Zinc Complexes of Piperazinyl-Derived Aminephenolate Ligands: Synthesis, Characterization and Ring-Opening Polymerization Activity

A series of zinc complexes was prepared from 4,6-disubstituted (R,R') o-[(4-methyl-1-piperazinyl)methyl]phenols ([ONNR',R]H) and characterized by elemental analysis, 1H and 13C{1H} NMR spectroscopy, and X-ray crystallography. Reaction of these products with diethylzinc gave Zn[ONNtBu,tBu]2 (1) as a monometallic complex and {[mu-ONNR',R]ZnEt}2 [R' = Me, R = tBu, (2); R' = R = tBu (3); R' = R = tAm (4)] bimetallic species that have distorted tetrahedral environments about the Zn centres. Reaction of 3 and 4 with alcohols gave {[ONNtAm,tAm]Zn(mu-OR?)}2 [R? = Bn (5); R? = Et (6)] bimetallic species in which the Zn centres are bridged by benzyl alkoxide and ethoxide groups, respectively. A morpholinyl derived ligand was also synthesized and characterized (L4H), and its 1:1 stoichiometric reaction with ZnEt2 resulted in complex 7, {[mu-ONOtBu,tBu]ZnEt}2. The reactivity of complexes 27 in the ring-opening polymerization of rac-lactide (LA) and ?-caprolactone (?-CL) was studied. Reactions of carbon dioxide with cyclohexene oxide in the presence of 6 or 7/ROH (R = Bn, Et) afforded cyclohexene carbonate.