Group 4 metal complexes with new chiral pincer NHC-ligands: synthesis, structure and catalytic activity

Abstract

Chiral group 4 NHC–metal complexes were prepared in good yields by amine elimination from M(NR₂)₄ (M = Ti, Zr, Hf; R = Me, Et) and chiral pincer NHC-ligands, L4 (L4a and L4b), L5 and L6, which are derived from (S,S)-diphenyl-1,2-ethanediamine. Treatment of M(NR₂)₄ with 1 equiv. of L4 in THF gives, after recrystallization from a benzene solution, the chiral titanium amides (L4)Ti(NMe₂)(Br)(THF) (7) and (L4)Ti(NMe₂)(Cl)(THF) (11), zirconium amides (L4)Zr(NMe₂)(Br)(THF) (8), (L4)Zr(NEt₂)(Br)(THF) (10), (L4)Zr(NMe₂)(Cl)(THF) (12) and (L4)Zr(NEt₂)(Cl)(THF) (14), and hafnium amides (L4)Hf(NMe₂)(Br)(THF) (9) and (L4)Hf(NMe₂)(Cl)(THF) (13), respectively. Similarly, the reactions of L5 or L6 with 1 equiv. of M(NR₂)₄ yield the titanium amide (L6)Ti(NMe₂)(Cl)(THF) (16), the zirconium amides (L5)Zr(NMe₂)(Cl)(THF) (15), (L6)Zr(NMe₂)(Cl)(THF) (17) and (L6)Zr(NEt₂)(Cl)(THF) (19), and the hafnium amide (L6)Hf(NMe₂)(Cl)(THF) (18), respectively. Complexes 7–19 were characterized by various spectroscopic techniques and elemental analyses. The molecular structures of 10 and 14–19 were also established by X-ray diffraction analyses, which represent the first example of the structurally characterized group 4 chiral NHC–metal complex. Furthermore, 7–19 are active catalysts for the polymerization of rac-lactide in the presence of isopropanol, leading to the heterotactic-rich polylactides.