Chiral Group 4 Cyclopentadienyl Complexes and Their Use in Polymerization of Lactide Monomers

A family of group 4 alkoxide and aryloxide complexes of a chiral cyclopentadienyl-derived (hydro)­permethylpentalenyl ligand (C8Me6H; Pn*­(H)) have been prepared and fully characterized. Both racemic and enantiopure complexes of all group 4 congeners were prepared with a wide variety of alkoxide and aryloxide ligands. The complexes Pn*­(H)­Ti­(OtBu)3 (1), Pn*­(H)­Ti­(O-2,6-Me-C6H3)3 (2), Pn*­(H)­Zr­(OtBu)3 (3), Pn*­(H)­Zr­(OCH2Ph)3 (4), Pn*­(H)­Zr­(S-OCH­{CH3}­C6H5)3 (5), Pn*­(H)­Zr­(rac-OCH­{CH3}­C6H5)3 (6), Pn*­(H)­Zr­(O-2,6-Me-C6H3)3 (7), Pn*­(H)­Zr­(O-2,6-iPr-C6H3)3 (8), Pn*­(H)­ZrCl2(O-2,6-tBu-C6H3) (9), Pn*­(H)­Hf­(O-2,6-Me-C6H3)3 (10), Pn*­(H)­HfCl­(O-2,6-iPr-C6H3)2, (11), and Pn*­(H)­HfCl2(O-2,6-tBu-C6H3) (12) were prepared by the reaction of Pn*­(H)­MCl3 complexes with the corresponding potassium alkoxides and aryloxides. Single-crystal X-ray diffraction studies implied that, despite multiple diastereomers being possible for each complex, the diastereomers isolated are limited to configurations in which the methyl group at the chiral center is always oriented anti to the metal center in order to minimize steric hindrance (R,RP and S,SP). The complexes were investigated as initiators for the ring-opening polymerization of l- and rac-lactide in order to ascertain if these mixtures of diastereomers could exert any stereocontrol on the resulting polymerization. Kinetic studies were completed to explore the effects of the metal cation, chiral (hydro)­permethylpentalenyl ligand, ancillary ligands, initiator concentration and temperature. Both Pn*­(H)­Zr­(S–OCH­{CH3}­C6H5)3 and Pn*­(H)­Zr­(rac-OCH­{CH3}­C6H5)3 demonstrated very high rates of propagation for l- and rac-lactide (1.885 kobs –1) at 100 °C. The observed propagation rates using Pn*­(H)­Zr­(rac-OCH­{CH3}­C6H5)3 are around 70% faster for l-lactide and rac-lactide in comparison to those using Pn*­(H)­Zr­(S-OCH­{CH3}­C6H5)3. The polymers were characterized by NMR spectroscopy, GPC, and MALDI-ToF mass spectrometry in order to investigate the tacticities and polydispersities of the polymerizations.