Synthesis, Characterization, and Lactide Polymerization Activity of Group 4 Metal Complexes Containing Two Bis(phenolate) Ligands

A series of group 4 metal complexes Zr-(1)2, Zr-(2)2, Zr-(3)2, Zr-(4)2, Zr-(5)2, Hf-(1)2, and Hf-(4)2 containing two bridged bis­(phenolate) ligands
of the (OSSO)-type were prepared by the reaction of the corresponding
bis­(phenol) and group 4 metal precursor MX4 (X = OiPr, CH2Ph) and isolated as robust,
colorless crystals. NMR spectra indicate D2 symmetry, in agreement with the solid state structure determined
by single crystal X-ray diffraction study of the complexes Zr-(1)2, Hf-(1)2, Zr-(3)2, Zr-(4)2, and Zr-(5)2. The complexes with the 1,4-dithiabutanediyl bridged ligands
exhibit a highly symmetric coordination around the metal center. The
introduction of the rigid trans-1,2-cyclohexanediyl
bridged ligands led to a distorted coordination around the metal center
in Zr-(4)2 and Zr-(5)2 when the ortho substituent
is tert-butyl and the para substituent
is larger than methyl. The complexes Zr-(1)2, Zr-(2)2, Zr-(3)2, Zr-(4)2 as well as Hf-(1)2 and Hf-(4)2 initiated the ring-opening
polymerization of meso-lactide at 100 °C to
give heterotactic polylactide with pronounced heterotacticity (>70%)
and varying polydispersity (1.05 < Mw/Mn < 1.61). As shown by kinetic studies,
zirconium complex Zr-(1)2 polymerized meso-lactide faster than the homologous hafnium complex Hf-(1)2.