High-valent iron commonly denotes compounds and intermediates in which
iron is found in a formal
oxidation state > 3 that show a
number of bonds > 6 with a
coordination number ≤ 6. The term is rather uncommon for hepta-coordinate compounds of iron. It has to be distinguished from the terms
hypervalent and hypercoordinate, as high-valent iron compounds neither necessarily violate the
18-electron rule nor necessarily show coordination numbers > 6. The
ferrate(VI)
Ferrate(VI) is the inorganic anion with the chemical formula eO4sup>2−. It is photosensitive, contributes a pale violet colour to compounds and solutions containing it and is one of the strongest water-stable oxidizing species known. Although ...
ion
4">eO4sup>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 commonly proposed as intermediates in
catalytic cycle
In chemistry, a catalytic cycle is a multistep reaction mechanism that involves a catalyst. The catalytic cycle is the main method for describing the role of catalysts in biochemistry, organometallic chemistry, bioinorganic chemistry, materials ...
s, especially biological systems in which O
2 activation is required. Diatomic oxygen has a high reduction potential (E
0 = 1.23 V), but the first step required to harness this potential is a thermodynamically unfavorable one electron reduction E
0 = -0.16 V. This reduction occurs in nature by the formation of a
superoxide
In chemistry, a superoxide is a compound that contains the superoxide ion, which has the chemical formula . The systematic name of the anion is dioxide(1−). The reactive oxygen ion superoxide is particularly important as the product of th ...
complex in which a reduced metal is oxidized by O
2. 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
Iodosobenzene or iodosylbenzene is an organoiodine compound with the empirical formula . This colourless solid compound is used as an oxo transfer reagent in research laboratories examining organic and coordination chemistry.
Preparation and str ...
:
[Que et al.; Journal of Inorganic Biochemistry Volume 100, Issue 4, April 2006, Pages 421-433;]
:
:
symbolic oxidation of an iron compound using iodosobenzene
Iodosobenzene or iodosylbenzene is an organoiodine compound with the empirical formula . This colourless solid compound is used as an oxo transfer reagent in research laboratories examining organic and coordination chemistry.
Preparation and str ...
; L denotes the supporting ligand
Fe(IV)O
Several syntheses of oxoiron(IV) species have been reported. These compounds model biological complexes such as
cytochrome P450
Cytochromes P450 (CYPs) are a superfamily of enzymes containing heme as a cofactor that functions as monooxygenases. In mammals, these proteins oxidize steroids, fatty acids, and xenobiotics, and are important for the clearance of various comp ...
,
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,
II(TMCS)">eII(TMCS)PF
6) (TMCS = 1-mercaptoethyl-4,8,11-trimethyl-1,4,8,11-tetraza cyclotetradecane), and 3-5 equivalents of H
2O
2 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
Electrospray ionization (ESI) is a technique used in mass spectrometry to produce ions using an electrospray in which a high voltage is applied to a liquid to create an aerosol. It is especially useful in producing ions from macromolecules becaus ...
mass spectrometry (ESI-MS),
X-ray absorption spectroscopy
X-ray absorption spectroscopy (XAS) is a widely used technique for determining the local geometric and/or electronic structure of matter. The experiment is usually performed at synchrotron radiation facilities, which provide intense and tunable ...
,
extended X-ray absorption fine structure
Extended X-ray absorption fine structure (EXAFS), along with X-ray absorption near edge structure (XANES), is a subset of X-ray absorption spectroscopy ( XAS). Like other absorption spectroscopies, XAS techniques follow Beer's law. The X-ray a ...
(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 = CF
3SO
3, with iodosylbenzene (PhIO) in CH
3CN 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 H
2O
2 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
Bispidine (3,7-diazabicyclo .3.1onane) is an organic compound that is classified as a bicyclic diamine. Although synthetic, it is related structurally to natural alkaloid sparteine. It is a white crystalline solid. It has been widely investigated ...
complexes can oxidize
cyclohexane to
cyclohexanol
Cyclohexanol is the organic compound with the formula HOCH(CH2)5. The molecule is related to cyclohexane by replacement of one hydrogen atom by a hydroxyl group. This compound exists as a deliquescent colorless solid with a camphor-like odor, ...
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
III(TAML)(H2O)">eIII(TAML)(H2O)PPh
4) 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
In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistr ...
anion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
of
chemical formula 4">eO4sup>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
A weak base is a base that, upon dissolution in water, does not dissociate completely, so that the resulting aqueous solution contains only a small proportion of hydroxide ions and the concerned basic radical, and a large proportion of undissociat ...
, concentrated solutions of ferrate(VI) are only stable at high
pH.
Electronic structure
The electronic structure of porphyrin oxoiron compounds has been reviewed.
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
In chemistry, azide is a linear, polyatomic anion with the formula and structure . It is the conjugate base of hydrazoic acid . Organic azides are organic compounds with the formula , containing the azide functional group. The dominant applic ...
complex.
:
:
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 Fe
VI species apart from the
ferrate(VI)
Ferrate(VI) is the inorganic anion with the chemical formula eO4sup>2−. It is photosensitive, contributes a pale violet colour to compounds and solutions containing it and is one of the strongest water-stable oxidizing species known. Although ...
ion,
3cy-ac)FeN">Me3cy-ac)FeNPF
6)
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 Mossbauer, EXAFS, IR, and DFT calculations. Unlike the ferrate(VI) ion, compound 5 is
diamagnetic.
μ-Nitrido compounds and oxidation catalysis
Bridged μ-nitrido di-iron
phthalocyanine
Phthalocyanine () is a large, aromatic, macrocyclic, organic compound with the formula and is of theoretical or specialized interest in chemical dyes and photoelectricity.
It is composed of four isoindole units linked by a ring of nitrogen atom ...
compounds such as
iron(II) phthalocyanine
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in fron ...
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.
[Dey, A.; Ghosh, A.; J. Am. Chem. Soc., 2002, 124 (13), pp 3206–3207; ]
See also
*
Jacobsen's catalyst
Jacobsen's catalyst is the common name for N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III) chloride, a coordination compound of manganese and a salen-type ligand. It is used as an asymmetric catalyst in the Jaco ...
(high-valent manganese)
*
Iron(IV) oxytetrafluoroborate
Iron(IV) oxytetrafluoroborate is a chemical compound with the rare Fe(IV) ion. It was first reported in 1998 but was not confirmed. Then in 2006, a more reliable production was reported. It was created by reacting iron(II) tetrafluoroborate with ...
References
Further reading
*Solomon et al.; Angewandte Chemie International Edition Volume 47, Issue 47, pages 9071–9074, November 10, 2008; {{doi, 10.1002/anie.200803740
Iron
Ferrates
Iron_complexes
Oxidizing agents
Inorganic chemistry
Cluster chemistry
Coordination complexes
Iron, high-valent