Inorganic Chemistry, Vol.49, No.2, 419-426, 2010
Chiral Indium Alkoxide Complexes as Initiators for the Stereoselective Ring-Opening Polymerization of rac-Lactide
The indium complex InL2N '' has been prepared from the reaction of 2 equiv of (Bu-t)(2)P(O)CH2CH(Bu-t)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 H and P-31 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 LnL(3) 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 raclactide, and shows the best tacticity control. The polylactide samples have high molecular weights M-n,M-exp (between 44 000 and 270 000 g/mol at completion) and narrow polydispersities (as low as 1.25 at completion).