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

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posted on 07.08.2006, 00:00 by Jan-Uwe Rohde, Audria Stubna, Emile L. Bominaar, Eckard Münck, Wonwoo Nam, Lawrence Que
Treatment of [FeIV(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 [FeIV(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 (rFe-Cl = 2.29 Å) and Br (rFe-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 [FeIV(O)(TPA)]2+ complexes. These anion-substituted complexes model the cis−X−FeIVO units proposed in the mechanisms of oxygen-activating nonheme iron enzymes.