Reactivity of biomimetic iron(II)-2-aminophenolate complexes toward dioxygen: Mechanistic investigations on the oxidative C-C bond cleavage of substituted 2-aminophenols

Biswarup Chakraborty, Sourav Bhunya, Ankan Paul, Tapan Kanti Paine

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31 Scopus citations

Abstract

The isolation and characterization of a series of iron(II)-2-aminophenolate complexes [(6-Me3-TPA)FeII(X)]+ (X = 2-amino-4-nitrophenolate (4-NO2-HAP), 1; X = 2-aminophenolate (2-HAP), 2; X = 2-amino-3-methylphenolate (3-Me-HAP), 3; X = 2-amino-4-methylphenolate (4-Me-HAP), 4; X = 2-amino-5-methylphenolate (5-Me-HAP), 5; X = 2-amino-4-tert-butylphenolate (4-tBu-HAP), 6 and X = 2-amino-4,6-di-tert-butylphenolate (4,6-di-tBu-HAP), 7) and an iron(III)-2-amidophenolate complex [(6-Me3-TPA)FeIII(4,6- di-tBu-AP)]+ (7Ox) supported by a tripodal nitrogen ligand (6-Me3-TPA = tris(6-methyl-2-pyridylmethyl)amine) are reported. Substituted 2-aminophenols were used to prepare the biomimetic iron(II) complexes to understand the effect of electronic and structural properties of aminophenolate rings on the dioxygen reactivity and on the selectivity of C-C bond cleavage reactions. Crystal structures of the cationic parts of 5·ClO4 and 7·BPh4 show six-coordinate iron(II) centers ligated by a neutral tetradentate ligand and a monoanionic 2-aminophenolate in a bidentate fashion. While 1·BPh 4 does not react with oxygen, other complexes undergo oxidative transformation in the presence of dioxygen. The reaction of 2·ClO 4 with dioxygen affords 2-amino-3H-phenoxazin-3-one, an auto-oxidation product of 2-aminophenol, whereas complexes 3·BPh 4, 4·BPh4, 5·ClO4 and 6·ClO4 react with O2 to exhibit C-C bond cleavage of the bound aminophenolates. Complexes 7·ClO4 and 7 Ox·BPh4 produce a mixture of 4,6-di-tert-butyl-2H- pyran-2-imine and 4,6-di-tert-butyl-2-picolinic acid. Labeling experiments with 18O2 show the incorporation of one oxygen atom from dioxygen into the cleavage products. The reactivity (and stability) of the intermediate, which directs the course of aromatic ring cleavage reaction, is found to be dependent on the nature of ring substituent. The presence of two tert-butyl groups on the aminophenolate ring in 7·ClO4 makes the complex slow to cleave the C-C bond of 4,6-di-tBu-HAP, whereas 4·BPh4 containing 4-Me-HAP displays fastest reactivity. Density functional theory calculations were conducted on [(6-Me 3-TPA)FeIII(4-tBu-AP)]+ (6 Ox) to gain a mechanistic insight into the regioselective C-C bond cleavage reaction. On the basis of the experimental and computational studies, an iron(II)-2-iminobenzosemiquinonate intermediate is proposed to react with dioxygen resulting in the oxidative C-C bond cleavage of the coordinated 2-aminophenolates.

Original languageEnglish
Pages (from-to)4899-4912
Number of pages14
JournalInorganic Chemistry
Volume53
Issue number10
DOIs
StatePublished - 19 May 2014
Externally publishedYes

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

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