Xue et al. 10.1073/pnas.0708516105. |
Fig. 6. Mössbauer spectrum of 2b (dash marks) recorded at 100 K in a parallel applied field of 8.0 T. The solid lines are spectral simulations for the FeIVFeIV complex assuming antiferromagnetic coupling. The theoretical spectrum in A was generated for J = 130 cm-1 whereas J = 70 cm-1 was used for B.
Fig. 7. (A) Raman spectra of 1b (black line) and 18O-labeled 1b (red line, prepared by exchange with 300 equivalents of H218O at -30°C for 10 min). Excitation: 647.1 nm. (B) Raman spectra of 2b samples after exchange with 300 equivalents of H218O at -30°C for 30 min (black line), 60 min (blue line), and 180 min (green line). The red line represents the sample prepared from 18O-labeled 1b. Excitation: 514.5 nm. All spectra measured with frozen MeCN solutions (77 K) in the presence of 100 mM KPF6.
Fig. 8. UV-visible spectroscopic change upon reaction of 0.17 mM 1b (green dashed line) with 10 mM DHA in MeCN at -30°C under Ar. The blue line represents the spectrum of anthracene measured after removing the iron complex. The concentration of anthracene was estimated as 0.088 mM (52% yield) based on the absorbance at 377 nm (e= 7,700 M-1·cm-1). There was no detectable amount of anthrone/anthraquinone by GC analysis. The red solid line represents the spectrum of 0.17 mM 3b in MeCN at -30° C. The fact that the final spectrum of the reaction system (black solid line) showed the 320-nm and 480-nm features similar to that of 3b suggested that 1b was converted to a diiron(III) species after DHA oxidation. The time trace (at 620 nm) was fitted with a pseudo-first-order model (Inset).
Fig. 9. UV-visible spectroscopic change upon reaction of 0.13 mM 2b (red dashed line) with 10 mM DHA in MeCN at -30°C under Ar in the presence of 5 equivalents of 2,6-lutidine. 2b was reduced to 1b in quantitative yield, as indicated by the absorbance at 620 nm. In the absence of a base such as 2,6-lutidine, less than 50% of 1b was obtained, suggesting that a proton was produced in the oxidation of DHA and caused the decomposition of 1b. The blue line represents the spectrum of anthracene measured after reducing 1b with 1 equivalent of ferrocene and removing the iron complexes. The concentration of anthracene was estimated as 0.018 mM (14% yield) based on the absorbance at 377 nm. GC/GC-mass analysis also revealed that anthraquinone was produced with 13% yield. The product mixture accounts for all the iron oxidizing equivalents consumed. The time trace (at 485 nm) was fitted with a pseudo-first-order model (Inset).
Fig. 10. UV-visible spectroscopic change upon reaction of 1.0 mM 4b (green dashed line) with 10 mM DHA in MeCN at -30°C under Ar. Features at 450 nm, 486 nm, and 501 nm, which are similar with that of the [FeIII2(O)(O2CR)(TPA)2] complex (6), grew during the reaction and suggest that the corresponding diiron(III) species of Lb was formed. The blue line represents the spectrum of anthracene (diluted to 1/5 of the original concentration) measured after removing the iron complex. The concentration of anthracene was estimated as 0.42 mM (42% yield) based on the absorbance at 377 nm. There was no detectable amount of anthrone/anthraquinone by GC analysis. The time trace (at 720 nm) was fitted with a pseudo-first-order model (Inset).
Table 3. XANES Characteristics of 1b and 2b
Sample | Peak No.* | Height† | Area, | E 1s"3d,eV‡ | E 0, eV§ |
1a | 1 | 0.056 | 14.7 | 7,113.2 | 7,129.0 |
| 2 | 0.019 | 5.0 | 7,115.7 |
|
2b | 1 | 0.047 | 13.8 | 7,114.7 | 7,130.1 |
| 2 | 0.018 | 5.2 | 7,117.9 |
|
*Both samples required two peaks to adequately fit the pre-edge feature.
†
Pre-edge peak heights were normalized on Fe-edge heights.‡
Positions of E1s"3d were calculated after curve fitting by SSExafs.§
Observed first inflection point of the Fe edge.SI Text
Synthesis and Characterization of Lb, 3b, 1b, 1a, and 4b
Tris(4-methoxy-3,5-dimethylpyridyl-2-methyl)amine (Lb).
Lb was synthesized following a procedure described for the synthesis of Tris(4-methoxypyridyl-2-methyl)amine (1) with commercial available 2-chloromethyl-4-methoxy-3,5-dimethylpyridine hydrochloride as the starting material. The ligand was obtained as a white solid. 1H-NMR (MeOH-d4): 8.14 (3H, s); 3.72 (9H, s); 3.67 (6H, s); 2.26 (9H, s); 1.66 (9H, s). ESI-MS (MeOH): 465 (M + H+), 487 (M + Na+).[Fe2(m-O)(H2O)(OH)(Lb)2](ClO4)3 (3b).
200 mg (0.43 mmol) Lb and 230 mg (0.43 mmol) Fe(ClO4)3.10 H2O was dissolved in 10 ml MeOH. 1.3 ml of 500 mM NaOH solution in MeOH (0.65 mmol) were added dropwise upon stirring to afford a brown solution. Upon dropwise addition of 10 ml H2O, a red power precipitated out eventually. The mixture was stored in refrigerator overnight and the product as red powder was then collected by filtration and washed with 2:1 H2O-MeOH. Anal. Calcd. for [Fe2(m-O)(H2O)(OH)(Lb)2](ClO4)3. 3H2O (C54H81N8Cl3Fe2O24): C, 44.90; H, 5.65; N, 7.76; Cl, 7.36. Found: C, 44.84; H, 5.29; N, 7.78; Cl, 7.96.[FeIIIFeIV(m-O)2(Lb)2](ClO4)3 (1b).
100 mg complex 3b was dissolved in 2 ml dry MeCN and filtered to afford the clear red solution that was then cooled to -40°C. Upon addition of 30 ml H2O2 (mixed with 300 ml MeCN), a deep green solution was obtained. The solution was stirred at -40°C for two hours and 2 ml Et2O was added to precipitate a green solid, which was then collected by filtration (at -40°C) and washed with 2:1 Et2O-MeCN. UV-visible (in MeCN at -40°C): 620 nm (5,300 M-1·cm-1), 350 nm (sh, 7,700 M-1·cm-1). Raman (647.1 nm excitation, 77 K in frozen MeCN): a doublet at 655 cm-1 and 667 cm-1, shifted to one peak at 628 cm-1 upon exchange with 300 equivalents of H218O (Fig. 7). Mössbauer (at 100 K in frozen MeCN): d = 0.11(1) mm·s-1, DEQ = 0.44(2) mm·s-1.Two other complexes were also synthesized for comparison. Complex 1a ([FeIIIFeIV(m-O)2(La)2]3+) was synthesized following the published procedure (2). Complex 4b ([FeIV(O)Lb(MeCN)]2+) was prepared by treating a solution of [FeIILb(MeCN)2](OTf)2 in MeCN with 1 equivalent of peracetic acid (32 wt. % in acetic acid) at -30°C. It exhibits a near-IR absorbance maximum at 720 nm (e = 300 M-1·cm-1 and a Mössbauer quadrupole doublet with d = 0.01 mm·s-1 and DEQ = 0.95 mm·s-1, parameters similar to those of the oxoiron(IV) complex with the parent TPA ligand (3).
8.0 T Mössbauer Spectrum of 2b at 100 K
We have analyzed the 8.0 T spectrum of 2b with the 2SPIN option of WMOSS for the spin Hamiltonian
for S1 = S2 = 1. HQ(i) describes the quadrupole interactions; h(i) = (Vxx(i) - Vyy(i))/Vzz(i) is the asymmetry parameter of the electric field gradient tensor. For the simulations we have used D1 = D2 = 30 cm-1 (the 100 K spectrum is essentially independent of Di), Ax = Ay = -30 MHz and Az = -6 MHz for both iron sites, DEQ = 2.02 mm/s, and h = 0 for both sites. The chosen A-values are typical for S = 1 FeIV = O complexes (4, 5). For these A-values and J = 70 cm-1 the internal field in the x-y plane is Bint(x, y) = -0.6 T, leading to the mismatch with the experimental magnetic splitting in the low energy region of the spectrum in Fig. 6B. Only the low-energy feature in Fig. 6 is unaffected by the presence of the contaminants; taking only the position of this feature into account, we estimate 80 cm-1 < J <180 cm-1.
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