Synthesis, structure, and catalytic activity of trinuclear zinc complexes with new chiral biaryl-based NO2 ligands

Three new trinuclear organozinc complexes (4H)2Zn3Et2 (7), (5H)2Zn3Et2 (8) and (6H)2Zn3Et2 (9) have been prepared in good yields by alkane elimination between ZnEt2 and chiral biaryl-based NO2 ligands, (S)-2-(4,6-di-tert-butylhydroxyphenyl-2-methylamino)-2′-hydroxy-1,1′-binaphthyl (4H3), (S)-5,5′,6,6′,7,7′,8,8′-octahydro-2-(4,6-di-tert-butylhydroxyphenyl-2-methylamino)-2′-hydroxy-1,1′-binaphthyl (5H3) and (S)-2-(6-tert-butylhydroxyphenyl-2-methylamino)-2′-hydroxy-6,6′-dimethyl-1,1′-biphenyl (6H3), which are derived from (S)-2-amino-2′-hydroxy-1,1′-binaphthyl or (S)-2-amino-2′-hydroxy-6,6′-dimethyl-1,1′-biphenyl, respectively. Complexes 7–9 have been characterized by various spectroscopic techniques, elemental analyses, and X-ray diffraction analyses. Complex 7 derived from 4H3, has a C2v quasi-symmetric EtZnO2ZnN2(OPh)2ZnEt core structure, while complexes 8 and 9 derived from 5H3 or 6H3, adopt a C2 symmetric/quasi-symmetric EtZnO(OPh)ZnN2O(OPh)ZnEt core structure, indicating that the steric effect of the ligand plays an important role in the complex formation. Complexes 7–9 are active catalysts for the polymerization of rac-lactide, leading to the heterotactic-rich polylactides.