Solution and Solid-State Structural Studies of Epoxide Adducts of Cadmium Phenoxides. Chemistry Relevant to Epoxide Activation for Ring-Opening Reactions

The reaction of Cd[N(SiMe₃)₂]₂ with 2 equiv of the corresponding phenol in toluene has led to the isolation of [Cd(O-2,6-R₂C₆H₃)₂]₂ derivatives, where R represents the sterically bulky ^tBu and Ph substituents. The dimeric nature of these complexes in the solid state has been established via X-ray crystallography, i.e., trigonal geometry around cadmium is observed in 1 (R = ^tBu) where the two cadmium centers are bridged by two phenoxides with each metal containing a terminal phenoxide. Complex 2 (R = Ph) contains an additional interaction of the metal centers with carbon atoms of the aromatic substituents on the phenoxide ligands. These dimeric structures are maintained in weakly coordinating solvents as revealed by ¹¹³Cd NMR in d₂-methylene chloride, which displays ¹¹¹Cd−¹¹³Cd coupling. Nevertheless, because of the excessive steric requirements of these phenoxide ligands, these dimers are easily disrupted in solution by weak donor ligands such as epoxides. Three bisepoxide adducts have been isolated as crystalline solids and characterized by X-ray crystallography. As previously observed in other Cd(O-2,6-^tBu₂C₆H₃)₂·L₂ complexes, these epoxide adducts adopt a crystallographically imposed square-planar geometry about the cadmium centers, with the exception of the exo-2,3-epoxynorbornane derivative, which displays a distorted tetrahedral geometry. Temperature-dependent ¹¹³Cd NMR studies have established that there is little difference in the binding abilities of these epoxides with either complex 1 or complex 2. Importantly, it is concluded from these studies that the lack of reactivity of α-pinene oxide and exo-2,3-epoxynorbornane toward copolymerization reactions with carbon dioxide, in the presence of zinc bisphenoxide catalysts, is not due to differences in epoxide metal binding. This is further affirmed by the isolation and crystallographic characterization of the very stable Zn(O-2,6-^tBu₂C₆H₃)₂·(exo-2,3-epoxynorbornane)₂ derivative.