High-valent iron

Oxoiron compounds

Oxoferryl species are commonly proposed as intermediates in catalytic cycles, especially biological systems in which O₂ activation is required. Diatomic oxygen has a high reduction potential (E⁰ = 1.23 V), but the first step required to harness this potential is a thermodynamically unfavorable one electron reduction E⁰ = -0.16 V. This reduction occurs in nature by the formation of a superoxide complex in which a reduced metal is oxidized by O₂. The product of this reaction is a peroxide radical that is more readily reactive. A widely applicable method for the generation of high-valent oxoferryl species is the oxidation with iodosobenzene:

${\displaystyle (\mathrm{L})\mathrm{F}{\mathrm{e}}^{\mathrm{n}}+\mathrm{O}\mathrm{I}\mathrm{P}\mathrm{h}⟶(\mathrm{L})\mathrm{F}{\mathrm{e}}^{\mathrm{n}+2}\mathrm{O}+\mathrm{I}\mathrm{P}\mathrm{h}}$

symbolic oxidation of an iron compound using iodosobenzene; L denotes the supporting 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=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, [Fe^II(TMCS)](PF₆) (TMCS = 1-mercaptoethyl-4,8,11-trimethyl-1,4,8,11-tetraza cyclotetradecane), and 3-5 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 Fe^IV(=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 Fe^II(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 Fe^II(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

Fe^VTAML(=O), TAML = tetra-amido macrocyclic ligand, is formed by the reaction of [Fe^III(TAML)(H₂O)](PPh₄) with 2-5 equivalents of meta-chloroperbenzoic acid at -60 ˚C in n-butyronitrile. This deep green compound (two λ_max 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 an inorganic anion of chemical formula [FeO₄]²⁻. It is photosensitive and contributes a pale violet colour to its compounds and solutions. It is one of the strongest water-stable oxidising species known. Although it is classified as a weak base, concentrated solutions of ferrate(VI) are only stable at high pH.

Electronic structure

The electronic structure of porphyrin oxoiron compounds has been reviewed.