Zinc Complexes Supported
by Maltolato Ligands: Synthesis,
Structure, Solution Behavior, and Application in Ring-Opening Polymerization
of Lactides
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Abstract
A series of novel zinc alkoxides supported by chelating
maltolato
(MalO; MalOH = maltol) ligands were successfully synthesized and characterized.
Reaction of MalOH with ZnEt<sub>2</sub> (3:4) gives a trinuclear cluster
[Zn<sub>3</sub>(Et)<sub>2</sub>(MalO)<sub>4</sub>] (<b>1</b>), which spontaneously disproportionates in solution to mononuclear
species [Zn(MalO)<sub>2</sub>] (<b>1a</b>) and [Zn(Et)(MalO)]
(<b>1b</b>); (<b>1a</b>) and (<b>1b</b>) form [Zn(MalO)<sub>2</sub>(py)] (<b>2</b>), [Zn(Et)(MalO)(py)] (<b>3</b>), [Zn(MalO)<sub>2</sub>]<sub>2</sub> (<b>(1a)<sub>2</sub></b>), [Zn(MalO)(OBn)]<sub>2</sub> (<b>(1c)<sub>2</sub></b>), and
[Zn<sub>4</sub>(Et)<sub>2</sub>(OEt)<sub>2</sub>(MalO)<sub>4</sub>] (<b>4</b>) on addition of pyridine, benzyl alcohol (BnOH),
or dry O<sub>2</sub>, respectively. Compounds <b>1</b>, <b>2</b>, <b>(1a)</b><sub><b>2</b></sub>, and <b>4</b> were characterized by elemental analysis, NMR, ESI-MS, and single-crystal
X-ray structural analysis. Variable-temperature NMR experiments showed
that <b>(1a)</b><sub><b>2</b></sub> and <b>(1c)</b><sub><b>2</b></sub> are in equilibrium with the monomeric form
in solution. The addition of l-lactide (l-LA) to
a combination of <b>1</b> 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)] (<b>1c</b>) is first-order with respect to both the monomer and <b>1c</b> concentrations; <b>1a</b> 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
