Spectroscopic and Reactivity Comparisons of a Pair of bTAML Complexes with FeV═O and FeIV═O Units

Abstract

In this report we compare the geometric and electronic structures and reactivities of [Fe^V(O)]^- and [Fe^IV(O)]^2- species supported by the same ancillary nonheme biuret tetraamido macrocyclic ligand (bTAML). Resonance Raman studies show that the Fe═O vibration of the [Fe^IV(O)]^2- complex 2 is at 798 cm^-1, compared to 862 cm^-1 for the corresponding [Fe^V(O)]^- species 3, a 64 cm^-1 frequency difference reasonably reproduced by density functional theory calculations. These values are, respectively, the lowest and the highest frequencies observed thus far for nonheme high-valent Fe═O complexes. Extended X-ray absorption fine structure analysis of 3 reveals an Fe═O bond length of 1.59 Å, which is 0.05 Å shorter than that found in complex 2. The redox potentials of 2 and 3 are 0.44 V (measured at pH 12) and 1.19 V (measured at pH 7) versus normal hydrogen electrode, respectively, corresponding to the [Fe^IV(O)]^2-/[Fe^III(OH)]^2- and [Fe^V(O)]^-/[Fe^IV(O)]^2- couples. Consistent with its higher potential (even after correcting for the pH difference), 3 oxidizes benzyl alcohol at pH 7 with a second-order rate constant that is 2500-fold bigger than that for 2 at pH 12. Furthermore, 2 exhibits a classical kinteic isotope effect (KIE) of 3 in the oxidation of benzyl alcohol to benzaldehyde versus a nonclassical KIE of 12 for 3, emphasizing the reactivity differences between 2 and 3.

Figure 1

bTAML complexes discussed in this report with tetraethylammonium cations as counterions for all complexes: 1 = chloroiron(III) complex; 2 = oxoiron(IV) complex, and 3 = oxoiron(V) complex.

Figure 2

Resonance Raman spectra of 2 in CH₃CN at room temperature (left) and 3 in CD₃CN at 77 K (right). Blue and red lines represent¹⁶O- and ¹⁸O-labeled samples, respectively. λ_ex = 476.5 nm; power ≈ 40 mW. (#) Solvent-derived features.

Figure 3

Observed X-ray absorption pre-edge regions of 1 (left), 2 (middle), and 3 (right). Experimental data are represented by black dotted lines, with the best fits as blue solid lines, the modeled baselines as red dashed lines, the fitted component peaks as red solid lines, and the residuals as green solid lines.

Figure 4

(left) Fourier transforms of the EXAFS data (black dotted) with best fit (solid red) for 1 (top), 2 (middle), 3 (bottom), k range = 2−15 Å⁻¹. (right) Unfiltered EXAFS data (black dotted) with best fit (red solid) for 1 (top), 2 (middle), and 3 (bottom).

Figure 5

Optimized geometries (top) and MO diagrams (bottom) for [(bTAML)Fe^IV(O)]²⁻ (2), left and [(bTAML)Fe^V(O)]⁻ (3), right. ψ_Fe and ψ_o indicate contributions of Fe and oxo centers in the individual orbitals. Note that only d orbitals of 3 are shown for clarity, and the energy of the nonbonding d_xy orbital is set to zero for 2 and 3.

Figure 6

(A) CV of 2 synthesized from 1 and NaOCl in a pH 12 aqueous solution (conditions: GC working electrode, Pt counter electrode, 0.2 M KNO₃ as supporting electrolyte, scan rate 50 mV s⁻¹; arrow indicates the direction of potential scanning). (B) Plot of E_1/2 vs pH for 1 in water.

Figure 7

Plots of k_obs vs benzyl alcohol concentration for reactions with (A) 2 (1.5 × 10⁻⁴ M) and (B) 3 (5 × 10⁻⁵ M) performed in a 80:20 acetonitrile−water solvent mixtures at room temperature.

Scheme 1

Reactivity of 2 and 3 with BnOH