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

Abstract

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), 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-(t)Bu-HAP), 6 and X = 2-amino-4,6-di-tert-butylphenolate (4,6-di-(t)Bu-HAP), 7) and an iron(III)-2-amidophenolate complex [(6-Me3-TPA)Fe(III)(4,6-di-(t)Bu-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·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 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 (18)O2 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-(t)Bu-HAP, whereas 4·BPh4 containing 4-Me-HAP displays fastest reactivity. Density functional theory calculations were conducted on [(6-Me3-TPA)Fe(III)(4-(t)Bu-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.

Dehydrogenation of anhydrous methanol at room temperature by o-aminophenol-based photocatalysts

“…Accordingly, the axial ligands should be MeOH ligands. Moreover, the Fe–N1, Fe–O2 and Fe–O1 bond distances of 2.214(4) Å, 2.229(3) Å and 2.048(3) Å, respectively, suggest the presence of a high-spin Fe II centre43444546. These observations, in their entirety, suggest that 1 adopts a structure that is consistent with trans -[Fe II (apH) 2 (MeOH) 2 ].…”

“…The assignment of the two chelating ligands requires great care, as these could be apH − , isq − or o -iminobenzoquinone (ibq) ligands (Fig. 1)4243444546. In complex 1 , the observed C–N and C–O bond distances of 1.461(5) Å and 1.342(5) Å, respectively, suggest a single-bond character for these bonds414344454649.…”

“…While apH 2 contains one hydroxyl and two amino protons, which can be deprotonated by TBAOH (1 equiv.) to give monoanionic C 6 H 4 NOH 2 − , previous reports on Fe(II) complexes of apH − have shown that it is possible to deprotonate the hydroxyl proton in apH 2 selectively43444546. As the Fe–O1 (2.048(3) Å) and Fe–N1 (2.214(4) Å) bond distances in 1 are comparable to those in these examples (Fe–O: 1.931–1.994(2) Å; Fe–N: 2.212–2.299(7) Å), the structure of the chelating ligands should be commensurate with that of apH − .…”

“…1)4243444546. Pino and co-workers47 have reported the 1 π-π*-initiated photochemical activation of the O–H bond of o -aminophenol (apH 2 ) in the gas phase.…”

“…Recently, successful examples on the elucidation of electron- and proton-transfer properties of transition metal complexes of apH − , together with the characterization of corresponding complexes with o -iminosemiquinonate (isq − ) have been reported, and these studies are currently under extensive investigation as models for non-haem-type iron dioxygenase4243444546. In contrast to these sophisticated examples for the reactivity of apH 2 derivatives, which proceed under the exclusion of light, examples on photochemical reactions still remain elusive.…”