Structural characterization of several (CO)(3)(dppp)MnX derivatives, dppp=1,3-bis(diphenylphosphino)propane and X = H, OTs, OC2H5, Cl, Br, or N-3. An assessment of their efficacy for catalyzing the coupling of carbon dioxide and epoxides
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The X-ray structures of a series of (CO) 3(dppp)MnX complexes, X = H, OTs, OEt, Cl, Br, and N 3, and dppp = 1,3-bis(diphenylphosphino)propane, prepared by literature methods are reported. Several of these derivatives have been examined for their ability to serve as catalysts for the coupling of cyclohexene oxide and carbon dioxide. Although, these organometallic complexes do catalyze the formation of polycarbonate, their activity is not competitive with other very effective catalysts for this coupling reaction. Nevertheless, findings from these studies contribute significantly to our understanding of this important process. The complex containing the alkoxide ligand is the most active, yet it possesses a TON (mol epoxide consumed/mol Mn) of only 50 for a 24 h reaction. In this instance the (CO) 3(dppp)MnOEt complex exists as the carbonate species, (CO) 3(dppp)MnOC(O)OEt, because of rapid CO 2 insertion into the Mn-OEt bond. The order of epoxide ring-opening by the X group (initiator) was found to be -OC(O)OR > Cl N 3 > Br. Furthermore, since these complexes are substitutionally inert, this process is best defined as occurring via an associative interchange process and is rate-limiting.