Synthesis, Characterization, and Reactivity of Iron(III) Complexes Supported by a Trianionic ONO^3– Pincer Ligand

Synthetic and characterization results of a new family of Fe­(III) compounds stabilized by a trianionic [CF3–ONO]3– pincer-type ligand are reported. The ligand possesses three negatively charged donors constrained to the meridional positions that provide sufficient electron density to stabilize high-valent metal complexes. Using the redox-insulated [CF3–ONO]3–, pentacoordinated square-pyramidal {[CF3–ONO]­FeCl2}­{LiTHF2}2 (3), dimeric μ-DME­{[CF3–ONO]­FeDME}2 (4), trigonal bipyramidal [CF3–ONO]­Fe­(bpy) (5), and octahedral [CF3–ONO]­Fe­(bpy)­H2O (5·H2O) complexes are synthesized. An interesting feature of the [CF3–ONO]3– pincer-type ligand is its ability to coordinate the metal center in both the more common meridional positions or occupying a face of a trigonal bipyramidal complex. The molecular structure of 3 contains structural features similar to those of a rare square-planar high-spin Fe­(II) complex, and the important role of the counterions in stabilizing a square-plane is emphasized. SQUID magnetometry measurements of 3 reveal its high-spin character, and cyclic voltammetry measurements indicate high oxidation state species are unstable. However, all compounds can be reduced, and in particular 5 displays a reversible reduction event at −2255 mV versus ferrocene (Fc+/Fc) that can be assigned to either the FeI/Fe0 couple or 2,2′-bipyridine reduction.