ja020184c_si_001.pdf (126.52 kB)
Solution and Solid-State Structural Studies of Epoxide Adducts of Cadmium Phenoxides. Chemistry Relevant to Epoxide Activation for Ring-Opening Reactions
journal contribution
posted on 2002-05-25, 00:00 authored by Donald J. Darensbourg, Jacob R. Wildeson, Samuel J. Lewis, Jason C. YarbroughThe 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.