Syntheses, structures, and binding constants of cyclic ether and thioether adducts of soluble cadmium(II) carboxylates. Intermediates in the homopolymerization of oxiranes and thiiranes and in carbon dioxide coupling processes

A synthetic methodology for the preparation of a large variety of eta(3)-HB(3-Phpz)(3)Cd(acetate) adducts is presented which involves replacement of toluene in the eta(3)-HB (3-Phpz)(3)Cd(acetate) solvate complex by the appropriate cyclic ether or cyclic thioether. In this manner, adducts of THF, dioxane, propylene oxide, cyclohexene oxide, and propylene sulfide were isolated. The solid-state structures of several of these complexes were determined by X-ray crystallography, revealing a six-coordinate complex where the acetate ligand is shown to be fairly symmetrically bonded to the cadmium center. In methylene chloride solution, the cyclic ether or thioether readily dissociates to afford the five-coordinate complex, as demonstrated by Cd-113 NMR. A quantitative assessment of the binding of these base adducts of eta(3)-HB(3-Phpz)(3)Cd(acetate) was determined by measuring the temperature dependence of the equilibrium constants for the five- and six-coordinate derivatives. The presence of one sharp Cd-113 resonance in this equilibrium mixture is indicative of rapid intermolecular exchange between the five- and six-coordinate complexes when compared to the chemical shift differences in these two species (similar to 6600 Hz at 89 MHz). The order established for ether binding is THF > dioxane > propylene sulfide > cyclohexene oxide greater than or equal to propylene oxide, with Delta H degrees and Delta S degrees spanning the ranges -27.7 to 24.3 kJ/mol and -89.7 to -94.1 J/(mol K). The epoxide and thioepoxide adducts were shown to serve as models for the initiation step in the copolymerization of epoxides with carbon dioxide catalyzed by metal carboxylates. That is, the carboxylate ligand was shown to ring-open the epoxide or thioepoxide, subsequently affording polyethers or polythioethers with ester end groups. By way of contrast, in the presence of CO2 and epoxides, this system led to cyclic carbonate production.