Aldimine-Thioether-Phenolate Based Mono- and Bimetallic Zinc Complexes as Catalysts for the Reaction of CO2 with Cyclohexene Oxide

A synthetic strategy for the preparation of a new class of ligands is introduced. The bis(aldimine-thioether-phenolate) ligands bear two anionic oxygen donors from phenolate moieties, two neutral sulfur donors and two neutral nitrogen donors belonging, respectively, to thioether and aldimine functionalities. The so designed OSNNSO ligands show two coordinative pockets and thus should be able to host two metallic centers. Instead, the aldimine-thioether-phenolate OSN ligands are tridentate ligands which may form monometallic complexes. Two OSNNSO and one OSN ligands of this class, have been synthesized. Satisfyingly, by direct reaction of the ligands with one or two equivalents of the zinc precursor, the corresponding bimetallic Zn(II) complexes were prepared from the bis(aldimine-thioether-phenolate) ligands, while the monometallic zinc complex was prepared from the tridentate ligand. These zinc amido complexes act as single component catalysts in the reaction of CO2 with cyclohexene oxide by furnishing polycyclohexene carbonate, while adding PPNCl as cocatalyst to the reaction medium, cyclohexene carbonates were obtained as the main products.