Generation of a μ-1,2-hydroperoxo FeIIIFeIII and a μ-1,2-peroxo FeIVFeIII Complex

Abstract

Abstractμ-1,2-Peroxo-diferric intermediates (P) of non-heme diiron enzymes are proposed to convert upon protonation either to high-valent active species or to activated P′ intermediates via hydroperoxo-diferric intermediates. Protonation of synthetic μ-1,2-peroxo model complexes occurred at the μ-oxo and not at the μ-1,2-peroxo bridge. Here we report a stable μ-1,2-peroxo complex {FeIII(μ-O)(μ-1,2-O2)FeIII} using a dinucleating ligand and study its reactivity. The reversible oxidation and protonation of the μ-1,2-peroxo-diferric complex provide μ-1,2-peroxo FeIVFeIII and μ-1,2-hydroperoxo-diferric species, respectively. Neither the oxidation nor the protonation induces a strong electrophilic reactivity. Hence, the observed intramolecular C-H hydroxylation of preorganized methyl groups of the parent μ-1,2-peroxo-diferric complex should occur via conversion to a more electrophilic high-valent species. The thorough characterization of these species provides structure-spectroscopy correlations allowing insights into the formation and reactivities of hydroperoxo intermediates in diiron enzymes and their conversion to activated P′ or high-valent intermediates.

Dinuclear Diferrous Complexes of a Bis(tetradentate) Dinucleating Ligand: Influence of the Exogenous Ligands on the Molecular and Electronic Structures

“…Some structural parameters of the central core of [(susan){Fe II ( μ ‐OH) 2 Fe II }] 2+ are compared to those of the {Fe II ( μ ‐X) 2 Fe II } cores (X=OH − , F − ) of the 6‐methyl derivates [(susan 6−Me ){Fe II ( μ ‐OH) 2 Fe II }](ClO 4 ) 2 [24] and [(susan 6‐Me ){Fe II ( μ ‐F) 2 Fe II ](ClO 4 ) 2 [22] and of tpa [29] and tpa 6−Me[30] complexes (Table 2). The Fe II ‐O bond lenghts in [(tpa)Fe II 2 ( μ ‐OH) 2 (tpa)] 2+ ([(tpa 6−Me )Fe II 2 ( μ ‐OH) 2 (tpa 6−Me )] 2+ ) are 1.99 (2.00) Å and 2.15 (2.19) Å demonstrating the asymmetric core with alternating short/long bonds.…”

“…In this respect, we could synthesize the stable peroxo complex [(susan 6À Me ){Fe III (μ-O 2 )(μ-O)Fe III }](ClO 4 ) 2 and fully characterize it not only in solution but also in the solid state. [24] This peroxo complex can be reversibly oxidized to an unprecedented high-valent peroxo complex [(susan 6À Me ){Fe IV (μ-O 2 )(μ-O)Fe III }] 3 + and reversibly protonated to an also unprecedented μ-1,2-hydroperoxo complex [(susan 6À Me )-{Fe III (μ-OOH)(μ-O)Fe III }] 3 + . [24] This protonation mimics the activation of peroxo intermediates in NHFe 2 enzymes for the conversion to a high-valent intermediate.…”

“…In this respect, we could synthesize the stable peroxo complex [(susan 6−Me ){Fe III ( μ ‐O 2 )( μ ‐O)Fe III }](ClO 4 ) 2 and fully characterize it not only in solution but also in the solid state [24] . This peroxo complex can be reversibly oxidized to an unprecedented high‐valent peroxo complex [(susan 6−Me ){Fe IV ( μ ‐O 2 )( μ ‐O)Fe III }] 3+ and reversibly protonated to an also unprecedented μ ‐1,2‐hydroperoxo complex [(susan 6−Me ){Fe III ( μ ‐OOH)( μ ‐O)Fe III }] 3+ [24] . This protonation mimics the activation of peroxo intermediates in NHFe 2 enzymes for the conversion to a high‐valent intermediate.…”

“…The peroxo complex [(susan 6−Me ){Fe III ( μ ‐O 2 )( μ ‐O)Fe III }](ClO 4 ) 2 is stable in CH 3 CN solution at −40 °C and exhibits no electrophilic reactivity ( e. g . with PPh 3 or 9,10‐dihydroanthracene (DHA)) and even the oxidized complex [(susan 6−Me ){Fe IV ( μ ‐O 2 )( μ ‐O)Fe III }] 3+ only reacts with the weak O‐H bond of 1‐hydroxy‐2,2,6,6‐tetramethyl‐piperidine (TEMPO−H) [24] . This demonstrates that the ligand susan 6−Me is indeed capable for a strong stabilization of the {Fe III ( μ ‐1,2‐O 2 )( μ ‐O)Fe III } core.…”

“…Hence, we assumed that susan 6−Me could stabilize a peroxo {Fe III ( μ ‐O 2 )( μ ‐O)Fe III } core as the conversion to the high‐valent species is impeded by the longer Fe‐N 6−Me−py bonds and the driving force for the conversion to the thermodynamic sink {Fe III X( μ ‐O)Fe III X} is absent. In this respect, we could synthesize the stable peroxo complex [(susan 6−Me ){Fe III ( μ ‐O 2 )( μ ‐O)Fe III }](ClO 4 ) 2 and fully characterize it not only in solution but also in the solid state [24] . This peroxo complex can be reversibly oxidized to an unprecedented high‐valent peroxo complex [(susan 6−Me ){Fe IV ( μ ‐O 2 )( μ ‐O)Fe III }] 3+ and reversibly protonated to an also unprecedented μ ‐1,2‐hydroperoxo complex [(susan 6−Me ){Fe III ( μ ‐OOH)( μ ‐O)Fe III }] 3+ [24] .…”