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

The indium complex InL2N′′ has been prepared from the reaction of 2 equiv of (tBu)2P(O)CH2CH(tBu)OH (HL) with InN′′3 (N′′ = N(SiMe3)2). This complex reacts with a further equivalent of 2,6-di-tert-butylphenol or HL to afford the adducts InL2(OAr) and InL3, respectively. Confirmation that the anion L− exhibits “ligand self-recognition” in the formation of predominantly homochiral complexes RR-InL2N′′ and SS-InL2N′′ is obtained from 1H and 31P NMR spectroscopic data. However, the self-recognition is less effective at the indium cation, and mixtures of InL3 complexes with different configurations are observed. Single-crystal X-ray diffraction data confirm the five-coordinate, distorted bipyramidal In center in InL2N′′ and InL2(OAr) as anticipated. Selected crystals of InL3 show two of the possible configurations: one is the fac-RRR-InL3 complex, analogous to the lanthanide complexes LnL3 reported previously (Ln = Y, Eu, Er, Yb); another is the alternative, homochiral mer form RRR′-InL3. 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, InL3 is the fastest initiator among the three complexes for the polymerization of rac-lactide, and shows the best tacticity control. The polylactide samples have high molecular weights Mn,exp (between 44 000 and 270 000 g/mol at completion) and narrow polydispersities (as low as 1.25 at completion).