X-Ray crystal structures of five-coordinate (salen)MnN3 derivatives and their binding abilities towards epoxides: chemistry relevant to the epoxide-CO2 copolymerization process.
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The synthesis of several (salen)MnN3 complexes in good yields and purities were achieved by the reaction of manganese(III) acetate and H2salen, followed by metathesis of the remaining acetate ligand with an aqueous solution of NaN3. The X-ray structures of two derivatives, where salen=N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-ethylenediamine and N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexenediamine respectively, were determined. The complexes were shown to be monomeric 5-coordinate derivatives displaying a distorted square pyramidal geometry, and to be d4 high-spin derivatives by solution magnetic moment measurements using the Evans method. Binding studies of the (salen)MnN3 derivatives with added azide ions or cyclohexene oxide showed these complexes to have modest affinities for binding a sixth ligand. These observations are used to rationalize the low activity exhibited by manganese(III) complexes relative to their chromium(III) and cobalt(III) analogs for serving as catalysts for the copolymerization of carbon dioxide and epoxides.