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)­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·BPh4 does not react
with oxygen, other complexes undergo oxidative transformation in the
presence of dioxygen. The reaction of 2·ClO4 with dioxygen affords 2-amino-3H-phenoxazin-3-one,
an auto-oxidation product of 2-aminophenol, whereas complexes 3·BPh4, 4·BPh4, 5·ClO4 and 6·ClO4 react with O2 to exhibit C–C bond cleavage
of the bound aminophenolates. Complexes 7·ClO4 and 7Ox·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-Me3-TPA)­FeIII(4-tBu-AP)]+ (6Ox) 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.