Oxoiron(IV) units are often implicated as intermediates in the catalytic cycles of non-heme iron oxygenases and oxidases. The most reactive synthetic analogues of these intermediates are supported by tetradentate tripodal ligands with N-methylbenzimidazole or quinoline donors, but their instability precludes structural characterization. Herein we report crystal structures of two [Fe (O)(L)] complexes supported by pentadentate ligands incorporating these heterocycles, which show longer average Fe-N distances than the complex with only pyridine donors. These longer distances correlate linearly with log k ' values for O- and H-atom transfer rates, suggesting that weakening the ligand field increases the electrophilicity of the Fe=O center. The sterically bulkier quinoline donors are also found to tilt the Fe=O unit away from a linear N-Fe=O arrangement by 10°.

This work directly compares the spectroscopic and reactivity properties of an oxoiron(IV) and an oxoiron(V) complex that are supported by the same neutral tetradentate N-based PyNMe 3 ligand. A complete spectroscopic characterization of the oxoiron(IV) species (2) reveals that this compound exists as a mixture of two isomers. The reactivity of the thermodynamically more stable oxoiron(IV) isomer (2b) is directly compared to that exhibited by the previously reported 1e − -oxidized analogue [Fe V (O)(OAc)(PyNMe 3 )] 2+ (3). Our data indicates that 2b is 4 to 5 orders of magnitude slower than 3 in hydrogen atom transfer (HAT) from C−H bonds. The origin of this huge difference lies in the strength of the O−H bond formed after HAT by the oxoiron unit, the O−H bond derived from 3 being about 20 kcal•mol −1 stronger than that from 2b. The estimated bond strength of the Fe IV O−H bond of 100 kcal•mol −1 is very close to the reported values for highly active synthetic models of compound I of cytochrome P450. In addition, this comparative study provides direct experimental evidence that the lifetime of the carbon-centered radical that forms after the initial HAT by the high valent oxoiron complex depends on the oxidation state of the nascent Fe−OH complex. Complex 2b generates long-lived carbon-centered radicals that freely diffuse in solution, while 3 generates short-lived caged radicals that rapidly form product C−OH bonds, so only 3 engages in stereoretentive hydroxylation reactions. Thus, the oxidation state of the iron center modulates not only the rate of HAT but also the rate of ligand rebound.

A new BODIPY derivative with 2,2'-(ethane-1,2-diylbis(oxy))bis(N,N-bis(pyridine-2-ylmethyl)aniline unit as the metal receptor has been designed and synthesized. The dye selectively detects either Cd(2+) or Hg(2+) ions in the presence of hosts of other biologically important and environmentally relevant metal ions in aqueous medium at physiological pH. Binding of metal ions causes a change in the emission behavior of the dye from weakly fluorescent to highly fluorescent. Confocal microscopic experiments validate that the dye can be used to identify changes in either Hg(2+) or Cd(2+) levels in living cells.

Oxoiron(IV) units are often implicated as intermediates in the catalytic cycles of non‐heme iron oxygenases and oxidases. The most reactive synthetic analogues of these intermediates are supported by tetradentate tripodal ligands with N‐methylbenzimidazole or quinoline donors, but their instability precludes structural characterization. Herein we report crystal structures of two [FeIV(O)(L)]2+ complexes supported by pentadentate ligands incorporating these heterocycles, which show longer average Fe–N distances than the complex with only pyridine donors. These longer distances correlate linearly with log k2′ values for O‐ and H‐atom transfer rates, suggesting that weakening the ligand field increases the electrophilicity of the Fe=O center. The sterically bulkier quinoline donors are also found to tilt the Fe=O unit away from a linear N‐Fe=O arrangement by 10°.

Soluble methane monooxygenase (sMMO) carries out methane oxidation at 4 °C and under ambient pressure in a catalytic cycle involving the formation of a peroxodiiron­(III) intermediate (P) from the oxygenation of the diiron­(II) enzyme and its subsequent conversion to Q, the diiron­(IV) oxidant that hydroxylates methane. Synthetic diiron­(IV) complexes that can serve as models for Q are rare and have not been generated by a reaction sequence analogous to that of sMMO. In this work, we show that [FeII(Me3NTB)­(CH3CN)]­(CF3SO3)2 (Me3NTB = tris­((1-methyl-1H-benzo­[d]­imidazol-2-yl)­methyl)­amine) (1) reacts with O2 in the presence of base, generating a (μ-1,2-peroxo)­diiron­(III) adduct with a low O–O stretching frequency of 825 cm–1 and a short Fe···Fe distance of 3.07 Å. Even more interesting is the observation that the peroxodiiron­(III) complex undergoes O–O bond cleavage upon treatment with the Lewis acid Sc3+ and transforms into a bis­(μ-oxo)­diiron­(IV) complex, thus providing a synthetic precedent for the analogous conversion of P to Q in the catalytic cycle of sMMO.