the coordination of one molecule of THF to the magnesium atom leads to dissociation of the methoxy group of the ligand from the metal center. The homoleptic calcium complex <b>2b</b> has a six-coordinate metal core ligated by all six donor atoms of two iminophenolate ligands. The heteroleptic magnesium complexes <b>1a</b>–<b>8a</b> and calcium complex <b>4b</b> proved to be efficient initiators for the ring-opening polymerization of <i>rac</i>-lactide at ambient temperature in THF or at 70 °C in toluene, and the polymerizations were better controlled in the presence of 2-propanol. The introduction of a bulky ortho substituent on the phenoxy unit of the ligand resulted in an increase of the catalytic activity of the corresponding metal complex. Microstructure analysis of the resultant poly­(<i>rac</i>-lactide) samples via homonuclear-decoupled ¹H NMR spectroscopy revealed <i>P</i>_r values ranging from 0.60 to 0.81, which closely depended on the employed catalyst and polymerization conditions