Sulfur versus iron oxidation in an iron-thiolate model complex

Abstract

In the absence of base, the reaction of [Fe(II)(TMCS)]PF6 (1, TMCS = 1-(2-mercaptoethyl)-4,8,11-trimethyl-1,4,8,11-tetraazacyclotetradecane) with peracid in methanol at -20 °C did not yield the oxoiron(IV) complex (2, [Fe(IV)(O)(TMCS)]PF6), as previously observed in the presence of strong base (KO(t)Bu). Instead, the addition of 1 equiv of peracid resulted in 50% consumption of 1. The addition of a second equivalent of peracid resulted in the complete consumption of 1 and the formation of a new species 3, as monitored by UV-vis, ESI-MS, and Mössbauer spectroscopies. ESI-MS showed 3 to be formulated as [Fe(II)(TMCS) + 2O](+), while EXAFS analysis suggested that 3 was an O-bound iron(II)-sulfinate complex (Fe-O = 1.95 Å, Fe-S = 3.26 Å). The addition of a third equivalent of peracid resulted in the formation of yet another compound, 4, which showed electronic absorption properties typical of an oxoiron(IV) species. Mössbauer spectroscopy confirmed 4 to be a novel iron(IV) compound, different from 2, and EXAFS (Fe═O = 1.64 Å) and resonance Raman (ν(Fe═O) = 831 cm(-1)) showed that indeed an oxoiron(IV) unit had been generated in 4. Furthermore, both infrared and Raman spectroscopy gave indications that 4 contains a metal-bound sulfinate moiety (ν(s)(SO2) ≈ 1000 cm (-1), ν(as)(SO2) ≈ 1150 cm (-1)). Investigations into the reactivity of 1 and 2 toward H(+) and oxygen atom transfer reagents have led to a mechanism for sulfur oxidation in which 2 could form even in the absence of base but is rapidly protonated to yield an oxoiron(IV) species with an uncoordinated thiol moiety that acts as both oxidant and substrate in the conversion of 2 to 3.

Figure 1

UV-Vis spectral changes upon titration of 1 with m-CPBA in MeOH. Red solid line: 0 equiv.; red dashed line: 1 equiv.; green dashed line: 2 equiv. to form 3; blue solid line: 3 equiv. to form 4.

Figure 2

Plot of titration of oxidant (m-CPBA) with 1 (0.85 mM) against intensity of absorption features assigned to 1 (•, λ_max = 320 nm, left) and 4 (■, λ_max = 830 nm, right).

Figure 3

The 4.2-K Mössbauer spectra of 1, 3, and 4 recorded in zero field (1.5 mM Fe, MeOH solvent). Spectra are shown as hash-marks, while spectral simulations are shown as lines: (A) 1; (B) 1 + 1 equiv. m-CPBA (50% 1, 50% 3); (C) 1 + 2 equiv. m-CPBA (92% 3, 8% 5); (D) 1 + 3 equiv. m-CPBA (86% 4, 14% 3)

Figure 4

ESI-MS spectra following the conversion of 1 (m/z = 357.1) to 3 (389.2) and 4 (405.2): (A) 1; (B) 1 + 1 equiv. m-CPBA; (C) 1 + 2 equiv. m-CPBA; (D) 1 + 3 equiv. m-CPBA

Figure 5

Resonance Raman spectrum of 4 obtained from an 8 mM CH₃CN frozen solution of 4 using 406.7 nm excitation (10 mW power, 32 scans of 16 s each). S = solvent peak.

Figure 6

ORTEP plot of 6 (50% probability level). Hydrogen atoms and the O₂SPh counteranion omitted for clarity. Selected inter-atomic distances (Å): Fe–O1, 1.996(2); Fe–S1, 3.1749(9); Fe–S2, 5.3605(11).

Figure 7

Comparison of the Fe K-edge X-ray absorption edge and pre-edge (inset) features of species 1 (black, –––), 3 (red, ), 4 (green, ), and 6 (blue, — — —).

Figure 8

Fourier transforms of the Fe K-edge EXAFS data (k³χ(k)) and unfiltered EXAFS spectra (k³χ(k), inset) obtained for 1, 3, 4, and 6. Experimental data is shown with dotted lines (•••••), while fits are shown with solid lines (——). Fourier transformation ranges are as follows: k = 2 – 15 Å⁻¹ (1), k = 2 – 14 Å⁻¹ (3), k = 2 – 14.95 Å⁻¹ (4), k = 2 – 14.3 Å⁻¹ (6). Fit parameters are shown in bold italics in Table 4.

Figure 9

ESI-MS spectra of compound 7 (A) and after reaction with 2 equiv. m-CPBA (B).

Figure 10

UV-vis spectral changes observed in the reaction of 2 (0.5 mmol/L, blue solid line) with 6 equivalents PhCO₂H in MeOH at −20 °C over a 3-min period yielding a mixture of compounds 3 and 5 (yellow solid line).

Figure 11

4.2 K Mössbauer spectrum obtained after adding ~6 equivalents pyridinium triflate at −40 °C to 1.3 mM 2. 38% of the iron belongs to complex 5 and 47% to 3; the remainder of the iron belongs to broad and unresolved Fe^III species.

Scheme 1

Structures of 1, 2, and some synthetic oxoiron(IV) complexes (TMC, TPA, N4Py).

Scheme 2

Reaction of 1 with m-CPBA.

Scheme 3

Postulated mechanism for the conversion of 1 to 3 in the reaction between 1 and m-CPBA without added base. m-CPBA could react either at the iron site (pathway A) or the sulfur site (pathway B) of 1, evidence for both pathways is provided.