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-Me-3-TPA)Fe-II(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-Bu-t-HAP), 6 and X = 2-amino-4,6-di-tert-butylphenolate (4,6-di-Bu-t-HAP), 7) and an iron(III)-2-amidophenolate complex [(6-Me-3-TPA)Fe-III(4,6-di-Bu-t-AP)](+) (7(Ox)) 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 center dot ClO4 and 7 center dot BPh4 show six-coordinate iron(II) centers ligated by a neutral tetradentate ligand and a monoanionic 2-aminophenolate in a bidentate fashion. While 1 center dot BPh4 does not react with oxygen, other complexes undergo oxidative transformation in the presence of dioxygen. The reaction of 2 center dot ClO4 with dioxygen affords 2-amino-3H-phenoxazin-3-one, an auto-oxidation product of 2-aminophenol, whereas complexes 3 center dot BPh4, 4 center dot BPh4, 5 center dot ClO4 and 6 center dot ClO4 react with O-2 to exhibit C-C bond cleavage of the bound aminophenolates. Complexes 7 center dot ClO4 and 7(Ox)center dot 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 O-18(2) 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 center dot ClO4 makes the complex slow to cleave the C-C bond of 4,6-di-Bu-t-HAP, whereas 4 center dot BPh4 containing 4-Me-HAP displays fastest reactivity. Density functional theory calculations were conducted on [(6-Me-3-TPA)Fe-III(4-Bu-t-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.