Bisazo-Salen Lanthanum Complexes for Stereoselectivity-Switchable Ring-Opening Polymerization of <i>rac</i> -Cyclic Esters

The development of catalysts featuring precisely designed ligands that simultaneously deliver high catalytic activity, precise stereocontrol, and conformational adaptability remains an important issue in catalytic chemistry. Herein, we report novel Salen-like bisazobisphenols, termed bisazo-Salen ligands and lanthanum (La) bisazo-Salen catalysts, where the conventional imine (C═N) groups in Salen are replaced with diazene (N═N) groups. Because of the electronic donating ability of N═N groups, one catalyst achieves exceptionally high activity (TOF = 1.4 × 104 h–1) for the ring-open polymerization (ROP)s of racemic lactide (rac-LA) at room temperature, outperforming structurally analogous C═N-bridged La catalysts by 380-fold at 0.1 mol % loading. Remarkably, this catalytic system exhibits a high isoselectivity of Pm = 0.92 at −40 °C in toluene but dramatically switches to a high heteroselectivity of Pr = 0.98 at −30 °C in THF. This switchable stereoselectivity behavior extends to other racemic glycolide monomers (Pm up to 0.98 at −30 °C in toluene and Pr up to 0.98 at −20 °C in THF). Mechanistic insights reveal that THF dynamic coordination alters the chiral pocket geometry, switching the stereocontrol mode from the enantiomorphic site to chain-end control. This work establishes a new type of bisazo-Salen as a platform for designing catalysts with dynamically tunable stereoselectivity in precision polyester synthesis.