Chalcogens as Terminal Ligands to Iron:  Synthesis and Structure of Complexes with Fe^III−S and Fe^III−Se Motifs

Metal complexes with terminal chalcogenido ligands are known for the early transition-metal complexes, yet for the heavier congeners (e.g., sulfido and selenido), there are no analogous examples for the late 3d metal ions. Reported herein is the isolation and characterization of monomeric iron(III) complexes containing sulfido and selenido ligands; isolation was accomplished using the tripodal ligand tris[(N‘-tert-butylureaylato)-N-ethylene]aminato ([H₃buea]^3-). The Fe^III−E (E = S^2-, Se^2-) complexes were prepared from the iron(II) precursor, [Fe^II(H₃buea)]^2-, and the elemental forms of the chalogen. The formulation of [Fe^IIIH₃buea(S)]^2- and [Fe^IIIH₃buea(Se)]^2- as monomeric complexes with Fe−E units is supported by spectroscopic, analytical, and X-ray diffraction studies. For instance, X-band EPR spectra contain well-resolved axial signals, which are consistent with each complex having S = 5/2 ground states. The solid-state molecular structures reveal Fe^III−E bond lengths of 2.211(1) and 2.355(1) Å for [Fe^IIIH₃buea(S)]^2- and [Fe^IIIH₃buea(Se)]^2-, respectively. The primary coordination sphere for each complex also contains three deprotonated urea nitrogen atoms from [H₃buea]^3-; the apical amine nitrogen atom weakly interacts with the iron centers at distances of greater than 2.6 Å. The terminal chalcogenido ligands appear to weakly hydrogen-bond with the urea NH groups of the [H₃buea]^3-; however, open H-bond cavities are observed for [Fe^IIIH₃buea(S)]^2- and [Fe^IIIH₃buea(Se)]^2-, which may contribute to their observed long-term instability.