Characterization and Subsequent Reactivity of an Fe-Peroxo Porphyrin Generated by Electrochemical Reductive Activation of O₂

Abstract

Reductive activation of O is achieved by using the [Fe(FTPP)Cl] (FTPP = 5,10,15,20-tetrakis(pentafluorophenyl) porphyrinate) porphyrin through electrochemical reduction of the [Fe(FTPP)(O)] superoxo complex. Formation of the [Fe(FTPP)(OO)] peroxo species is monitored by using low-temperature electronic absorption spectroscopy, electron paramagnetic resonance, and cyclic voltammetry. Its subsequent protonation to yield the [Fe(FTPP)(OOH)] hydroperoxo intermediate is probed using low-temperature electronic absorption spectroscopy and electron paramagnetic resonance.

Probing the Activity of Iron Peroxo Porphyrin Intermediates in the Reaction Layer during the Electrochemical Reductive Activation of O2

“…The best fit of the experimental LSVs at G and the C UME for Fe(F 20 TPP) is obtained for K O 2 = 25 AE 10 s À1 , a value comparable but lower to previous estimate based only on the analysis of the CVs at G (see Section 4, SI), [14] and k d = 1 AE 0.3 s À1 as shown in Figure 3 A,B. k d is 20 times higher than the apparent Fe III (OO 2À ) decomposition rate constant determined by the thin-layer spectroelectrochemical study at 253 K (Section 2, SI), a difference that could be accounted for by the 40 K temperature difference between the two experiments.…”

“…We focus here on the first intermediate produced in nonacidic medium, the Fe III (OO 2À ) (peroxo) adduct, for three porphyrins showing, based on earlier work, [7,14]…”

“…Under Ar, the LSV at C probes a single oxidation event at the half-wave potential 2,C % 0.4 V. This new wave is attributed to the oxidation of Fe III (OO 2À ) probed directly in the diffusion layer of G corroborating its relative stability. [14] First, the absence of a collection current at the SECM tip for E C close to E p Fe III ðOO C À Þ=Fe III ðOO 2À Þ indicates that the reoxidation of Fe III -(OO 2À ) does not yield Fe III (OOC À ), likely owing to the change in O 2 coordination within the porphyrin complex during the electron transfer steps. [5a,d, 14] As a consequence, the Fe III -(OOC À )/Fe III (OO 2À ) couple does not behave here as a reversible system, except at low temperature (210 K).…”

Synthesis and Characterization of Iron(II) Complexes with a BPMEN‐Type Ligand Bearing π‐Accepting Nitro Groups

“…This clearly reflects the harder Lewis acidity of the Fe II center when it is coordinated to the electron‐withdrawing L 4 2 NO 2 ligand. As stated above, such a property is required for a more efficient coordination of dioxygen to the Fe II center and an easier subsequent activation , …”

“…A thorough mechanistic investigation of this reaction through cyclic voltammetry revealed that the coordination of O 2 to an Fe II center, which leads to a putative Fe III –superoxo intermediate, is disfavored with this family of polydentate aza ligands . To develop more reactive Fe II precursors that facilitate the formation of the Fe III –superoxo intermediate and avoid the reduction of the latter, it appeared crucial to make the metal center more Lewis acidic , . Herein, we report the synthesis, characterization, and stability studies of Fe II complexes prepared with the ligand N , N′ ‐dimethyl‐ N , N′ ‐bis(4‐nitropyridin‐2‐ylmethyl)ethane‐1,2‐diamine ( L 4 2 NO 2 ), in which the pyridyl rings are functionalized with electron‐withdrawing para ‐nitro groups to provide the desired electronic effects without impacting the accessibility to the metal center.…”