Synthetic, Spectroscopic, and DFT Studies of Iron Complexes with Iminobenzo(semi)quinone Ligands: Implications for o‐Aminophenol Dioxygenases

Abstract

The oxidative C-C bond cleavage of o-aminophenols by nonheme Fe dioxygenases is a critical step in both human metabolism (the kynurenine pathway) and the microbial degradation of nitroaromatic pollutants. The catalytic cycle of o-aminophenol dioxygenases (APDOs) has been proposed to involve formation of an Fe(II)/O2/iminobenzosemiquinone complex, although the presence of a substrate radical has been called into question by studies of related ring-cleaving dioxygenases. Recently, we reported the first synthesis of an iron(II) complex coordinated to an iminobenzosemiquinone (ISQ) ligand, namely, [Fe(Ph2Tp)(ISQtBu)] (2a; where Ph2Tp = hydrotris(3,5-diphenylpyrazol-1-yl)borate and ISQtBu is the radical anion derived from 2-amino-4,6-di-tert-butylphenol). In the current manuscript, density functional theory (DFT) calculations and a wide variety of spectroscopic methods (electronic absorption, Mössbauer, magnetic circular dichroism, and resonance Raman) were employed to obtain detailed electronic-structure descriptions of 2a and its one-electron oxidized derivative [3a]+. In addition, we describe the synthesis and characterization of a parallel series of complexes featuring the neutral supporting ligand tris(4,5-diphenyl-1-methylimidazol-2-yl)phosphine (Ph2TIP). The isomer shifts of ~0.97 mm/s obtained via Mössbauer experiments confirm that 2a (and its Ph2TIP-based analogue [2b]+) contain Fe(II) centers, and the presence of an ISQ radical was verified by analysis of the absorption spectra in light of time-dependent DFT calculations. The collective spectroscopic data indicate that one-electron oxidation of the Fe2+–ISQ complexes yields complexes ([3a]+ and [3b]2+) with electronic configurations between the Fe3+–ISQ and Fe2+–IBQ limits (IBQ = iminobenzoquinone), highlighting the ability of o-amidophenolates to access multiple oxidation states. The implications of these results for the mechanism of APDOs and other ring-cleaving dioxygenases are discussed.

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.…”