Synthesis and Structure of Homo- and Heterometallic Lithium–Magnesium Complexes and Their Reactivity in the ROP of rac-Lactide

The homometallic azo complexes of lithium [Li2{(η2-O(C10H6)N═N(C6H5)}2]n and magnesium [Mg{(η2-O(C10H6)N═N(C6H5)}2]n and the heterometallic lithium–magnesium derivative [Li2Mg2{(η2-O(C10H6)N═N(C6H5)}6] have been synthesized. The heterometallic complex exhibits a novel structure described as two truncated cubes that share a Mg2O2 ring as one of the cube sides. These complexes have been studied in the polymerization of rac-lactide. In order to gain insight into the behavior of these compounds as catalysts, 1H NMR experiments, diffusion-ordered spectroscopy, and X-ray studies have been used. DOSY NMR experiments support the notion that the structure of the mixed-metal complex 3 persists in benzene solution, and 1H NMR studies show that it follows an activated monomer mechanism.