Isolation and Characterization of a Dihydroxo-Bridged Iron(III,III)(μ-OH)2 Diamond Core Derived from Dioxygen

Dioxygen addition to coordinatively unsaturated [Fe(II)((ON4)-N-Me2(6-Me-DPEN))](PF6) (1) is shown to afford a complex containing a dihydroxo-bridged Fe(III)(2)(mu-OH)(2) diamond core, [Fe-III((ON4)-N-Me2(6-Me-DPEN))](2)(mu-OH)(2)(PF6)(2). (CH3CH2CN)(2) (2). The diamond core of 2 resembles the oxidized methane monooxygenase (MMOox) resting state, as well as the active site product formed following H-atom abstraction from Tyr-OH by ribonucleotide reductase (RNR). The Fe-OH bond lengths of 2 are comparable with those of the MMOHox suggesting that MMOHox contains a Fe(III)(2)(mu-OH)(2) as opposed to Fe(III)(2)(mu-OH)(mu-OH2) diamond core as had been suggested. Isotopic labeling experiments with O-18(2) and CD3CN indicate that the oxygen and proton of the mu-OH bridges of 2 are derived from dioxygen and acetonitrile. Deuterium incorporation (from CD3CN) suggests that an unobserved intermediate capable of abstracting a H-atom from CH3CN forms en route to 2. Given the high C H bond dissociation energy (BDE = 97 kcal/mol) of acetonitrile, this indicates that this intermediate is a potent oxidant, possibly a high-valent iron oxo. Consistent with this, iodosylbenzene (PhIO) also reacts with 1 in CD3CN to afford the deuterated Fe(III)(2)(mu-OD)(2) derivative of 2. Intermediates are not spectroscopically observed in either reaction (O-2 and PhIO) even at low-temperatures (-80 degrees C), indicating that this intermediate has a very short lifetime, likely due to its highly reactive nature. Hydroxo-bridged 2 was found to stoichiometrically abstract hydrogen atoms from 9,10-dihydroanthracene (C-H BDE = 76 kcal/mol) at ambient temperatures.