113Cd NMR determination of the binding parameters of alicyclic epoxides to [hydrotris(3-phenylpyrazol-1-yl)borate]Cd(II) acetate

The thermodynamic parameters for the binding of a series of epoxides to the complex [hydrotris(3-phenylpyrazol-1-yl)borate]Cd(II) acetate have been determined via Cd-113 NMR spectroscopy. As the activation of an epoxide by interaction with a metal center is a key step in the copolymerization of carbon dioxide and epoxides to produce polycarbonates, the tendency for epoxides to bind to metal centers may play a role in the propensity for epoxides to ring-open. The epoxides examined include (+)-limonene oxide, 1,4-dihydronaphthalene oxide, [2-(3,4-epoxycyclohexyl)ethyl]trimethoxysilane, exo-norbornene oxide, and alpha-pinene oxide. DFT calculations were performed to give optimized geometries of the epoxide adducts of the cadmium complex. Our investigations show that the binding abilities of these alicyclic epoxides vary over the range -22.1 to -38.7 kJ mol(-1) for DeltaHdegrees and do not differ significantly from the corresponding values previously determined for the easily ring-opened epoxides (cyclohexene oxide and propylene oxide). The optimized geometries give a metal centerepoxide (Cd-O) bond distance range of 2.401-2.448 Angstrom, which is in good agreement with values obtained experimentally for selected derivatives. In addition, comparative studies involving the more strongly binding phosphine and amine ligands are reported. From these observations we conclude that the binding of the epoxide by the metal complex is not necessarily the determining factor in the ability of the epoxide to be ring-opened by a nucleophile.