Halogen Transfer to Carbon Radicals by High-Valent Iron Chloride and Iron Fluoride Corroles

Abstract

High-valent iron halide corroles were examined to determine their reactivity with carbon radicals and their ability to undergo radical rebound-like processes. Beginning with Fe(Cl)(ttppc) (1) (ttppc = 5,10,15-tris(2,4,6-triphenylphenyl)corrolato^3-), the new iron corroles Fe(OTf)(ttppc) (2), Fe(OTf)(ttppc)(AgOTf) (3), and Fe(F)(ttppc) (4) were synthesized. Complexes 3 and 4 are the first iron triflate and iron fluoride corroles to be structurally characterized by single crystal X-ray diffraction. The structure of 3 reveals an Ag^I-pyrrole (η²-π) interaction. The Fe(Cl)(ttppc) and Fe(F)(ttppc) complexes undergo halogen transfer to triarylmethyl radicals, and kinetic analysis of the reaction between (p-OMe-C₆H₄)₃C• and 1 gave k = 1.34(3) × 10³ M^-1 s^-1 at 23 °C and 2.2(2) M^-1 s^-1 at -60 °C, ΔH^⧧ = +9.8(3) kcal mol^-1, and ΔS^⧧ = -14(1) cal mol^-1 K^-1 through an Eyring analysis. Complex 4 is significantly more reactive, giving k = 1.16(6) × 10⁵ M^-1 s^-1 at 23 °C. The data point to a concerted mechanism and show the trend X = F^- > Cl^- > OH^- for Fe(X)(ttppc). This study provides mechanistic insights into halogen rebound for an iron porphyrinoid complex.

Figure 1.

UV-vis spectra of 1 (blue, dashed line) and 2 (red, solid line) in toluene at 23 °C.

Figure 2.

¹H NMR spectra of 2 (top) and 3 (bottom) in C₆D₅CD₃ at 23 °C. Inset: expanded region from 6 – 9 ppm. Chemical shifts labeled in red highlight the peak at −7.4 ppm in 2 which splits into two peaks in the presence of AgOTf in 3.

Figure 3.

Zero-field ⁵⁷Fe Mössbauer spectra of 2 at 125 K (top), 2 at 80 K (middle), 3 at 80 K (bottom) in toluene. Experimental data = black circles, best fit = red line.

Figure 4.

Displacement ellipsoid plot (40% probability) of 3 at 110 K. Hydrogen atoms omitted for clarity. Inset: chemical structure of 3.

Figure 5.

UV-vis spectra of Fe(X)(ttppc) where X = Cl (blue, dashed line) or F (red, solid line) in toluene at 23 °C.

Figure 6.

Displacement ellipsoid plot (50% probability) of 4 at 110 K. Hydrogen atoms are omitted for clarity.

Figure 7.

¹H NMR (400 MHz) spectrum of Fe(X)(ttppc) with X = F (top) and Cl (bottom) in C₆D₅CD₃ at 23 °C.

Figure 8.

Zero-field ⁵⁷Fe Mössbauer spectra of Fe(X)(ttppc) with X = Cl (top), and F (bottom) in toluene at 80 K. Experimental data = black circles, best fit = red line.

Figure 9.

Cyclic voltammograms of 1 (2 mM) with a scan rate of 50 mV/s (black) and 200 mV/s (blue) (top), and 4 (3 mM) with a scan rate of 100 mV/s (black) in benzonitrile (bottom). The solution contains Bu₄N⁺PF₆⁻ (0.25 M) supporting electrolyte.

Figure 10.

UV-vis spectra for the reaction of Fe(X)(ttppc) where X = Cl (top) or F (bottom) with Ph₃C=C₆H₅CPh₂ in toluene at 23 °C. Spectra before (blue) and after (red) the addition of excess Ph₃C=C₆H₅CPh₂.

Figure 11.

Zero-field ⁵⁷Fe Mössbauer spectrum (C₆H₅CH₃, 80 K) of 1 (top), and 1 + (p-MeO-C₆H₄)₃C• (bottom). Experimental data = black circles, best fit = red line.

Figure 12.

(a) Overlay of UV–vis spectra for 1 (blue, solid line), 1 + (p-MeO-C₆H₄)₃C• (red, dashed line), and (p-MeO-C₆H₄)₃C• (green, dotted line) in toluene. (b) UV-vis spectra for reaction of 1 (32 μM) with (p-MeO-C₆H₄)₃C• (62 equiv) in toluene from 0 (blue) to 2 sec (red) at 0.04 s intervals (grey) at 23 °C. Inset: Changes in absorbance versus time for the formation of Fe^III(ttppc) (black dots) fit to a single exponential expression (red line). (c) Plot of k_obs (23 °C) versus [(p-MeO-C₆H₄)₃C•] with best-fit line. (d) Eyring plot of (ln(kh/k_BT) versus 1/T (−70 °C to 23 °C) with best-fit line.

Figure 13.

(a) UV-vis spectra for reaction of 4 (12.5 μM) with (p-MeO-C₆H₄)₃C• (31 equiv) in toluene from 0 (blue) to 0.1 s (red) at 0.0015 s intervals (grey) at 23 °C. Inset: Changes in absorbance versus time for the formation of Fe^III(ttppc) (black dots) fit to a single exponential expression (red line). (b) Plot of k_obs versus [(p-MeO-C₆H₄)₃C•] with best-fit line.

Scheme 1.

C-H Bond Activation by Cytochrome P450

Scheme 2.

C-H Bond Chlorination by αKG-Dependent Enzymes

Scheme 3.

Synthesis of Complex 2

Scheme 4.

Synthesis of Complex 4

Scheme 5.

Reaction of Fe(X)(ttppc) (X = Cl or F) with Triphenylmethyl Radical

Scheme 6.

Reaction of 4 with Tris-(para-Methoxyphenyl)methyl Radical