Issue 5, 2021

Tuning the stereoelectronic factors of iron(ii)-2-aminophenolate complexes for the reaction with dioxygen: oxygenolytic C–C bond cleavage vs. oxidation of complex

Abstract

Oxidative C–C bond cleavage of 2-aminophenols mediated by transition metals and dioxygen is a topic of great interest. While the oxygenolytic C–C bond cleavage reaction relies on the inherent redox non-innocent property of 2-aminophenols, the metal complexes of 2-aminophenolates often undergo 1e/2e oxidation events (metal or ligand oxidation), instead of the direct addition of O2 for subsequent C–C bond cleavage. In this work, we report the isolation, characterization and dioxygen reactivity of a series of ternary iron(II)-2-aminophenolate complexes [(TpPh,Me)FeII(X)], where X = 2-amino-4-tert-butylphenolate (4-tBu-HAP) (1); X = 2-amino-4,6-di-tert-butylphenolate (4,6-di-tBu-HAP) (2); X = 2-amino-4-nitrophenolate (4-NO2-HAP)(3); and X = 2-anilino-4,6-di-tert-butylphenolate (NH-Ph-4,6-di-tBu-HAP) (4) supported by a facial tridentate nitrogen donor ligand (TpPh,Me = hydrotris(3-phenyl-5-methylpyrazol-1-yl)borate). Another facial N3 ligand (TpPh2 = hydrotris(3,5-diphenyl-pyrazol-1-yl)borate) has been used to isolate an iron(II)-2-anilino-4,6-di-tert-butylphenolate complex (5) for comparison. Both [(TpPh,Me)FeII(4-tBu-HAP)] (1) and [(TpPh,Me)FeII(4,6-di-tBu-HAP)] (2) undergo regioselective oxidative aromatic ring fission reaction of the coordinated 2-aminophenols to the corresponding 2-picolinic acids in the reaction with dioxygen. In contrast, complex [(TpPh,Me)FeII(4-NO2-HAP)] (3) displays metal based oxidation to form an iron(III)-2-amidophenolate complex. Complexes [(TpPh,Me)FeII(NH-Ph-4,6-di-tBu-HAP)] (4) and [(TpPh2)FeII(NH-Ph-4,6-di-tBu-HAP)] (5) react with dioxygen to undergo 2e oxidation with the formation of the corresponding iron(III)-2-iminobenzosemiquinonato radical species implicating the importance of the –NH2 group in directing the C–C bond cleavage reactivity of 2-aminophenols. The systematic study presented in this work unravels the effect of the electronic and structural properties of the redox non-innocent 2-aminophenolate ring and the supporting ligand on the C–C bond cleavage reactivity vs. the metal/ligand oxidation of the complexes. The study further reveals that proper modulation of the stereoelectronic factors enables us to design a well synchronised proton transfer (PT) and dioxygen binding events for complexes 1 and 2 that mimic the structure and function of the nonheme enzyme 2-aminophenol-1,6-dioxygenase (APD).

Graphical abstract: Tuning the stereoelectronic factors of iron(ii)-2-aminophenolate complexes for the reaction with dioxygen: oxygenolytic C–C bond cleavage vs. oxidation of complex

Supplementary files

Article information

Article type
Paper
Submitted
23 Sep 2020
Accepted
19 Dec 2020
First published
23 Dec 2020
This article is Open Access
Creative Commons BY license

Dalton Trans., 2021,50, 1901-1912

Tuning the stereoelectronic factors of iron(II)-2-aminophenolate complexes for the reaction with dioxygen: oxygenolytic C–C bond cleavage vs. oxidation of complex

S. Chatterjee, S. Banerjee, R. D. Jana, S. Bhattacharya, B. Chakraborty and S. A. V. Jannuzzi, Dalton Trans., 2021, 50, 1901 DOI: 10.1039/D0DT03316B

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