Ring-opening polymerization of cyclic esters by phenoxy-thioether complexes derived from biocompatible metals

A series of novel Mg(II) and Zn(II) complexes of the form LMX or L2M, supported by phenoxy-thioether ligands bearing different substituents at the ortho position of the thiophenol rings [L- = 4,6-tBu(2)-OC6H2-2-CH2S(2-R-C6H4)(-)] [M = Zn, R = H, X = N(SiMe3)(2) (1) and X = Et (2); M = Mg, X = n-Bu, R = H (3), R = CH3 (4), R = Br (5); M = Mg, R = H (6)], were synthesized and characterized. Reaction of the proper zinc precursor (Zn[N(SiHMe2)(2)](2) or ZnEt2) with one equivalent of the phenoxy-thioether proligand produced the corresponding amido (1) and ethyl (2) zinc complexes in high yields. The monoalkylmagnesium complexes (3-5) were all obtained by butane elimination reaction of the dialkylmagnesium reagent, Mg(n-Bu)(2), with one equivalent of the proligand in good yields. The homoleptic complex 6 was obtained by reaction of 0.5 equivalent of Mg(n-Bu)(2) and the proper proligand. Variable temperature H-1 NMR studies performed on 2 and 3 demonstrated that the named complexes are involved in fluxional processes concerning a fast conformational change of the six-membered metallacycle. DOSY (Diffusion Ordered SpectroscopY) H-1 experiments and ligand scrambling strongly suggested that complexes 1-6 exist as dimeric species in solution. All complexes were active as catalysts in the ring-opening polymerization of cyclic esters. In particular, magnesium complexes showed superior epsilon-caprolactone and lactide ROP behavior in terms of activity, control of molecular weights and molecular weight distributions.