High-valent iron
Encyclopedia
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 numers > 6. The ferrate
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.
complex in which a reduced metal is oxidized by O2. The product of this reaction is a peroxide radical that is more readily reactive. The abundance of these species in nature and the chemistry that is available to them are the reasons why the study of these compounds is important. A widely applicable method for the generation of high-valent oxoferryl species is the oxidation with iodosobenzene
.
Thiolate-ligated oxoiron(IV) is formed by the oxidation of a precursor, [FeII(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 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, x-ray absorption fine structure (EXAFS), UV-visible spectroscopy (UV-vis), Fourier transform infrared spectroscopy (FT-IR), and results were compared to 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 are able to oxidize cyclohexane
to cyclohexanol
and cyclohexanone
in 35% yield with an alcohol to ketone ratio up to 4.
= tetraamido macrocyclic ligand, is formed by the reaction of [FeIII(TAML)(H2O)](PPh4) with 2-5 equivalents of meta-chloroperbenzoic acid at -60 ˚C in n-butyronitrile. This compound has a deep green color with absorption maxima at 445 and 630 nm. It is reported to be stable for at least 1 month at 77 K. FeVTAML(=O) was characterized with electron paramagnetic resonance (EPR), Mössbauer, EXAFS, ESI-MS, reactivity studies, and DFT calculations. This compound is capable of 1 electron oxidation at the metal as opposed to at the ligand. This is due to the stronger –donor capacity of deprotonated amide nitrogens on TAML.
. 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 elemental nitrogen from an azide complex.
at low temperatures ,.
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 Mossbauer, EXAFS, IR, and DFT calculations. Unlike the ferrate ion compound 5 is diamagnetic.
to methanol
, formaldehyde
and formic acid
using hydrogen peroxide
as sacrificial oxidant.
See also:
Solomon et al.; Angewandte Chemie International Edition Volume 47, Issue 47, pages 9071–9074, November 10, 2008;
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
commonly denotes compounds and intermediates in which iron is found in a formal oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
> 3 that show a number of bonds
Valence (chemistry)
In chemistry, valence, also known as valency or valence number, is a measure of the number of bonds formed by an atom of a given element. "Valence" can be defined as the number of valence bonds...
> 6 with a coordination number
Coordination number
In chemistry and crystallography, the coordination number of a central atom in a molecule or crystal is the number of its nearest neighbours. This number is determined somewhat differently for molecules and for crystals....
≤ 6. The term is rather uncommon for hepta-coordinate compounds of iron. It has to be distinguished from the terms hypervalent and hypercoordinate
Hypervalent molecule
A hypervalent molecule is a molecule that contains one or more main group elements formally bearing more than eight electrons in their valence shells...
, as high-valent iron compounds neither necessarily violate the 18-electron rule
18-Electron rule
The 18-electron rule is a rule of thumb used primarily for predicting formulas for stable metal complexes. The rule rests on the fact that valence shells of a transition metal consists of nine valence orbitals, which collectively can accommodate 18 electrons either as nonbinding electron pairs or...
nor necessarily show coordination numers > 6. The ferrate
Ferrate
In chemistry, ferrate refers either to the anion , in which iron is in the +6 oxidation state, or to a salt containing this anion. The term ferrate is often used to mean ferrate, although according to IUPAC naming conventions, it may also refer to other iron-containing oxyanions, such as ferrate...
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 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 superoxideSuperoxide
A superoxide, also known by the obsolete name hyperoxide, is a compound that possesses the superoxide anion with the chemical formula O2−. The systematic name of the anion is dioxide. It is important as the product of the one-electron reduction of dioxygen O2, which occurs widely in nature...
complex in which a reduced metal is oxidized by O2. The product of this reaction is a peroxide radical that is more readily reactive. The abundance of these species in nature and the chemistry that is available to them are the reasons why the study of these compounds is important. A widely applicable method for the generation of high-valent oxoferryl species is the oxidation with iodosobenzene
Iodosobenzene
Iodosobenzene or iodosylbenzene is an organoiodine compound with the empirical formula C6H5IO. This colourless solid compound is used as an oxo transfer reagent in research laboratories examining organic and coordination chemistry...
.
- symbolic oxidation of an iron compound using iodosobenzeneIodosobenzeneIodosobenzene or iodosylbenzene is an organoiodine compound with the empirical formula C6H5IO. This colourless solid compound is used as an oxo transfer reagent in research laboratories examining organic and coordination chemistry...
, 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, 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) (TMCS = 1-mercaptoethyl-4,8,11-trimethyl-1,4,8,11-tetraza cyclotetradecane), and 3-5 equivalents of H2O2 at -60 ˚C in methanol
Methanol
Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
. 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, x-ray absorption fine structure (EXAFS), UV-visible spectroscopy (UV-vis), Fourier transform infrared spectroscopy (FT-IR), and results were compared to DFT-calculations
Density functional theory
Density functional theory is a quantum mechanical modelling method used in physics and chemistry to investigate the electronic structure of many-body systems, in particular atoms, molecules, and the condensed phases. With this theory, the properties of a many-electron system can be determined by...
.
Tetramethylcyclam
Cyclam
Cyclam is macrocyclic ligand which is capable of binding to metal cations. It is a close relative of cyclen....
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
Raman spectroscopy
Raman spectroscopy is a spectroscopic technique used to study vibrational, rotational, and other low-frequency modes in a system.It relies on inelastic scattering, or Raman scattering, of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range...
, and FT-IR.
High-valent iron bispidine complexes are able to oxidize cyclohexane
Cyclohexane
Cyclohexane is a cycloalkane with the molecular formula C6H12. Cyclohexane is used as a nonpolar solvent for the chemical industry, and also as a raw material for the industrial production of adipic acid and caprolactam, both of which being intermediates used in the production of nylon...
to cyclohexanol
Cyclohexanol
Cyclohexanol is the organic compound with the formula 5CHOH. The molecule is related to cyclohexane ring by replacement of one hydrogen atom by a hydroxyl group. This compound exists as a deliquescent colorless solid, which, when very pure, melts near room temperature...
and cyclohexanone
Cyclohexanone
Cyclohexanone is the organic compound with the formula 5CO. The molecule consists of six-carbon cyclic molecule with a ketone functional group. This colorless oil has an odor reminiscent of peardrop sweets as well as acetone. Over time, samples assume a yellow color due to oxidation...
in 35% yield with an alcohol to ketone ratio up to 4.
Fe(V)O
FeVTAML(=O), TAMLTAML
The synthetic molecule tetra-amido macrocyclic ligand is a catalyst which speeds oxidation reactions. It was designed by Terrence J. Collins and associates at Carnegie Mellon University, and is touted as one example of an environmentally-friendly catalyst...
= tetraamido macrocyclic ligand, is formed by the reaction of [FeIII(TAML)(H2O)](PPh4) with 2-5 equivalents of meta-chloroperbenzoic acid at -60 ˚C in n-butyronitrile. This compound has a deep green color with absorption maxima at 445 and 630 nm. It is reported to be stable for at least 1 month at 77 K. FeVTAML(=O) was characterized with electron paramagnetic resonance (EPR), Mössbauer, EXAFS, ESI-MS, reactivity studies, and DFT calculations. This compound is capable of 1 electron oxidation at the metal as opposed to at the ligand. This is due to the stronger –donor capacity of deprotonated amide nitrogens on TAML.
Electronic structure
The electronic structure of porphyrin oxoiron compounds has been reviewed.Nitridoiron & imidoiron compounds
Nitrido- and imidoiron-compounds are closely related to iron-dinitrogen chemistryHaber process
The Haber process, also called the Haber–Bosch process, is the nitrogen fixation reaction of nitrogen gas and hydrogen gas, over an enriched iron or ruthenium catalyst, which is used to industrially produce ammonia....
. 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 elemental nitrogen from an azide 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 generated and characterized in 1988/1989 by Wagner and Nakamoto using photolysis and Raman spectroscopyRaman spectroscopy
Raman spectroscopy is a spectroscopic technique used to study vibrational, rotational, and other low-frequency modes in a system.It relies on inelastic scattering, or Raman scattering, of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range...
at low temperatures ,.
Fe(VI)N
A second FeVI species apart from the ferrateFerrate
In chemistry, ferrate refers either to the anion , in which iron is in the +6 oxidation state, or to a salt containing this anion. The term ferrate is often used to mean ferrate, although according to IUPAC naming conventions, it may also refer to other iron-containing oxyanions, such as ferrate...
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 Mossbauer, EXAFS, IR, and DFT calculations. Unlike the ferrate ion compound 5 is diamagnetic.
µ-nitrido compounds & oxidation catalysis
Bridged µ-nitrido di-iron phtalocyanine compounds catalyze the oxidation of methaneMethane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
to methanol
Methanol
Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
, formaldehyde
Formaldehyde
Formaldehyde is an organic compound with the formula CH2O. It is the simplest aldehyde, hence its systematic name methanal.Formaldehyde is a colorless gas with a characteristic pungent odor. It is an important precursor to many other chemical compounds, especially for polymers...
and formic acid
Formic acid
Formic acid is the simplest carboxylic acid. Its chemical formula is HCOOH or HCO2H. It is an important intermediate in chemical synthesis and occurs naturally, most notably in the venom of bee and ant stings. In fact, its name comes from the Latin word for ant, formica, referring to its early...
using hydrogen peroxide
Hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
as sacrificial oxidant.
Further reading
- high-valent manganese: Jacobsen's catalystJacobsen's catalystJacobsen's catalyst is the common name for N,N'-bis-1,2-cyclohexanediaminomanganese chloride, a coordination compound of manganese and a salen-type ligand. It is used as a asymmetric catalyst in the Jacobsen epoxidation, which is renowned for its ability to enantioselectively transform prochiral...
See also:
Solomon et al.; Angewandte Chemie International Edition Volume 47, Issue 47, pages 9071–9074, November 10, 2008;