Zinc Complexes Supported by Maltolato Ligands: Synthesis, Structure, Solution Behavior, and Application in Ring-Opening Polymerization of Lactides

A series of novel zinc alkoxides supported by chelating
maltolato
(MalO; MalOH = maltol) ligands were successfully synthesized and characterized.
Reaction of MalOH with ZnEt2 (3:4) gives a trinuclear cluster
[Zn3(Et)2(MalO)4] (1), which spontaneously disproportionates in solution to mononuclear
species [Zn­(MalO)2] (1a) and [Zn­(Et)­(MalO)]
(1b); (1a) and (1b) form [Zn­(MalO)2(py)] (2), [Zn­(Et)­(MalO)­(py)] (3), [Zn­(MalO)2]2 ((1a)2), [Zn­(MalO)­(OBn)]2 ((1c)2), and
[Zn4(Et)2(OEt)2(MalO)4] (4) on addition of pyridine, benzyl alcohol (BnOH),
or dry O2, respectively. Compounds 1, 2, (1a)2, and 4 were characterized by elemental analysis, NMR, ESI-MS, and single-crystal
X-ray structural analysis. Variable-temperature NMR experiments showed
that (1a)2 and (1c)2 are in equilibrium with the monomeric form
in solution. The addition of l-lactide (l-LA) to
a combination of 1 and 2 equiv of BnOH in dichloromethane
at room temperature in different molar ratios leads to rapid and efficient
generation of poly­(l-LA) with end-capped BnO groups. According
to kinetic studies, propagation by [Zn­(OBn)­(MalO)] (1c) is first-order with respect to both the monomer and 1c concentrations; 1a in ring-opening polymerization of l-LA shows no activity. These results suggest a single-site
active species in the ring-opening polymerization of l-LA.