Heterolytic O-O Bond Cleavage Upon Single Electron Transfer to a Nonheme Fe(III)-OOH Complex

Abstract

The one-electron reduction of the nonheme iron(III)-hydroperoxo complex, [Fe^III (OOH)(L₅ ² )]²⁺ (L₅ ² =N-methyl-N,N',N'-tris(2-pyridylmethyl)ethane-1,2-diamine), carried out at -70 °C results in the release of dioxygen and in the formation of [Fe^II (OH)(L₅ ² )]⁺ following a bimolecular process. This reaction can be performed either with cobaltocene as chemical reductant, or electrochemically. These experimental observations are consistent with the disproportionation of the hydroperoxo group in the putative Fe^II (OOH) intermediate generated upon reduction of the Fe^III (OOH) starting complex. One plausible mechanistic scenario is that this disproportionation reaction follows an O-O heterolytic cleavage pathway via a Fe^IV -oxo species.

Figure 1

(A) Electronic absorption spectrum of [Fe^III(OOH)(L₅ ²)](PF₆)₂ with its schematical representation (1 mM, PrCN+0.2 M TBAPF₆ at −70 °C). (B) X‐band EPR spectrum of the same sample recorded at 90 K. The inset shows the full spectral range (0–5000 Gauss). The complete assignment is given in the Supporting Information, Figure S1. Microwave frequency 9.65 GHz, microwave power 1.0 mW, modulation amplitude 8 G, gain 50 dB, modulation frequency 100 kHz.

Figure 2

Cyclic Voltammogram of [Fe^III(OOH)(L₅ ²)](PF₆)₂ (1 mM) in PrCN+0.2 M TBAPF₆ at −70 °C, 0.1 V s⁻¹. The arrows indicate the initial direction of the scan.

Figure 3

(A) UV‐visible monitoring of the reaction of [Fe^III(OOH)(L₅ ²)](PF₆)₂ (−50 °C, 0.15 mM in PrCN/MeCN 1 : 1, blue trace) with 1 equiv. Co(Cp)₂ (black to grey to red trace). The inset shows the time traces of [Fe^III(OOH)(L₅ ²)]²⁺ disappearance at 530 nm (blue) and [Fe^II(OH)(L₅ ²)]⁺ formation at 390 nm (red). (B) Parallel mode X‐band EPR spectra at 10 K of aliquots taken from the same reaction mixture (with corresponding color lines). Microwave frequency 9.38 GHz, microwave power 2.0 mW, modulation amplitude 8 G, gain 50 dB, modulation frequency 100 kHz, temperature 10 K. Experiments were done in the presence of 2 equivalents of potassium tetrakis(pentafluorophenyl)borate to avoid cobaltocenium precipitation.

Scheme 1

Possible mechanisms accounting for the disproportionation reaction of the peroxo group without (A) or with (B) accumulation of a high‐valent iron‐oxo intermediate.