Manganese(III) Schiff Base Complexes: Chemistry Relevant to the Copolymerization of Epoxides and Carbon Dioxide

Schiff base complexes of the form (acacen)MnIIIX (acacen = N,N‘-bis(acetylacetone)-1,2-ethylenediimine), where
X = OAc, Cl, or N3, have been evaluated for their ability to couple CO2 and cyclohexene oxide in the presence
of a variety of cocatalysts to provide cyclic or polycarbonates. These complexes proved to be ineffective at catalyzing
this process; however, valuable information related to the coordination chemistry of these manganese Schiff bases
was elucidated. Of importance, mechanistic findings as revealed by comprehensive studies involving structurally
related (salen)CrX and (salen)CoX complexes strongly support the requirement of six-coordinate metal species for
the effective copolymerization of CO2 and epoxides. In the case of these Mn(III) complexes, it was determined that
in chloroform or toluene solution a five-coordinate species was greatly favored over a six-coordinate species even
in the presence of 20 equiv or more of various Lewis bases. Significantly epoxide monomers such as propylene
oxide and cyclohexene oxide displayed no tendency to bind to these (acacen)MnX derivatives, even when used as
solvents. Only in the case of excessive quantities of heterocyclic amines such as pyridine, DMAP, and DBU was
spectral evidence of a six-coordinate Mn derivative observed in solution. X-ray crystal structures are provided for
many of the complexes involved in this study, including the one-dimensional polymeric structures of [(acacen)MnOAc·2H2O]n, [(acacen)MnN3]n (μ1,3-N3), and a rare mixed bridging species [(acacen)MnN3]n (μ1,3-N3/μ1,1-N3). In
addition, a structure was obtained in which the unit cell contains both a (acacen)MnN3(DMAP) and a (acacen)MnN3 species.