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

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.

以-40 °C于乙腈中用NR4X（X = CF3CO2、Cl或Br）3当量与[FeIV(O)(TPA)(NCMe)](CF3SO3)2和N,N,N-三(2-吡啶甲基)胺（TPA）进行反应，可得到一系列亚稳态的[FeIV(O)(TPA)(X)]+络合物。S = 1的氧合铁(IV)单位的某些特征属性对赤道平面的配体替换反应相当不敏感，例如Fe-O距离（1.65−1.66 Å）、X射线吸收光谱中1s至3d跃迁的能量（约7114.5 eV）和强度[25(2)单位]，以及莫塞巴乌尔同位素位移（0.01−0.06 mm·s^-1）和四极分裂（0.92−0.95 mm·s^-1）。然而，阴离子配体X的配位可通过特征近红外配体场带（720−800 nm）的红移以及X = CF3CO2时通过19F NMR观察到的配位阴离子，以及X = Cl（rFe-Cl = 2.29 Å）和Br（rFe-Br = 2.43 Å）时通过EXAFS分析得到证实。密度泛函理论计算得到的莫塞巴乌尔参数和键长与实验数据吻合良好，并产生了符合配体场带观察趋势的激发态能量。尽管减轻了铁(IV)中心的较高有效电荷，但用单阴离子配体X-替换MeCN配体会降低[FeIV(O)(TPA)]2+络合物的热稳定性。这些阴离子取代络合物模拟了在非血红素铁酶氧活化机理中提出的顺式-X-FeIVO单位。