X-Ray crystal structures of five-coordinate (salen)MnN3 derivatives and their binding abilities towards epoxides: chemistry relevant to the epoxide–CO2copolymerization process

Abstract

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.