Heteroscorpionate Rare-Earth Metal Zwitterionic Complexes: Syntheses, Characterization, and Heteroselective Catalysis on the Ring-Opening Polymerization of rac-Lactide
作者：Zhichao Zhang and Dongmei Cui
关键字：C-H activation, heteroscorpionates, polymerization, rare earths, zwitterions
Novel neutral phosphine-modified heteroscorpionate ligand (3,5-Me2Pz)2CHPPh2(1) and its derivatives oxophosphine (2) and iminophosphine (3) heteroscorpionates were synthesized for the first time. These neutral heteroscorpionate ligands displayed unique chemistry towards rare-earth metal tris(alkyl)s [Ln(CH2SiMe3)3(thf)2] (Ln=Y, Lu, Sc). The reaction between compound 1 and [Ln(CH2SiMe3)3(thf)2] afforded heteroscorpionate rare-earth metal trialkyl adduct complexes 4?a–c. Compounds 2 and 3 were treated with [Ln(CH2SiMe3)3(thf)2] to give the unprecedented zwitterionic heteroscorpionate rare-earth metal dialkyls 5 and 6, respectively. In the process, the heteroscorpionate transferred to the carbanions by means of methine C-H bond cleavage that was attributed to the presence of the electron-withdrawing groups. In addition the ligand and central metal showed a concerted effect on both the catalytic activity and specific selectivity of complexes 4–6 for the ring-opening polymerization (ROP) of rac-lactide (rac-LA). All the adduct complexes 4 were nonselective and gave atactic polylactide (PLA), probably due to the dissociation of ligand 1 from the active metal center during the polymerization. Strikingly, zwitterionic complexes 5 catalyzed rapid ROP of rac-LA to produce PLAs with heterotacticity up to 0.87. However, the zwitterionic complexes 6 were less active and less selective than 5, which might be on account of the stronger coordination of the tetradentate ligand. Complexes 5 represent rare examples of the selective ROP of rac-LA mediated by rare-earth metal complexes supported by non-bisphenolate ligands.