One-electron oxidation of an oxoiron(IV) complex to form an [O═FeV═NR]+ center

Abstract

Oxoiron(V) species are postulated to be involved in the mechanisms of the arene cis-dihydroxylating Rieske dioxygenases and of bioinspired nonheme iron catalysts for alkane hydroxylation, olefin cis-dihydroxylation, and water oxidation. In an effort to obtain a synthetic oxoiron(V) complex, we report herein the one-electron oxidation of the S = 1 complex [Fe(IV)(O)(TMC)(NCCH(3))](2+) (1, where TMC is tetramethylcyclam) by treatment with tert -butyl hydroperoxide and strong base in acetonitrile to generate a metastable complex 2 at -44 °C, which has been characterized by UV-visible, resonance Raman, Mössbauer, and EPR methods. The defining spectroscopic characteristic of 2 is the unusual x/y anisotropy observed for the (57)Fe and (17)O A tensors associated with the high-valent Fe═O unit and for the (14)N A tensor of a ligand derived from acetonitrile. As shown by detailed density functional theory (DFT) calculations, the unusual x/y anisotropy observed can only arise from an iron center with substantially different spin populations in the d(xz) and d(yz) orbitals, which cannot correspond to an Fe(IV)═O unit but is fully consistent with an Fe(V) center, like that found for [Fe(V)(O)(TAML)](-) (where TAML is tetraamido macrocyclic ligand), the only well-characterized oxoiron(V) complex reported. Mass spectral analysis shows that the generation of 2 entails the addition of an oxygen atom to 1 and the loss of one positive charge. Taken together, the spectroscopic data and DFT calculations support the formulation of 2 as an iron(V) complex having axial oxo and acetylimido ligands, namely [Fe(V)(O)(TMC)(NC(O)CH(3))](+).

Fig. 1.

Proposed steps in the one-electron oxidation of 1.

Fig. 2.

Spectroscopic data for 2 and 2-H⁺. (A) UV-visible spectra of 1 (black dashed line), 2 (thick red line), and 2-H⁺ (blue solid line) collected at -44 °C. (Inset) EPR spectra of 2 in CD₃CN and 2-H⁺ in 1∶9 CH₃CN∶ CH₂Cl₂. (B) Resonance Raman spectra of 2 (Left) and 2-H⁺ (Right) collected at 77 K using 413-nm laser excitation for frozen solution samples prepared with ¹⁶O (black line) and ¹⁸O (red line). Acetonitrile solvent peaks are marked as “s”.

Fig. 3.

A 4.2 K Mössbauer spectra of 2 and 2-H⁺. Mössbauer spectra of samples containing 2 (A) and 2-H⁺ (C) recorded in a 50 mT field applied parallel to the observed γ rays. (B) Difference spectrum parallel minus perpendicular, representing 2, obtained with 50 mT applied fields. Red lines are simulations for 2 and 2-H⁺ based on Eq. 1, using the parameters listed below and in Table 1. The major Fe^IV═O contaminants are shown by the green (1) and blue (1-OH) lines. The black solid line in A is a spectral simulation for the sum of 2 (55% of Fe), 1 (8%), and 1-OH (16%). The doublets in C represent 27% of 1 and 2% of 1-OH. The arrows in C point to absorption due to a high-spin Fe^III contaminant (30%). The δ, ΔE_Q, and η values used in the simulations are +0.10(4) mm/s, -0.5 mm/s, and -3, respectively, for 2 and +0.10(4) mm/s, -0.2 mm/s, and -3, respectively, for 2-H⁺.

Fig. 4.

X-band EPR spectra of 2 and 2-H⁺. Experimental data (solid black lines) obtained in 1∶3 CH₃CN∶butyronitrile (A, B, D, E) or 1∶3 CH₃CN∶ CH₂Cl₂ (C) and recorded at 40 K. The conditions are, as follows: 9.62 GHz; microwave power, 20 μW; modulation, 0.3 mT. The red lines are theoretical curves generated with the parameters listed in Table 1. The ¹⁷O enrichment in E, obtained from EPR, is ≈30%. For 2 only the simulation for the majority species is shown. F shows the spectral simulation of 2-H⁺ assuming 100% ¹⁷O enrichment.

Fig. 5.

Spin density plots of geometry optimized BP86 solutions for , , and [Fe(O)(TAML)]^-. The plot for , shown in two views, reveals the contours of the orbitals carrying spin density. From top to bottom, p_x (N_am), d_yz (Fe), and p_y (¹⁷O). For , the TMC ligand has been rotated by approximately 90° around the Fe═O bond relative to the orientation shown for . Majority spin α in blue; minority spin β in red.

Fig. 6.

Calculated geometric and spectral parameters for the energy-minimized BP86 models 1_ox, 2, and 2-H⁺. Bond lengths are reported in angstroms “L” denotes the TMC ligand.