Nonheme Oxoiron(IV) Complexes of Tris(2-pyridylmethyl)amine with cis-Monoanionic Ligands

Treatment of [Fe^IV(O)(TPA)(NCMe)](CF₃SO₃)₂ [TPA, N,N,N-tris(2-pyridylmethyl)amine] with 3 equiv of NR₄X (X = CF₃CO₂, Cl, or Br) in MeCN at −40 °C affords a series of metastable [Fe^IV(O)(TPA)(X)]⁺ complexes. Some characteristic features of the S = 1 oxoiron(IV) unit are quite insensitive to the ligand substitution in the equatorial plane, namely, the Fe−O distances (1.65−1.66 Å), the energy (∼7114.5 eV) and intensity [25(2) units] of the 1s-to-3d transition in the X-ray absorption spectra, and the Mössbauer isomer shifts (0.01−0.06 mm·s^-1) and quadrupole splittings (0.92−0.95 mm·s^-1). The coordination of the anionic X ligand, however, is evidenced by red shifts of the characteristic near-IR ligand-field bands (720−800 nm) and spectroscopic observation of the bound anion by ¹⁹F NMR for X = CF₃CO₂ and by EXAFS analysis for X = Cl (r_Fe-Cl = 2.29 Å) and Br (r_Fe-Br = 2.43 Å). Density functional theory calculations yield Mössbauer parameters and bond lengths in good agreement with the experimental data and produce excited-state energies that follow the trend observed in the ligand-field bands. Despite mitigating the high effective charge of the iron(IV) center, the substitution of the MeCN ligand with monoanionic ligands X^- decreases the thermal stability of [Fe^IV(O)(TPA)]²⁺ complexes. These anion-substituted complexes model the cis−X−Fe^IVO units proposed in the mechanisms of oxygen-activating nonheme iron enzymes.