A synthetic high-spin oxoiron(IV) complex: generation, spectroscopic characterization, and reactivity

Abstract

High versus low: The high-yield generation of a synthetic high-spin oxoiron(IV) complex, [Fe(IV)(O)(TMG(3)tren)](2+) (see picture, TMG(3)tren = 1,1,1-tris{2-[N2-(1,1,3,3-tetramethylguanidino)]ethyl}amine), has been achieved by using the very bulky tetradentate TMG(3)tren ligand, in order to both sterically protect the oxoiron(IV) moiety and enforce a trigonal bipyramidal geometry at the iron center, for which an S = 2 ground state is favored.

Figure 1

(A) Schematic structure of the TMG₃tren ligand. (B) Thermal ellipsoid drawing of [Fe^II(TMG₃tren)(OTf)]⁺ (2), showing 50% probability ellipsoids. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å): Fe-O, 2.156(2); Fe-N_axial, 2.118(3); Fe-N_guanidine(ave), 2.094. (C) Ball-and-stick and (D) space-filling models of the DFT geometry optimized structure of 3. Selected bond distances (Å): Fe=O, 1.648; Fe-N_axial, 2.121; Fe-N_guanidine(ave), 2.034. Atom color scheme: C, gray; N, blue, O, red; S, yellow; F, light blue; Fe, magenta.

Figure 2

Main: electronic spectrum of 3 in CH₃CN solution. Inset: resonance Raman spectra (λ_ex = 514.5 nm, power = 10 mW) of ¹⁶O-3 (solid line) and ¹⁸O-3 (dashed line) recorded in frozen CH₃CN solution.

Figure 3

4.2 K Mössbauer spectra of 3 in CH₃CN recorded in parallel applied magnetic fields, B, as indicated. Downward arrows indicate nuclear Δm = 0 transitions of the M_S = 0 ground state spectrum. Upward arrows mark outer absorption features of the spectrum associated with the M_S = −1 excited state. Solid lines are spectral simulations using the parameters listed in Table 1 with the S = 2 spin Hamiltonian H = D(S_z²−2) + E(S_x² – S_y²) + 2β S·B + S·A·I – g_nβ_n B·I + H_Q. In B, C and D a high-spin Fe^III impurity, representing 12% of the iron, has been subtracted from the raw data. In E, this impurity has not been subtracted and exhibits weak absorption bands at ~−6.5 and ~+7 mm/s.

Figure 4

Top: X-ray absorption edge spectra of 2 (– – –) and 3 (–––). The inset shows an expansion of the pre-edge region. Bottom: Fe K-edge unfiltered EXAFS data (k³χ(k), inset) and the corresponding Fourier transform of 3. Experimental data are shown with dotted (•••) lines and fits with solid (–––) lines. Please see Supporting Information for further details of the EXAFS analysis.