Bis 2,6-difluorophenoxide Dimeric Complexes of Zinc and Cadmium and Their Phosphine Adducts:  Lessons Learned Relative to Carbon Dioxide/Cyclohexene Oxide Alternating Copolymerization Processes Catalyzed by Zinc Phenoxides (2000)

Dimeric phenoxide derivatives of zinc and cadmium have been synthesized from the reaction of the corresponding metal bistrimethylsilylamide and two equivalents of 2,6-F2C6H3OH in tetrahydrofuran. The zinc analogue, [Zn(O-2,6-F2C6H3)2·THF]2 (1), has been characterized in the solid state via X-ray crystallography, where the zinc centers are shown to possess distorted tetrahedral geometry containing two bridging phenoxides and a terminal phenoxide and THF ligand. The distance between the metal centers (Zn···Zn) was found to be 3.059 Å, and the THF ligands lie on opposite sides of the plane formed by the two zinc atoms and two bridging phenoxide ligands' oxygen atoms. There are several Zn···F nonbonding distances involving the bridging phenoxide ligands that are less than the van der Waals internuclear distance. In addition, both the zinc and cadmium dimeric derivatives have been prepared such that the labile THF ligands are replaced by the sterically encumbering basic phosphine, PCy3. The solid-state structures of [Zn(O-2,6-F2C6H3)2·PCy3]2 (2) and [Cd(O-2,6-F2C6H3)2·PCy3]2 (5) are similar to that of complex 1, where the tricyclohexylphosphine ligands, like the THF ligands, are accommodated in a trans configuration. The 31P NMR spectrum of complex 2 in C6D6 upon addition of free PCy3 exhibits sharp resonances assigned to both the complex (9.58 ppm) and free PCy3 (10.6 ppm), which is indicative of slow exchange of the phosphine ligands. On the other hand, the phosphine ligands on the cadmium derivative (5) are involved in an exchange process with free PCy3 via a rapid equilibrium between 5 and two equivalents of Cd(O-2,6-F2C6H3)2(PCy3)2. The equilibrium reaction strongly favors the monomer cadmium bisphosphine complex at low temperature (−80 °C). As expected, the 113Cd and 31P NMR spectra of complex 5 in solution in the absence of excess PCy3 is quite similar to that determined in the solid state by CP/MAS. Complex 1 and its chloro- and bromophenoxide analogues were shown to be effective catalysts for the copolymerization of cyclohexene oxide and CO2, the terpolymerization of cyclohexene oxide/propylene oxide/CO2, and the homopolymerization of cyclohexene oxide. In the case of the copolymerization process (80 °C and 55 bar), the polycarbonate copolymer that was produced is completely alternating, with no polyether linkages. At the same time, the homopolymerization of cyclohexene oxide to afford polyether in the presence of 1 as catalyst is much more facile than the copolymerization process. Importantly, for both copolymerization and homopolymerization processes catalyzed by complex 1, the initiator of the polymer chain growth is a difluorophenoxide unit, as revealed by 19F NMR, with both CO2 insertion and epoxide ring-opening being involved in the initiation step. At 80 °C and 55 bar, the coupling of propylene oxide and CO2 led almost exclusively to propylene carbonate. On the other hand, at lower temperatures (i.e., 40 °C), copolymer formation was favored over cyclic carbonate production. Because of the relative rates of copolymerization of cyclohexene oxide and carbon dioxide as a function of the halogen atom in the [Zn(O-2,6-X2C6H3)2·THF]2 catalysts, that is, F > Cl > Br, activation of epoxide by the zinc center is proposed to be rate-limiting relative to the CO2 insertion process.