Reactivity of Biomimetic Iron(II)-2-aminophenolate Complexes toward Dioxygen: Mechanistic Investigations on the Oxidative C–C Bond Cleavage of Substituted 2‑Aminophenols

The isolation and characterization of a series of iron­(II)-2-aminophenolate complexes [(6-Me₃-TPA)­Fe^II(X)]⁺ (X = 2-amino-4-nitrophenolate (4-NO₂-HAP), <b>1;</b> X = 2-aminophenolate (2-HAP), <b>2</b>; X = 2-amino-3-methylphenolate (3-Me-HAP), <b>3</b>; X = 2-amino-4-methylphenolate (4-Me-HAP), <b>4</b>; X = 2-amino-5-methylphenolate (5-Me-HAP), <b>5</b>; X = 2-amino-4-<i>tert</i>-butylphenolate (4-<i>^t</i>Bu-HAP), <b>6</b> and X = 2-amino-4,6-di-<i>tert</i>-butylphenolate (4,6-di-<i>^t</i>Bu-HAP), <b>7</b>) and an iron­(III)-2-amidophenolate complex [(6-Me₃-TPA)­Fe^III(4,6-di-<i>^t</i>Bu-AP)]⁺ (<b>7</b>^<b>Ox</b>) 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 <b>5</b>·ClO₄ and <b>7</b>·BPh₄ show six-coordinate iron­(II) centers ligated by a neutral tetradentate ligand and a monoanionic 2-aminophenolate in a bidentate fashion. While <b>1</b>·BPh₄ does not react with oxygen, other complexes undergo oxidative transformation in the presence of dioxygen. The reaction of <b>2</b>·ClO₄ with dioxygen affords 2-amino-3<i>H</i>-phenoxazin-3-one, an auto-oxidation product of 2-aminophenol, whereas complexes <b>3</b>·BPh₄, <b>4</b>·BPh₄, <b>5</b>·ClO₄ and <b>6</b>·ClO₄ react with O₂ to exhibit C–C bond cleavage of the bound aminophenolates. Complexes <b>7</b>·ClO₄ and <b>7</b>^Ox·BPh_<b>4</b> produce a mixture of 4,6-di-<i>tert</i>-butyl-2<i>H</i>-pyran-2-imine and 4,6-di-<i>tert</i>-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 <i>tert</i>-butyl groups on the aminophenolate ring in <b>7</b>·ClO₄ makes the complex slow to cleave the C–C bond of 4,6-di-<i>^t</i>Bu-HAP, whereas <b>4</b>·BPh₄ containing 4-Me-HAP displays fastest reactivity. Density functional theory calculations were conducted on [(6-Me₃-TPA)­Fe^III(4-<i>^t</i>Bu-AP)]⁺ (<b>6</b>^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