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.