Syntheses, Solid-state Structures and Catalytic Activity of Zinc Carboxylate Complexes in Lactide Polymerization

Three dinuclear zinc carboxylate complexes [L1-3Zn(mu,eta(2)-O2CPh)](2) (1, 2, 4) containing either the bidentate N,N'-chelating beta-diketiminate ligand RNC(Me)C(H)C(Me)NR (R = 2,6-iPr(2)-C6H3, L-1, complex 1), the tridentate 0,N,N-chelating ligand OC(Me)C(H)C(Me)NCH2CH2NMe2 (L-2, complex 2) or the bis-N,N'-chelating bis-beta-diketiminate ligand RNC(Me)C(H)C(Me)NNC(Me)C(H)C(Me)-NR (R = 2,6-iPr(2)-C6H3, L-3, complex 4) were synthesized and characterized including single-crystal X-ray diffraction. Reaction of the neutral bis-beta-diketimine (L-3(H)(2)) with two equivalents of ZnMe2 leads to the expected heteroleptic dinuclear zinc complex L-3(ZnMe)(2) 3 in 93% yield. Further reaction with benzoic acid PhCO2H leads to complex 4. Complex 2 forms a rather strong carboxylate-bridged dimer, whereas the carboxylate groups in complexes 1 and 4 act as asymmetrical bridges between both Zn atoms, pointing to the formation of a wealdy bonded dimer. The zinc atoms in 1 and 4 are tetrahedrally coordinated, whereas in 2 the coordination number is increased to five due to the coordination of the pendant donor arm. The ring opening polymerization (ROP) of rac-lactide was investigated with the zinc complexes 1-4 and diazabicycloundec-7-ene (DBU) as a co-catalyst. Complexes 2 and 3 are active polymerization catalysts, which in the presence of DBU converted 200 equiv. of rac-lactide into polylactide within 10 min at ambient temperature. The analysis of the crude polymer showed that the lactide polymerization with catalyst 2 occurs via a slightly modified activated-monomer mechanism.