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°.

CAN or CeIV(NH4)2(NO3)6 is often used in artificial water oxidation and generally considered to be an outer-sphere oxidant. Herein we report the spectroscopic and crystallographic characterization of [(N4Py)FeIII–O–CeIV(OH2)(NO3)4]+ (3), a complex obtained from the reaction of [(N4Py)FeII(NCMe)]2+ with 2 equiv. CAN or [(N4Py)FeIV=O]2+ (2) with CeIII(NO3)3 in MeCN. Surprisingly, the formation of 3 is reversible, the position of the equilibrium being dependent on the MeCN/water ratio of the solvent. These results suggest that the FeIV and CeIV centers have comparable reduction potentials. Moreover, the equilibrium entails a change in iron spin state, from S = 1 FeIV in 2 to S = 5/2 in 3, which is found to be facile despite the formal spin forbidden nature of this process. This observation suggests that FeIV=O complexes may avail of reaction pathways involving multiple spin states having little or no barrier.

Protons play essential roles in natural systems in controlling O−O bond cleavage of peroxoiron(III) species to give rise to the high-valent iron oxidants that carry out the desired transformations. Herein, we report kinetic and mechanistic evidence that acids can control the mode of O− O bond cleavage for a nonheme S = 1/2 Fe III −OOH species [(BnTPEN)Fe III (OOH)] 2+ (2, BnTPEN = N-benzyl-N,N′,N′tris(2-pyridylmethyl)-1,2-diaminoethane). Addition of acids having pK a values of >8.5 in CH 3 CN results in O−O bond homolysis, leading to the formation of hydroxyl radicals that give rise to alcohol/ketone (A/K) ratios of around 1 in the oxidation of cyclohexane. However, the introduction of acids with pK a values of <8.5 elicits a different outcome, namely the achievement of A/K ratios of as high as 9, the observation of rapid and catalytic hydroxylation of cyclohexane, and a million-fold acceleration in the decay rate of the Fe III −OOH intermediate at −40 °C. These results implicate the generation of a highly reactive Fe V O species via proton-assisted O−O bond heterolysis of the Fe III −OOH intermediate, which is unprecedented for nonheme iron complexes supported by neutral pentadentate ligands and serves as a nonheme analogue for heme enzyme compounds I.

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°.

Ceric ammonium nitrate (CAN) or Ce IV (NH 4 ) 2 -(NO 3 ) 6 is often used in artificial water oxidation and generally considered to be an outer-sphere oxidant. Herein we report the spectroscopic and crystallographic characterization of 2+ (2)w ith Ce III (NO 3 ) 3 in MeCN.S urprisingly,t he formation of 3 is reversible,t he position of the equilibrium being dependent on the MeCN/water ratio of the solvent. These results suggest that the Fe IV and Ce IV centers have comparable reduction potentials.M oreover,t he equilibrium entails achange in iron spin state,from S = 1Fe IV in 2 to S = 5/2 in 3,w hich is found to be facile despite the formal spinforbidden nature of this process.This observation suggests that Fe IV =Oc omplexes may avail of reaction pathwaysi nvolving multiple spin states having little or no barrier.