Stereoselective ROP of rac-Lactide Mediated by Enantiopure NNO-Scorpionate Zinc Initiators

The reaction of the enantiopure NNO-scorpionate ligand ( R, R )-bpzmmH with [ZnR 2 ] (R = Me, Et, CH 2 SiMe 3 ) in a 1:2 molar ratio afforded the dinuclear tris-alkyls [Zn(R)(κ 2 -NNμ-O)Zn(R) 2 ] (R = Me ( 1 ), Et ( 2 ), CH 2 SiMe 3 ( 3 )). The reaction of the bimetallic tris-alkyls 1 – 3 with an aromatic alcohol and thioalcohol yielded the dinuclear zinc complexes containing aryloxide/arylthioxide ligands [(ZnR) 2 (κN:κN-μ-O)(μ-EAr)] (Ar = 2,6-C 6 H 3 Me 2; E = O, R = Me ( 4 ), Et ( 5 ), CH 2 SiMe 3 ( 6 ); E = S, R = Me ( 7 ), Et ( 8 ), CH 2 SiMe 3 ( 9 )) by an alcoholysis or thioalcoholysis reaction. The structures of the different compounds were determined by spectroscopic methods and, in addition, the X-ray crystal structures of 2 and 5 were also unambiguously established. Alkyl- and aryloxide/thioaryloxide-containing enantiopure zinc complexes 3 – 6 and 9 can act as single-component living initiators for the ring-opening polymerization of lactides, affording materials with low molecular weights under mild conditions in only a few hours. Interestingly, microstructural analysis of poly( rac -lactide) by 1 H NMR spectroscopy revealed that the most sterically hindered initiator, [(ZnCH 2 SiMe 3 ) 2 ( R, R -bpzmm)(μ-OAr)] ( 6 ), is one of the first examples of a zinc complex capable of exerting a significant influence on the degree of enantioselectivity, with enriched-isotactic PLAs ( P i = 0.74) produced through an enantiomorphic site control mechanism.