Mechanistic aspects of the copolymerization of CO2 and epoxides by soluble zinc bis(phenoxide) catalysts as revealed by their cadmium analogues
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abstract
A family of monomeric cadmium(II) phenoxides of the general formula Cd(O-2,6-R2C6H3)2(base)2-3 [R = Ph, (t)Bu, Me; base = THF, THT, pyridine, propylene carbonate] has been synthesized. Determination of the solid-state structures of all complexes have uncovered a variety of geometries ranging from the expected distorted tetrahedral to square planar to trigonal bipyramidal, depending upon the identity of the phenoxide substituents and of the base ligands. By utilizing multinuclear NMR techniques available to cadmium, particularly 113Cd NMR, it was possible to discover the identities of these complexes in solution. Reactivities of these complexes with the small molecules CO2, COS, and CS2 were also carried out. While CO2 showed no reactivity with the sterically encumbered Cd(II) bis(phenoxides), insertion was observed spectroscopically to occur for COS and CS2. The product of CS2 insertion was isolated and characterized and found to be a dimeric xanthate complex, [Cd(S2CO- 2,6-Ph2C6H3)]2[-O-2,6-Ph2C6H3]2. These Cd(II) complexes are analogues to Zn(II) bis(phenoxides), which are highly active catalysts for the copolymerization of CO2 with epoxides. Although the Cd(II) bis(phenoxides) show only greatly reduced activity for this copolymerization reaction, the results of solution studies and reactivities have proven instructive toward better understanding the mechanism through which their Zn(II) counterparts work.
