View source for High-valent iron

[[File:Ferrate ion.svg|thumb|right|ferrate(VI) ion, [FeO<sub>4</sub>]<sup>2−</sup>]]
'''High-valent iron''' commonly denotes compounds and intermediates in which [[iron]] is found in a formal [[oxidation state]] > 3 that show a [[valence (chemistry)|number of bonds]] > 6 with a [[coordination number]] ≤ 6. The term is rather uncommon for hepta-coordinate compounds of iron.<ref>Craig et al. Dalton Trans., 2010, 39, 4874-4881 {{doi|10.1039/B927032A}}</ref> It has to be distinguished from the terms [[hypervalent molecule|hypervalent and hypercoordinate]], as high-valent iron compounds neither necessarily violate the [[18-electron rule]] nor necessarily show coordination numbers > 6. The [[ferrate(VI)]] ion [FeO<sub>4</sub>]<sup>2−</sup> 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<ref>Que et al.; Journal of Inorganic Biochemistry Volume 100, Issue 4, April 2006, Pages 421-433;{{doi|10.1016/j.jinorgbio.2006.01.014}}</ref>==
Oxoferryl species are commonly proposed as intermediates in [[catalytic cycle]]s, especially biological systems in which O<sub>2</sub> activation is required. Diatomic oxygen has a high reduction potential (E<sup>0</sup> = 1.23 V), but the first step required to harness this potential is a thermodynamically unfavorable one electron reduction E<sup>0</sup> = -0.16 V. This reduction occurs in nature by the formation of a [[superoxide]] complex in which a reduced metal is oxidized by O<sub>2</sub>. 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.{{Citation needed|date=December 2010}} A widely applicable method for the generation of high-valent oxoferryl species is the oxidation with [[iodosobenzene]]:
:<math>\mathrm{(L)Fe^{n} + OIPh \longrightarrow (L)Fe^{n+2}O + IPh}</math>
:<small>symbolic oxidation of an iron compound using [[iodosobenzene]]; L denotes the supporting ligand </small>

===Fe(IV)O===
[[File:Fe4TMCSOPF6.png|thumb|120px|right|thiolate-ligated oxoiron(IV)]]
Several syntheses of oxoiron(IV) species have been reported.<ref>
{{cite book
|first1=Gereon M.
|last1=Yee
|first2=William B.
|last2=Tolman
|editor=Peter M.H. Kroneck and Martha E. Sosa Torres
|title=Sustaining Life on Planet Earth: Metalloenzymes Mastering Dioxygen and Other Chewy Gases
|series=Metal Ions in Life Sciences
|volume=15
|year=2015
|publisher=Springer
|chapter=Chapter 5, Section 2.2.4 ''Fe(IV)-Oxo Intermediates''
|pages=145–146
|doi=10.1007/978-3-319-12415-5_5
}}
</ref> These compounds model biological complexes such as [[cytochrome P450]], [[nitric oxide synthase|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<sup>II</sup>(TMCS)](PF<sub>6</sub>) (TMCS = 1-mercaptoethyl-4,8,11-trimethyl-1,4,8,11-tetraza cyclotetradecane), and 3-5 equivalents of H<sub>2</sub>O<sub>2</sub> at -60 ˚C in [[methanol]]. The iron(IV) compound is deep blue in color and shows intense absorption features at 460&nbsp;nm, 570&nbsp;nm, 850&nbsp;nm, and 1050&nbsp;nm. This species Fe<sup>IV</sup>(=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.<ref>Bukowski, M. R., Koehntop, K. D., Stubna, A., Bominaar E. L., Halfen, J. A., Munck, E., Nam, W., Que, L., Science, 310, 1000-1002, 2005; {{doi|10.1126/science.111909}}</ref>
[[File:Fe4TMCOOTf2.png|thumb|120px|right|tetramethylcyclam-supported oxoiron(IV)]]
[[Cyclam|Tetramethylcyclam]] oxoiron(IV) is formed by the reaction of Fe<sup>II</sup>(TMC)(OTf)<sub>2</sub>, TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane; OTf = CF<sub>3</sub>SO<sub>3</sub>, with iodosylbenzene (PhIO) in CH<sub>3</sub>CN at -40 ˚C. A second method for formation of cyclam oxoiron(IV) is reported as the reaction of Fe<sup>II</sup>(TMC)(OTf)<sub>2</sub> with 3 equivalents of H<sub>2</sub>O<sub>2</sub> for 3 hours. This species is pale green in color and has an absorption maximum at 820&nbsp;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.<ref>Rohde, J.-U., In, J.-H., Lim, M. H., Brennessel, W. W., Bikowski, M. R., Stubna, A., Munck, E., Name, W., Que, L., Science, 299, 1037-1039, 2003; {{doi|10.1126/science.299.5609.1037}}</ref>

High-valent iron [[bispidine]] complexes can oxidize [[cyclohexane]] to [[cyclohexanol]] and [[cyclohexanone]] in 35% yield with an alcohol to ketone ratio up to 4.<ref>Comba, P. et al.; Inorg. Chem., 2009, 48 (21), pp 10389–10396; {{doi|10.1021/ic901702s}}</ref>

===Fe(V)O===
Fe<sup>V</sup>TAML(=O), TAML = [[tetra-amido macrocyclic ligand]], is formed by the reaction of [Fe<sup>III</sup>(TAML)(H<sub>2</sub>O)](PPh<sub>4</sub>) with 2-5 equivalents of meta-chloroperbenzoic acid at -60 ˚C in n-butyronitrile. This deep green compound (two λ<sub>max</sub> at 445 and 630&nbsp;nm respectively) is stable at 77 K.  The stabilization of Fe(V) is attributed to the strong π–donor capacity of deprotonated amide nitrogens.<ref>Oliveira, F. T., Chanda, A., Banerjee, D., Shan, X., Mondal, S., Que, L., Bominaar, E. L., Munck, E., Collins, T. J., Science, 315, 835-838, 2007; {{doi|10.1126/science.1133417}}</ref>

===Fe(VI)O===
{{main|Ferrate(VI)}}

Ferrate(VI) is an [[inorganic chemistry|inorganic]] [[anion]] of [[chemical formula]] [FeO<sub>4</sub>]<sup>2−</sup>. It is [[Photosensitivity#Interpretation in chemistry|photosensitive]] and contributes a pale violet colour to its compounds and solutions. It is one of the strongest water-stable [[oxidizing agent|oxidising species]] known. Although it is classified as a [[basicity|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.<ref>Fujii, H.; Coordination Chemistry Reviews Volume 226, Issues 1-2, March 2002, Pages 51-60; {{doi|10.1016/S0010-8545(01)00441-6}}</ref>

==Nitridoiron and imidoiron compounds==
[[File:wieghardt.png|thumb|240px|right|generation of an nitridoiron(VI) complex]]
[[Metal nitrido complex|Nitridoiron]]<ref>Berry, J.F.; Comments on Inorganic Chemistry, 30: 28–66, 2009; {{doi|10.1080/02603590902768875}}</ref> and imidoiron<ref>Peters, J.C., Mehn, M.P.; Journal of Inorganic Biochemistry Volume 100, Issue 4, April 2006, Pages 634-643; {{doi|10.1016/j.jinorgbio.2006.01.023}}</ref> compounds are closely related to [[Haber process|iron-dinitrogen chemistry]].<ref>Tyler, D. R., Crossland, J. E.; Coordination Chemistry Reviews 254 (2010) 1883–1894; {{doi|10.1016/j.ccr.2010.01.005}}</ref> The biological significance of nitridoiron(V) [[porphyrin]]s has been reviewed.<ref>Nakamoto, K.; Coordination Chemistry Reviews Volume 226, Issues 1-2, March 2002, Pages 153-165; {{doi|10.1016/S0010-8545(01)00425-8}}</ref><ref>Nakamoto, K.; Journal of Molecular Structure Volumes 408-409, 1 June 1997, Pages 11-16; {{doi|10.1016/S0022-2860(96)09670-6}}</ref> A widely applicable method to generate high-valent nitridoiron species is the thermal or photochemical oxidative elimination of molecular nitrogen from an [[azide]] complex.
:<math>\mathrm{(L)Fe^{n}N_{3} \longrightarrow (L)Fe^{n+2}N + N_{2}}</math>
:<small>symbolic oxidative elimination of nitrogen yields a nitridoiron complex; L denotes the supporting ligand. </small>

===Fe(IV)N===
Several structurally characterized nitridoiron(IV) compounds exist.<ref>Peters, Jonas C.; Que, Lawrence, Jr. et al.; Inorg. Chem., 2007, 46 (14), pp 5720–5726; {{doi|10.1021/ic700818q}}</ref><ref>Smith et al.; Angewandte Chemie International Edition Volume 48, Issue 17, pages 3158–3160, April 14, 2009; {{doi|10.1002/anie.200900381}}</ref><ref>Meyer et al.; Angewandte Chemie International Edition Volume 47, Issue 14, pages 2681–2684, March 25, 2008, April 14, 2009; {{doi|10.1002/anie.200800600}}</ref>

===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.<ref>Wagner, W.D.; Nakamoto, K.; J. Am. Chem. Soc., 1988, 110 (12), pp 4044–4045; {{doi|10.1021/ja00220a057}}</ref><ref>Wagner, W.D.; Nakamoto, K.; J. Am. Chem. Soc., 1989, 111 (5), pp 1590–1598; {{doi|10.1021/ja00187a010}}</ref>

===Fe(VI)N===
A second Fe<sup>VI</sup> species apart from the [[ferrate(VI)]] ion, [(Me<sub>3</sub>cy-ac)FeN](PF<sub>6</sub>)<sub>2</sub>, 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]].<ref>Berry, J. F., Bill, E., Bothe, E., George, S. D., Miener, B., Neese, F., Wieghardt, K., Science, 312, 1937-1941, 2006; {{doi|10.1126/science.1128506}}</ref>

===μ-Nitrido compounds and oxidation catalysis<ref>Review: Que, L., Tolman, W.B.; Nature 455, 333-340 (18 September 2008); {{doi|10.1038/nature07371}}</ref>===
Bridged μ-nitrido di-iron [[phthalocyanine]] compounds catalyze the oxidation of [[methane]] to [[methanol]], [[formaldehyde]], and [[formic acid]] using [[hydrogen peroxide]] as sacrificial oxidant.<ref>Sorokin, A.B.; Kudrik, E.V.; Bouchu, D.; Chem. Commun., 2008, 2562-2564; {{doi|10.1039/B804405H}}</ref>

===Electronic structure===
Nitridoiron(IV) and nitridoiron(V) species were first explored theoretically in 2002.<ref>Dey, A.; Ghosh, A.; J. Am. Chem. Soc., 2002, 124 (13), pp 3206–3207; {{doi|10.1021/ja012402s}}</ref>

==References==
{{Reflist}}

==See also==
*[[Jacobsen's catalyst]] (high-valent manganese)

==Further reading==
*Solomon et al.; Angewandte Chemie International Edition Volume 47, Issue 47, pages 9071–9074, November 10, 2008; {{doi|10.1002/anie.200803740}}

[[Category:Iron]]
[[Category:Oxidizing agents]]
[[Category:Inorganic chemistry]]
[[Category:Cluster chemistry]]
[[Category:Coordination compounds]]