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 < 3.442 h–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.