Synthesis and structural characterization of novel cyclam-based zirconium complexes and their use in the controlled ROP of rac-lactide: access to cyclam-functionalized polylactide materials

Abstract

Novel Bn₂Cyclam-based zirconium complexes of the type (Bn₂Cyclam)Zr(X)(X′) (3, X = X′ = OPh; 4, X = X′ = SPh; 5, X = Cl, X′ = OⁱPr) were synthesized in good yields via metathesis routes involving the reaction of the dichloro precursor (Bn₂Cyclam)Zr(Cl)₂ and the appropriate lithium salts. The molecular structures of compounds 3, 4 and 5, as determined by X-ray crystallographic studies, all confirmed the effective chelation of the Bn₂Cyclam ligand in a κ⁴-N₂N₂′ fashion, with the hexa-coordinated Zr center adopting a trigonal prismatic geometry. Complexes 3–5 as well as the diisopropoxide derivative (Bn₂Cyclam)Zr(OⁱPr)₂ (2) were all found to initiate the ring-opening polymerization (ROP) of rac-lactide in a controlled manner, as deduced from SEC data and linear correlations between molecular weight numbers (M_n) and monomer conversion as the ROP proceeds. While initiator 2 polymerizes rac-lactide to afford, as expected, an OⁱPr-ester-end PLA, the ROP of rac-lactide by species 3 or 4 affords an unusual cyclam-end group PLA, as deduced from MALDI-TOF data. The bonding and the electronic structures of the dialkoxides 2 and 3 were assessed by DFT and their possible influence on the polymerization mechanism is discussed.