The prevalence of iron in biological processes is due to its abundance and ability to participate in redox reactions. For example, iron–sulfur clusters and heme-containing proteins, which are ubiquitous components of electron transfer chains, function by cycling iron between its most common oxidations states: Fe(II) and Fe(III). Iron is also found in enzyme active sites as part of the heme cofactor, or directly attached to the protein framework in nonheme iron enzymes, and the mechanisms of such enzymes often require iron to access high-valent oxidation states of +4 or greater1. High-valent iron-oxo species are particularly potent oxidants capable of hydroxylating strong C-H bonds in enzymes like cytochrome P450 (ref. 2) and soluble methane monooxygenase3. Many synthetic catalysts also utilize high-valent iron intermediates to perform demanding functionalizations of organic compounds4.

Considerable effort has been devoted to generating and characterizing high-valent iron species owing to their importance in both biological and synthetic catalysis; however, they are often challenging to isolate because of their intrinsically reactive nature. Nevertheless, the past twenty-five years have seen an explosion in the number of structurally characterized nonheme iron(IV) and iron(V) complexes. These complexes typically feature axial π-donors, such as oxo (O2−) (ref. 5), imido (NR2−) (ref. 6), and nitrido (N3−) (refs. 6,7,8,9) ligands, that stabilize high oxidation states via iron-ligand multiple bonds.

Even though Fe(V) is the highest oxidation state routinely invoked for iron in a biological context, higher oxidation states for this metal are not unprecedented. For example, the Fe(VI) state is found in the ferrate ion, [FeO4]2−, which is stable in basic aqueous solution10. Synthetic chemists have also challenged this limit by synthesizing iron complexes featuring higher oxidation states than +V. For instance, an octahedral iron(VI)–nitrido species was spectroscopically detected by Wieghardt and co-workers in 2006 (ref. 9), and in 2020 Smith and co-workers reported the X-ray structure of a four-coordinate Fe(VI) bis(imido) complex featuring a bis(carbene)borate ligand11.

Now, writing in Nature Chemistry, Meyer, Munz, DeBeer and co-workers present the newest member of the Fe(VI) family — an octahedral nitrido complex supported by a tris(carbene)amine ligand (1 in Fig. 1a) (ref. 12). Even more remarkably, the team demonstrates that this complex undergoes one-electron oxidation to yield an Fe(VII) complex (2). Although 2 is metastable and needs to be generated at temperatures below –50°C, the researchers were able to characterize the ‘super-oxidized’ complex with multiple spectroscopic techniques.

Fig. 1: Structure and preparation of high-valent iron(VI, VII)–nitrido complexes.
figure 1

a, One-electron oxidation of [(TIMMNMes)FeVI(N)]2+ (1) yields a highly reactive iron(VII)–nitrido species (2). The thermal ellipsoid plot of 1 is shown at 50% probability level. b, Correlation between 57Fe isomer shift (δ, mm s−1) and iron oxidation state for TIMMNMes-based iron–nitrido complexes. Figure adapted with permission from ref. 12, Springer Nature Ltd.

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The current study employs a tripodal chelate (TIMMNMes) with three N-heterocyclic carbene (NHC) donors to stabilize the high iron oxidation states of 1 and 2. The Meyer group used this ligand in an earlier 2021 study to generate the analogous Fe(V) complex via one-electron oxidation of an iron(IV)–nitrido precursor13. The researchers have therefore prepared a remarkable series of four TIMMNMes-based iron–nitrido complexes with every oxidation state between +4 and +7. Although the carbene rings of TIMMNMes bear a superficial similarity to the imidazole of histidine — a common ligand in the active site of metalloenzymes — the NHC unit in these complexes coordinates to the Fe centre through its carbon atom, creating an organometallic bond. Thus, NHCs are considered strong-field ligands with donor properties that resemble trialkylphosphines14. The sterically bulky mesityl substituents of TIMMNMes further stabilize the iron–nitrido units by minimizing dimerization and other side reactions.

The conversion of [(TIMMNMes)FeV(N)]2+ to 1 is carried out using an AgIIF2 oxidant, which also supplies a fluoride anion to yield a six-coordinate complex. The X-ray crystal structure of 1 revealed a particularly short Fe(VI)≡N bond distance of 1.518 Å. Further oxidation to the Fe(VII) state was achieved by treating 1 with metal hexafluorides (MF6, M = Re or Os) or XeF2 (Fig. 1a). The handling of such powerful oxidants required exacting experimental conditions, such as the use of oxidation-resistant SO2 solvent and PFA tubing. The conversion of 1 to 2 causes a color change from lime green to reddish orange, but the latter complex is stable only below −50 oC, preventing crystallization. Nevertheless, the absence of a crystal structure is compensated by a battery of spectroscopic and theoretical studies that allow the unequivocal structural characterization of the complex.

The formal oxidation state of a transition-metal complex cannot be measured directly; instead, it must be inferred from structural and/or spectroscopic data. High-valent species pose a particular challenge owing to the high degree of metal–ligand covalency and the ‘noninnocent’ nature of some ligands, which raises the possibility that oxidation occurs elsewhere than the metal ion. To confirm the Fe(VI) and Fe(VII) states of 1 and 2, respectively, the team employed spectroscopic techniques, such as 57Fe Mössbauer, K-edge X-ray absorption, and electron paramagnetic resonance (EPR) spectroscopies, that are uniquely sensitive to the population of the Fe 3d orbitals. The plot in Fig. 1b reveals that the isomer shift (δ) parameter — an oxidation state marker derived from Mössbauer spectroscopy — shifts linearly to more negative values with each successive oxidation within the Fe-TIMMNMes series. Similarly, the pre-edge peaks and edge inflection points in the X-ray absorption spectra move to higher energies with increasing oxidation states, indicating that the effective nuclear charge of iron becomes more positive. Such trends are consistent with physical changes in the iron electron density that justify the formal oxidation assignments. Although 1 is EPR silent owing to its diamagnetic nature, the anisotropic S = 1/2 signal of 2 is evidence that the unpaired electron resides in an Fe(3d)-based orbital. The Fe(VI) and Fe(VII) states are further corroborated by quantum chemical calculations that converge to low-spin dxy2 and dxy1 electron configurations for 1 and 2, respectively.

Complex 2 decays through an intramolecular amination of the TIMMNMes ligand to yield an iron(V)-imido product (3) that was isolated and structurally characterized. The researchers elucidated the rearrangement mechanism assisted by DFT calculations, finding that the reaction is initiated by electrophilic attack of the iron(VII)–nitrido unit on the mesityl ring. The uniquely reactive nature of the superoxidized Fe(VII) state is evident in the fact that similar rearrangements are not observed for the Fe(V) or Fe(VI) analogues.

The synthesis and characterization of a bona fide Fe(VII) complex is a major step forward in the decades-long quest to prepare iron complexes with ever-higher oxidation states. The results hint that iron(VII)–nitrido complexes like 2 are potent oxidants, and future efforts will seek to direct this reactivity towards productive ends, for instance enabling organic transformations. It also remains to be seen whether ingenious chemists will soon reach the oxidation limit of iron by preparing an Fe(VIII) complex with an empty set of 3d orbitals.