Synthesis and characterization of zinc complexes supported by NHC-based CNN- and CNP-tridentate ligands and their catalysis in the ring-opening polymerization of rac-lactide and ε-caprolactone

A series of zinc complexes bearing NHC-based C,N,N- or C,N,P-tridentate ligands were synthesized and characterized and the catalysis of the complexes toward the ring-opening polymerization of rac-lactide was evaluated. The ligand precursors (2a, R = Me; 2b, R = iPr) prepared from (R = Me or iPr) and Ph2PCH2Py were successively treated with nBuLi and ZnEt2 to afford corresponding zinc complexes (3a, R = Me; 3b, R = iPr). Similar treatment of (4a) (prepared from and Ph2PCH2PPh2) with nBuLi and then ZnEt2 gave (5). Treatment of (4a, R = Me, X = I; 4b, R = Bn, X = Br; 4c, R = iPr, X = I) with p-MeC6H4N3 resulted in (6a, R = Me, X = I; 6b, R = Bn, X = Br; 6c, R = iPr, X = I). The reaction of 6a with nBuLi and ZnEt2 in sequence produced (7). Whereas the reaction of 6a–c with an equimolar amount of ZnEt2 gave (8a, R = Me, X = I; 8b, R = Bn, X = Br; 8c, R = iPr, X = I). All new compounds were characterized by 1H, 13C and 31P NMR spectroscopy and elemental analyses. The structures of complexes 3a, 5 and 8b were additionally characterized by single crystal X-ray diffraction. In the presence of BnOH complexes 3a, 3b, 5 and 7 catalyze the ring-opening polymerization of rac-lactide at room temperature and 3a, 3b, and 5 exhibited high activity. Complexes 8a–c are inactive under the same conditions. Complexes 3a and 5 were also tested for the catalysis in the ROP of ε-caprolactone and both showed good catalytic activity.