Mononuclear nonheme iron enzymes carry out a range of oxidative transformations via a common S = 2 oxoiron(IV) oxidant. Within the past two decades, many small-molecule analogs of these highly reactive species have been synthesized and under favorable circumstances, some have even been crystallized. These models shed light on the structural and functional properties of the FeIV=O unit in biological systems. Most of these model complexes have S = 1 ground states, which theoretical calculations predict to be less reactive than their S = 2 counterparts. This reactivity pattern is generally borne out by experiments. However, a notable exception to this trend is the [FeIV(O)(Me3NTB)]2+ complex (Me3NTB = tris(benzimidazolyl-2-methyl)amine), which is reported to be an S = 1 complex based on Mössbauer analysis at 4 K, but oxidizes hydrocarbon substrates at rates comparable to those found for the S = 2 complex [FeIV(O)(TQA)]2+ (TQA = tris(quinolyl-2-methyl)amine) reported by Que and co-workers. These two complexes represent two of the most reactive FeIV=O complexes synthesized thus far. The [FeIV(O)(Me3NTB)]2+ complex has been shown by Mössbauer spectroscopy to have an S = 1 ground spin state at 4 K but is proposed to become an S = 2 trigonal bipyramidal species at higher temperatures based on a theoretical model to rationalize its very high C–H bond cleavage reactivity. In this work, 1H NMR spectroscopy is used to determine that this highly reactive complex does not have C3-symmetry in solution and is not an S = 2 species. Our results show that it retains its S = 1 electronic configuration at 193 K and possess a pseudo-octahedral geometry in solution. The oxidation rates of [FeIV(O)(Me3NTB)]2+ are comparable to [FeIV(O)(TQA)]2+ despite the differences in the ground spin state of the iron center. More importantly, these rates are also comparable with the oxidation rates of [(TDCPP)+.FeIV(O)]+ (TDCPP = meso-tetrakis(2,6-dichlorophenyl)porphinato dianion)), a heme-based cytochrome P450 compound I model that has an S = 1 iron(IV) center coupled with an unpaired electron on the porphyrin ligand framework. The significance of our findings here is that an S = 2 iron(IV) center does not need to be invoked to explain high rates of C−H bond cleavage by nonheme oxoiron(IV) complexes.

Spectroscopic insights into the unusually high reactivity of the S = 1 [FeIV(O)(Me3NTB)]2+ complex that shed light on the oxidation mechanism of ferryl complexes

2019 Fall National Meeting

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