Chiral Indium Alkoxide Complexes as Initiators for the Stereoselective Ring-Opening Polymerization of rac-Lactide

The indium complex InL(2)N '' has been prepared from the reaction of 2 equiv of ((t)Bu)(2)P(O)CH(2)CH((t)Bu)OH (HL) with InN ''(3) (N = N(SiMe(3))(2)). This complex reacts with a further equivalent of 2,6-di-tert-butylphenol or HL to afford the adducts InL(2)(OAr) and InL(3), respectively. Confirmation that the anion L(-) exhibits ligand self-recognition in the formation of predominantly homochiral complexes RR-InL2N and SS-InL(2)N '' is obtained from H and (31)P NMR spectroscopic data. However, the self-recognition is less effective at the indium cation, and mixtures of InL(3) complexes with different configurations are observed. Single-crystal X-ray diffraction data confirm the five-coordinate, distorted bipyramidal In center in InL(2)N '' and InL(2)(OAr) as anticipated. Selected crystals of InL(3) show two of the possible configurations: one is the fac-RRR-InL(3) complex, analogous to the lanthanide complexes LnL(3) reported previously (Ln = Y, Eu, Er, Yb); another is the alternative, homochiral mer form RRR-InL(3). All three complexes are efficient single-component initiators for the ring-opening polymerization of rac-lactide over a wide range of temperatures and monomer-to-initiator ratios, exhibiting reasonable control over the synthesis of isotactic polylactide. Despite its poorly defined structure, InL(3) is the fastest initiator among the three complexes for the polymerization of raclactide, and shows the best tacticity control. The polylactide samples have high molecular weights M(n,exp) (between 44 000 and 270 000 g/mol at completion) and narrow polydispersities (as low as 1.25 at completion).