chemistry

High-valent iron

High-valent iron is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand High-valent iron rather than just read about it. In short: High-valent iron commonly denotes compounds and intermediates in which iron is found in a formal oxidation state greater than +3 that show a number of bonds above 6 with a coordination number no more than 6. The ferrate(VI) ion [FeO4]2− was the first structure in this class synthesized.

Key takeaways

High-valent iron belongs to chemistry; place it in that map before memorising details.
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Connect High-valent iron to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of High-valent iron from memory before moving on to harder problems.

Reference excerpt

High-valent iron commonly denotes compounds and intermediates in which iron is found in a formal oxidation state greater than +3 that show a number of bonds above 6 with a coordination number no more than 6. The ferrate(VI) ion [FeO4]2− was the first structure in this class synthesized. The synthetic compounds discussed below contain highly oxidized iron in general, as the concepts are closely related.

Oxoiron compounds Oxoferryl species are common examples of high-valent iron complexes. Such compounds are prepared by oxidation of ferrous complexes with iodosobenzene:

(mac)FeL2 + OIPh → (mac)Fe=O(L) + IPh + L (mac = tetradentate macrocyclic ligand)

Fe(IV)O

Several syntheses of oxoiron(IV) species have been reported. The simplest are mixed-metal oxides of the form MFeO3, with M being Ba, Ca, or Sr. However, those compounds do not have discrete iron anions. Isolated oxoiron(IV) species are known with more complicated ligands. These compounds model biological complexes such as cytochrome P450, NO synthase, and isopenicillin N synthase. Two such reported compounds are thiolate-ligated oxoiron(IV) and cyclam-acetate oxoiron(IV). Thiolate-ligated oxoiron(IV) is formed by the oxidation of a precursor, [FeII(TMCS)](PF6) (where TMCS is 1-mercaptoethyl-4,8,11-trimethyl-1,4,8,11-tetraazacyclotetradecane), and 3 to 5 equivalents of H2O2 at −60 °C in methanol. The iron(IV) compound is deep blue in color and shows intense absorption features at 460 nm, 570 nm, 850 nm, and 1050 nm. This species FeIV(=O)(TMCS)+ is stable at −60 °C, but decomposition is reported as temperature increases. Compound 2 was identified by Mössbauer spectroscopy, high resolution electrospray ionization mass spectrometry (ESI-MS), X-ray absorption spectroscopy, extended X-ray absorption fine structure (EXAFS), ultraviolet–visible spectroscopy (UV-vis), Fourier-transform infrared spectroscopy (FT-IR), and results were compared to density functional theory (DFT) calculations.

Tetramethylcyclam oxoiron(IV) is formed by the reaction of FeII(TMC)(OTf)2, TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane; OTf = CF3SO3, with iodosylbenzene (PhIO) in CH3CN at −40 °C. A second method for formation of cyclam oxoiron(IV) is reported as the reaction of FeII(TMC)(OTf)2 with 3 equivalents of H2O2 for 3 hours. This species is pale green in color and has an absorption maximum at 820 nm. It is reported to be stable for at least 1 month at −40 °C. It has been characterized by Mössbauer spectroscopy, ESI-MS, EXAFS, UV-vis, Raman spectroscopy, and FT-IR. High-valent iron bispidine complexes can oxidize cyclohexane to cyclohexanol and cyclohexanone in 35% yield with an alcohol-to-ketone ratio up to 4.

Fe(V)O FeVTAML(=O), where TAML is a tetra-amido macrocyclic ligand, is formed by the reaction of [FeIII(TAML)(H2O)](PPh4) with 2 to 5 equivalents of meta-chloroperbenzoic acid at −60 °C in n-butyronitrile. This deep green compound (two λmax at 445 and 630 nm respectively) is stable at 77 K. The stabilization of Fe(V) is attributed to the strong π-donor capacity of deprotonated amide nitrogens.

Fe(VI)O

Ferrate(VI) is found in the inorganic anion [FeO4]2−. It has been isolated as the potassium salt, potassium ferrate. It is a strong water-stable oxidizing agent. Its solutions are stable at high pH.

Nitridoiron and imidoiron compounds

Nitridoiron and imidoiron compounds are closely related to iron-dinitrogen chemistry. The biological significance of nitridoiron(V) porphyrins has been reviewed. A widely applicable method to generate high-valent nitridoiron species is the thermal or photochemical oxidative elimination of molecular nitrogen from an azide complex.

(L)FenN3 → (L)Fen+2N + N2 Symbolic oxidative elimination of nitrogen yields a nitridoiron complex; L denotes the supporting ligand.

Fe(IV)N Several structurally characterized nitridoiron(IV) compounds exist.

Fe(V)N The first nitridoiron(V) compound was synthesised and characterized by Wagner and Nakamoto (1988, 1989) using photolysis and Raman spectroscopy at low temperatures.

Fe(VI)N A second FeVI species apart from the ferrate(VI) ion, [(Me3cy-ac)FeN](PF6)2, has been reported. This species, is formed by oxidation followed by photolysis to yield the Fe(VI) species. Characterization of the Fe(VI) complex was done by Mössbauer, EXAFS, IR, and DFT calculations. Unlike the ferrate(VI) ion, compound 5 is diamagnetic.

μ-Nitrido compounds and oxidation catalysis Bridged μ-nitrido diiron phthalocyanine compounds such as iron(II) phthalocyanine catalyze the oxidation of methane to methanol, formaldehyde, and formic acid using hydrogen peroxide as sacrificial oxidant.

Electronic structure Nitridoiron(IV) and nitridoiron(V) species were first explored theoretically in 2002.

See also Jacobsen's catalyst (high-valent manganese)

References

Illustrations

Worked examples

Example 1 — a first encounter with High-valent iron

Start with the simplest possible case. Write down what High-valent iron claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to High-valent iron before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about High-valent iron ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of High-valent iron

In research: High-valent iron appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses High-valent iron in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: High-valent iron is common in secondary-school and first-year university syllabi. It links to neighbouring topics Chemical compounds by element, Cluster chemistry, Coordination complexes, so understanding it makes those chapters shorter.
In everyday life: Look for High-valent iron outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study High-valent iron in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what High-valent iron means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain High-valent iron out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is High-valent iron in simple terms?

Why does High-valent iron matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study High-valent iron?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on High-valent iron.