Synthesis and structural characterization of iron(III) salen complexes possessing appended anionic oxygen donor ligands

The synthesis and solid-state characterization of iron(III) salen complexes bearing monodentate and bidentate anionic oxygen donor ligands are reported. The synthesis of Fe(salen)OPh (OPh=2,6–diphenylphenoxide) (1) was accomplished using Fe[N(Si(CH3)3)2]3 with one equivalent of H2salen and the corresponding phenol. The complex was isolated as a crystalline sample by the slow diffusion of pentane into a concentrated solution of 1 at −20 °C. The solid-state structure of complex 1 reveals distorted square pyramidal geometry about the iron metal center. Alternatively the preparation of Fe(salen) complex 2 bearing the anionic, bidentate acetylacetonate ligand in the axial position was accomplished by thermally replacing two acetylacetonate ligands in Fe(acac)3 with one equivalent of H2salen. The complex was isolated and crystallographically characterized using X-ray quality crystals formed via the aforementioned protocol. The metal center adopts a very distorted octahedral geometry, indicating a preference for the square pyramidal species which is typical of other iron salen derivatives. Unfortunately, 1 and 2 were found to be inactive in the copolymerization of CO2 and cyclohexene oxide to afford polycarbonate in the presence of a Lewis base (i.e., N-MeIm and PCy3). The μ-[Fe(salen)]2O (3) derivative was also isolated via the protonation of Fe(salen)(N(Si(CH3)3)2). The complex is relatively air stable, forming large, dark crystals over a short period of time at −20 °C. The solid-state structure reveals a nearly linear Fe–O–Fe bond angle, which is due to the electronic repulsions associated with the sterically encumbering tert-butyl groups on the salen framework.